Heterosis is more important than propagule pressure for the establishment of invasive hybrid cattail (Typha x glauca) populations

Heterosis is more important than propagule pressure for the establishment of invasive hybrid cattail (Typha x glauca) populations | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Heterosis is more important than propagule pressure for the establishment of invasive hybrid cattail (Typha x glauca) populations Joanna Freeland, Olivia Kowalczyk, Margaret Brennan, Marcel Dorken This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4632132/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract A notable proportion of invasive plant taxa are interspecific hybrids, and their success can be influenced by both the frequency with which hybrids are formed (propagule pressure) and their ability to outcompete their parent species. A vast cattail hybrid zone in central Canada and the USA comprises T. latifolia, T. angustifolia , and their hybrid T. x glauca. The maternal parent is T. angustifolia , which in some regions is less common than T. latifolia or T. x glauca ; whether this translates into low T. x glauca propagule pressure will depend partly on whether T. angustifolia produces a high proportion of hybrids. The success of hybrids also depends on seedling establishment, and although T. x glauca exhibits heterosis at later life stages, little is known about its competitive ability at the seedling stage. We tested whether propagule pressure and/or competitive ability can help to explain the successful establishment of invasive T. x glauca. We collected fruit from 14 maternal T. angustifolia plants across 12 sites in and around Peterborough, Ontario, Canada, and grew seedlings from each plant both singly (without competition) and in groups (with competition). We used genetic data to assign a subset of seedlings to taxon, and found that overall, most seedlings (78%) were T. angustifolia , suggesting relatively low propagule pressure for hybrids. However, significantly more T. angustifolia seedlings (86%) grew singly - and thus without competition - compared to those grown in a group, competitive environment (71%). Typha hybrids dominate wetlands across a substantial area including the Laurentian Great Lakes and Prairie Pothole regions, and our data suggest that strong competitive ability is more important than propagule pressure for the establishment of these successful invaders. Typha wetlands hybrids propagule pressure competition Introduction Interspecific hybridization has been – and continues to be - an extremely important evolutionary force across taxa, for example an estimated 10% of both plant and animal species hybridize with heterospecifics, and hybridization has played an important role in the diversification of angiosperms (Schwenk et al. 2008; Soltis and Soltis 2009; Sætre 2013; Yakimowski and Rieseberg 2014). Hybridization combines two diverged genomes in a single organism, and therefore its outcomes are highly variable. On the one hand, novel gene combinations can lead to incompatibilities among genomes, proteins, or phenotypic traits, and hence inviable or sterile offspring (Piatkowska et al. 2013; Arnegard et al. 2014; Sloan et al. 2017). In other cases hybrids exhibit heterosis, also known as hybrid vigor, which occurs when hybrids have higher fitness than the parental species. While the molecular mechanisms underlying heterosis are not fully understood, it can be broadly explained by genomic and epigenetic processes that result from novel interactions between alleles that influence the growth, stress tolerance, and ultimately the fitness of hybrids (Chen 2013; Das et al. 2021). The success of hybrids may also depend on the frequency with which they are formed, because the likelihood of hybrid success should increase when more hybrids are produced. For example, hybridization between two ecotypes of Avena barbata resulted in hybrids that collectively showed a mix of hybrid vigor and hybrid breakdown (Johansen-Morris and Latta 2006), and under this scenario increased production of hybrid seeds should increase the likelihood of genetic combinations that lead to heterosis. This is analogous to the theory of propagule pressure in invasion biology, which refers to the number, size, and frequency with which propagules are introduced into a novel area: high propagule pressure should lead to a higher likelihood of successful establishment (Simberloff 2009). While largely discussed in the context of biological invasions, propagule pressure has also been identified as a potential factor limiting the establishment of local polyploid populations: if far fewer allo- or autopolyploid seeds are produced compared to diploid seeds, this should reduce the likelihood that novel polyploids will be among the subset of survivors in the next generation (Levin 2021). Similarly, high propagule pressure can lead to higher frequencies of hybrids between native and introduced species (Bennett et al. 2010), and thus may be relevant to the success of novel, invasive hybrids. A vast cattail hybrid zone around the St. Lawrence Seaway, Laurentian Great Lakes, midwestern USA, and Prairie Pothole regions of North America comprises T. latifolia, T. angustifolia, and their hybrid T. x glauca (Travis et al. 2010; Freeland et al. 2013; Pieper et al. 2020; Geddes et al. 2021; Tangen et al. 2022). Typha spp. are essential components of wetlands around the world, providing valuable ecosystem services and biodiversity habitat (Bansal et al. 2019). However, introductions of non-native Typha have led to biological invasions in some regions, the most well-studied example of this being T. x glauca, which in many regions dominates wetlands, alters ecosystem functioning, and reduces native biodiversity (Tuchman et al. 2009; Larkin et al. 2012b; Lishawa et al. 2014; Lawrence et al. 2016, 2017). F1 T. x glauca results from asymmetrical hybridization in which T. angustifolia is normally the maternal parent that is pollinated by T. latifolia (Kuehn et al. 1999; Ball and Freeland 2013; Pieper et al. 2017) . In addition, F1 hybrids can interbreed to produce advanced-generation hybrids, and can backcross to both parental species (Pieper et al. 2017). Heterosis has been observed at different growth stages in F1 hybrids (Bunbury-Blanchette et al. 2015; Zapfe and Freeland 2015; Szabo et al. 2018), although there is some evidence of breakdown in advanced-generation (F2) hybrids (Bhargav et al. 2022). The success of this hybrid is illustrated both by its ability to outcompete parental species and other taxonomic groups at the local level, and its ongoing range expansion across large areas of North America (Travis et al. 2010; Larkin et al. 2012a; Freeland et al. 2013; Pieper et al. 2020; Geddes et al. 2021; Tangen et al. 2022). In this study we tested two hypotheses that could help us understand the success of T. x glauca . The first of these refers to propagule pressure. Typha angustifolia is the maternal parent of F1 hybrids, and is also the least common taxon in numerous wetlands around the Laurentian Great Lakes, although can be locally abundant (Travis et al. 2010; Freeland et al. 2013; Bunbury-Blanchette et al. 2015; Pieper et al. 2020). One process that may be helping T. x glauca to outcompete T. angustifolia is genetic swamping: the relatively high abundance of hybrids and T. latifolia, combined with their abilities to successfully fertilize T. angustifolia (Pieper et al. 2017) , could mean that the majority of offspring produced by T. angustifolia are hybrids. Under this scenario, propagule pressure may be relatively high, which in turn could be facilitating the establishment and dispersal of T. x glauca. We therefore tested the hypothesis that T. angustifolia is being genetically swamped, and is producing a high proportion of hybrid offspring. Alternatively, the number of seeds produced may be less important than their competitive abilities. Wetlands are dynamic environments, and Typha stands often develop from seeds as opposed to clonal growth (Pieper et al. 2020). Repeated successful establishment of seedlings may therefore rely more on competitive ability, and the second hypothesis that we tested is that competition is more important for seedling establishment than propagule pressure. These two hypotheses are not mutually exclusive, and together could help us to better understand the processes driving the wetland domination and range expansion by T. x glauca. Methods Fieldwork and taxonomic verification From June 21 to July 12, 2022, we located 12 sites in and around Peterborough, Ontario, Canada, in which morphological characteristics (leaf width, spike gap, spike width, and spike length (Smith 1967; Grace and Harrison 1986) reflected combinations of T. angustifolia, T. latifolia, and T. x glauca . We identified up to three target, putative T. angustifolia at each site, recorded their GPS locations using a Bad Elf Flex (Bad Elf, LLC), and labelled plants on their leaves with permanent markers. Backcrossed and advanced-generation hybrids mean that morphological characteristics can overlap between hybrids and parental species (Geddes et al. 2021; Tangen et al. 2022), and we therefore collected a leaf fragment from each focal, putative T. angustifolia for genetic taxonomic verification, and placed these in coin envelopes within re-sealable plastic bags that contained Sorbead silica beads for desiccation. DNA was extracted from dried leaf samples and genotyped at five loci (one microsatellite locus and four PCR-RFLP loci) that harbour species-specific alleles, following the methods of (Chambers et al. 2024). We retained in our study 14 maternal plants from 12 sites, because these were homozygous for T. angustifolia alleles at all five loci. Seed germination, growth, and taxonomic identification On September 14-15, 2022, we returned to the field sites and collected fruit from focal plants confirmed as Typha angustifolia, and left fruit in paper bags to completely dry at room temperature. Once dried, we separated the seeds from the stem and processed these following the protocol of (Ahee et al. 2014). Processed seeds from each plant were transferred to individual petri dishes and left to germinate in the greenhouse for 7-10 days. Seeds were then transferred into pots that provided two types of growing conditions for each maternal plant: singles, and groups. Singles refers to germinated seedlings that were individually transplanted into the cells of a 200-cell plug tray (1 seed per cell; each cell 2.3 cm × 2.3 cm × 4.5 cm; T.O. Plastics, Clearwater MN), and which therefore grew without competition. In contrast, groups were created by the transfer of multiple seedlings (approximately 40-80 seedlings in each pot) from the same maternal plant into a single 4” pot, and thus grew in competition with one another. Both cells and pots were filled with Jiffy mix #1 soil (Jiffy Products of America, Lorain, Ohio, USA), and rested in plastic trays filled with water. Seedlings were fertilized after 45 days using 100 mL of 0.5 % water-soluble 20:20:20 N:P: K general-purpose fertilizer (Plant-Prod, Leamington, Ontario). When seedlings had reached ~ 8 cm (after approximately 20-30 days for singles, 80-90 days for groups), 10 single seedlings and ten group seedlings per maternal plant were harvested using tweezers to remove the entire plant. The leaves were detached from the roots using tweezers and the leaves were cleaned of any residual soil. The leaves were then placed in coin envelopes, dried in silica beads, and DNA was extracted following the same methods as described above. Taxonomic identification also followed the same methods described above but with one difference: we genotyped each offspring at four loci with species-specific alleles, which is sufficient to differentiate parent species, F1 hybrids, and backcrossed hybrids (Boecklen and Howard 1997). Offspring were identified as T. angustifolia if they had only T. angustifolia alleles at all loci; as F1 hybrids if they were heterozygous for T. latifolia and T. angustifolia alleles at all loci; and as backcrossed hybrids if they had one or more loci homozygous for T. angustifolia plus one or more loci heterozygous for T. angustifolia and T. latifiolia alleles. Five samples did not amplify at all loci, and so the final data set includes marker-based taxonomic IDs from a total of 275 seedlings. Data analysis We compared the proportion of seedlings emerging in the two growth conditions (singles versus groups) using a generalized linear model with quasi-binomial errors to account for overdispersion using the glm function in R (v. 4.4.1; (R Core Team 2024). In the model, treatment (two levels: singles and groups) was the independent variable and the number of T. angustifolia and hybrid offspring was the (compound) response variable. Analysis of deviance was calculated using the Anova function from the car package (Fox and Weisberg 2019). Data and R scripts are available at https://doi.org/10.6084/m9.figshare.26069629.v1 Results More than three quarters of all seedlings screened were identified as T. angustifolia (78% T. angustifolia versus 22% hybrid offspring). However, the proportion of T. angustifolia seedlings produced across sites depended on the conditions under which seeds were germinated. Among seedlings grown singly, 86% were identified as T. angustifolia , but there were only 71% T. angustifolia among seedlings sampled from group pots (one-way analysis of deviance: Likelihood ratio χ 2 = 4.28, df = 1, P < 0.05). Table 1 The proportion of Typha angustifolia seedlings that germinated and became established among the seeds produced by T. angustifolia maternal parents depended on how the seeds were germinated. There was a higher proportion of T. angustifolia seedlings among seeds grown singly within the cells of Treatment T. angustifolia T. × glauca Total Single 117 19 136 Grouped 98 41 139 Discussion While much of the research on invasive hybrids has focussed on heterosis, the varied outcomes of hybridization, even when limited to intraspecific crosses (Johansen-Morris and Latta 2006 ; Hahn and Rieseberg 2017 ; Irimia et al. 2021 ), could mean that propagule pressure plays an important role in determining the success of novel hybrids. The establishment of invasive hybrids could therefore be explained by high propagule pressure, heterosis, or a combination of the two (Luquet et al. 2011 ). An example of the former was found in Pyrus calleryana , an ornamental tree species introduced from China into North America. Intraspecific hybridization between different cultivars has led to invasive populations that have recently began to spread across the USA, and this is at least partly attributable to high propagule pressure as a result of substantial seed set and germination rates (Hardiman and Culley 2010 ). A constrasting situation occurs in southern Louisiana, where natural hybridization occurs among three Iris species ( Iris hexagona, Iris fulva and Iris brevicaulis ). In this hybrid zone, strong prezygotic isolation results in low propagule pressure, but the hybrids often show higher fitness measures than their parent species and thus can establish and persist despite being formed relatively infrequently (Carney et al. 1994 ; Arnold 1997 ; Burke et al. 2006 ; Martin et al. 2007 ; Taylor et al. 2009 ). In this study we found that only ~ 22% of T. angustifolia seedlings that we genotyped were hybrids. This was an unexpected finding because our study was conducted in a region where T. angustifolia is the least common Typha taxon, and hand-pollination experiments have demonstrated comparable seed production by T. angustifolia regardless of whether they are pollinated by T. angustifolia, T. latifolia , or T. x glauca (Pieper et al. 2017 ). This suggests alternative reproductive barriers to the production of hybrids such as differences in flowering phenology between T. angustifolia and both T. latifolia and T. x glauca . Our data therefore show that propagule pressure is lower in T. x glauca than in T. angustifolia , and therefore unlikely to play an important role in the successful establishment of invasive Typha hybrid populations. However, when T. angustifolia seedlings were grown in competition, there was a significant increase in the proportion of hybrids, which demonstrates superior competitive ability in hybrid seedlings compared to T. angustifolia seedlings. This evidence for heterosis at an early life stage adds to previous studies that reported heterosis in T. x glauca at later life stages (Travis et al. 2011 ; Bunbury-Blanchette et al. 2015 ; Zapfe and Freeland 2015 ; Szabo et al. 2018 ). Collectively these studies show that heterosis in the form of superior growth and survival can explain the persistence and establishment of T. x glauca despite low propagule pressure. Our finding that heterosis in hybrid seedlings is more important than propagule pressure for the establishment of T. x glauca populations has a number of important implications. Previous studies have suggested that T. angustifolia could be a limiting factor for the long-term persistence of T. x glauca because of its relative scarcity across much of the hybrid zone; future creation of F1 hybrids could thus be limited by the availability of maternal plants combined with the breakdown of advanced-generation hybrids (Bhargav et al. 2022 ). However, we found no evidence to suggest that genetic swamping by T. latifolia or T. x glauca pollen is threatening the persistence of T. angustifolia ; furthermore, even if T. angustifolia is relatively scarce it can reproduce following self-fertilization, which does not appear to lead to inbreeding depression (Whitehead et al. 2024 ). The persistence of T. angustifolia at low abundance and occupancy may have little to no impact on the continued establishment of T. x glauca populations. Recent studies have identified substantial areas of T. x glauca range expansion west of the Laurentian Great Lakes (Geddes et al. 2021 ; Tangen et al. 2022 ). Deb Joyee et al. (in review) identified a leading edge of this range expansion in the western Prairie Pothole Region of Canada, from where there are few to no historical reports of T. angustifolia. In this region T. angustifolia occurs in fewer sites, and across a smaller area, than T. x glauca , despite the fact that – similarly to this study - most of the genotyped seedlings (~ 87%) from nine flowering T. angustifolia were T. angustifolia. Heterosis is therefore likely more important than propagule pressure within both established and expanding regions of this hybrid zone. In conclusion, this and other studies collectively suggest that the spread of T. x glauca is unlikely to be limited by the availability of its maternal plant T. angustifolia. Typha latifolia produces very few seeds when pollinated by anything other than a conspecific (Pieper et al. 2017 ), and therefore should be producing almost entirely T. latifolia seeds. The Typha seed pool during the initial stages of T. x glauca establishment should therefore comprise primarily T. latifolia and – as identified in this study - T. angustifolia seeds. Despite the relative scarcity of T. x glauca in the total Typha seed pool of central and eastern North America, this invasive hybrid now dominates wetlands across a vast geographic area and is continuing to expand its range (Travis et al. 2010 ; Freeland et al. 2013 ; Pieper et al. 2020 ; Geddes et al. 2021 ; Tangen et al. 2022 ). This success can be attributed to heterosis, which will make future and ongoing management of invasive stands very challenging. Declarations Funding Funding for this project was provided by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada awarded to J. Freeland (RGPIN-2023-03305) and M. Dorken (RGPIN-2018-04866). Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contributions Study conception and design by Joanna Freeland and Marcel Dorken. Field work (sample collection) was performed by Olivia Kowalcyk, laboratory and greenhouse methods by Olivia Kowalcyk and Margaret Brennan (both students co-supervised by Joanna Freeland and Marcel Dorken). Data analysis was performed by Marcel Dorken. The first draft of the manuscript was written by Joanna Freeland and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgments Many thanks to Avery Chambers, Braidy Chambers, and Heather Wilcox for assistance in the field and lab. Funding for this project was provided by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada awarded to J. Freeland (RGPIN-2023-03305) and M. Dorken (RGPIN-2018-04866). References Ahee JE, Van Drunen WE, Dorken ME (2014) Analysis of pollination neighbourhood size using spatial analysis of pollen and seed production in broadleaf cattail ( Typha latifolia ) . 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Aquat Bot 125:44–47. https://doi.org/10.1016/j.aquabot.2015.05.004 Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4632132","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":319475383,"identity":"958f1d2e-9e67-4973-81e5-4bfa494fa220","order_by":0,"name":"Joanna Freeland","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABAUlEQVRIiWNgGAWjYBACxgYwJQfEzEAmmw3RWoyhTLY0CYQgfgDXcpiwFub25mePC2oM8vnZGxs/F5SdrzNv7z3+mIfBTh6nw3qOmRvPOGZgObPnYLP0jHO3JWTOnEts5mFINsRlFeOMBDNpHrY/BgY3EhukedtuS0hI5BgCtRzA6TrGGenfpHn+GRjY33/Y/Ju37ZyEhPwbsBZ73FpyzICGGxgYSDC2ARkHgLbwgLUk4tTSc6ZMmrcPqONMYps1z7lkyRk8OYYz5xgkJ+PSYtjevk2a55uBAX/74cO3ecrs+CXYzxh8eFNhZ4tTCw4JAxzqgQBn6I+CUTAKRsEogAMAj4xQWfBYjCEAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-5251-7680","institution":"Trent University Department of Biology","correspondingAuthor":true,"prefix":"","firstName":"Joanna","middleName":"","lastName":"Freeland","suffix":""},{"id":319475384,"identity":"938e91c9-aa11-4cd7-ac34-053bd40d9b2e","order_by":1,"name":"Olivia Kowalczyk","email":"","orcid":"","institution":"Trent University","correspondingAuthor":false,"prefix":"","firstName":"Olivia","middleName":"","lastName":"Kowalczyk","suffix":""},{"id":319475385,"identity":"678be00b-2cc6-42ca-948a-6173feca7fc5","order_by":2,"name":"Margaret Brennan","email":"","orcid":"","institution":"Trent University","correspondingAuthor":false,"prefix":"","firstName":"Margaret","middleName":"","lastName":"Brennan","suffix":""},{"id":319475386,"identity":"60c47418-49e5-4c42-832b-09c5d50969e0","order_by":3,"name":"Marcel Dorken","email":"","orcid":"","institution":"Trent University Department of Biology","correspondingAuthor":false,"prefix":"","firstName":"Marcel","middleName":"","lastName":"Dorken","suffix":""}],"badges":[],"createdAt":"2024-06-24 19:51:37","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-4632132/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4632132/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":65767758,"identity":"ec47214d-2f84-48da-9f25-3657350ebf88","added_by":"auto","created_at":"2024-10-02 12:01:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":370235,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4632132/v1/69878091-8526-40ae-93a5-9714fc166174.pdf"}],"financialInterests":"","formattedTitle":"Heterosis is more important than propagule pressure for the establishment of invasive hybrid cattail (Typha x glauca) populations","fulltext":[{"header":"Introduction","content":"\u003cp\u003eInterspecific hybridization has been \u0026ndash; and continues to be \u0026nbsp;- an extremely important evolutionary force across taxa, for example an estimated 10% of both plant and animal species hybridize with heterospecifics, and hybridization has played an important role in the diversification of angiosperms\u0026nbsp;(Schwenk et al. 2008; Soltis and Soltis 2009; S\u0026aelig;tre 2013; Yakimowski and Rieseberg 2014). \u0026nbsp;Hybridization combines two diverged genomes in a single organism, and therefore its outcomes are highly variable. \u0026nbsp;On the one hand, novel gene combinations can lead to incompatibilities among genomes, proteins, or phenotypic traits, and hence inviable or sterile offspring\u0026nbsp;(Piatkowska et al. 2013; Arnegard et al. 2014; Sloan et al. 2017). \u0026nbsp;In other cases hybrids exhibit heterosis, also known as hybrid vigor, which occurs when hybrids have higher fitness than the parental species. \u0026nbsp;While the molecular mechanisms underlying heterosis are not fully understood, it can be broadly explained by genomic and epigenetic processes that result from novel interactions between alleles that influence the growth, stress tolerance, and ultimately the fitness of hybrids\u0026nbsp;(Chen 2013; Das et al. 2021).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe success of hybrids may also depend on the frequency with which they are formed, because the likelihood of hybrid success should increase when more hybrids are produced. \u0026nbsp;For example, hybridization between two ecotypes of \u003cem\u003eAvena barbata\u003c/em\u003e resulted in hybrids that collectively showed a mix of hybrid vigor and hybrid breakdown\u0026nbsp;(Johansen-Morris and Latta 2006), and under this scenario increased production of hybrid seeds should increase the likelihood of genetic combinations that lead to heterosis. \u0026nbsp; This is analogous to the theory of propagule pressure in invasion biology, which refers to the number, size, and frequency with which propagules are introduced into a novel area: high propagule pressure should lead to a higher likelihood of successful establishment\u0026nbsp;(Simberloff 2009). \u0026nbsp;While largely discussed in the context of biological invasions, propagule pressure has also been identified as a potential factor limiting the establishment of local polyploid populations: if far fewer allo- or autopolyploid seeds are produced compared to diploid seeds, this should reduce the likelihood that novel polyploids will be among the subset of survivors in the next generation\u0026nbsp;(Levin 2021). \u0026nbsp;Similarly, high propagule pressure can lead to higher frequencies of hybrids between native and introduced species\u0026nbsp;(Bennett et al. 2010), and thus may be relevant to the success of novel, invasive hybrids. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA vast cattail hybrid zone around the St. Lawrence Seaway, Laurentian Great Lakes, midwestern USA, and Prairie Pothole regions of North America comprises \u003cem\u003eT. latifolia, T. angustifolia,\u0026nbsp;\u003c/em\u003eand their hybrid \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca\u0026nbsp;\u003c/em\u003e(Travis et al. 2010; Freeland et al. 2013; Pieper et al. 2020; Geddes et al. 2021; Tangen et al. 2022). \u0026nbsp;\u003cem\u003eTypha\u0026nbsp;\u003c/em\u003espp. are essential components of wetlands around the world, providing valuable ecosystem services and biodiversity habitat\u0026nbsp;(Bansal et al. 2019). \u0026nbsp;However, introductions of non-native \u003cem\u003eTypha\u003c/em\u003e have led to biological invasions in some regions, the most well-studied example of this being \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca,\u0026nbsp;\u003c/em\u003ewhich in many regions dominates wetlands, alters ecosystem functioning, and reduces native biodiversity\u0026nbsp;(Tuchman et al. 2009; Larkin et al. 2012b; Lishawa et al. 2014; Lawrence et al. 2016, 2017). \u0026nbsp;F1 \u003cem\u003eT. \u0026nbsp;\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca\u0026nbsp;\u003c/em\u003eresults from asymmetrical hybridization in which \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eis normally the maternal parent that is pollinated by \u003cem\u003eT. latifolia\u0026nbsp;\u003c/em\u003e(Kuehn et al. 1999; Ball and Freeland 2013; Pieper et al. 2017)\u003cem\u003e. \u0026nbsp;\u003c/em\u003eIn addition, F1 hybrids can interbreed to produce advanced-generation hybrids, and can backcross to both parental species\u0026nbsp;(Pieper et al. 2017). \u0026nbsp;Heterosis has been observed at different growth stages in F1 hybrids\u0026nbsp;(Bunbury-Blanchette et al. 2015; Zapfe and Freeland 2015; Szabo et al. 2018), although there is some evidence of breakdown in advanced-generation (F2) hybrids\u0026nbsp;(Bhargav et al. 2022). \u0026nbsp;The success of this hybrid is illustrated both by its ability to outcompete parental species and other taxonomic groups at the local level, and its ongoing range expansion across large areas of North America\u0026nbsp;(Travis et al. 2010; Larkin et al. 2012a; Freeland et al. 2013; Pieper et al. 2020; Geddes et al. 2021; Tangen et al. 2022).\u003c/p\u003e\n\u003cp\u003eIn this study we tested two hypotheses that could help us understand the success of \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca\u003c/em\u003e. \u0026nbsp;The first of these refers to propagule pressure. \u0026nbsp;\u003cem\u003eTypha angustifolia\u0026nbsp;\u003c/em\u003eis the maternal parent of F1 hybrids, and is also the least common taxon in numerous wetlands around the Laurentian Great Lakes, although can be locally abundant\u0026nbsp;(Travis et al. 2010; Freeland et al. 2013; Bunbury-Blanchette et al. 2015; Pieper et al. 2020). \u0026nbsp;One process that may be helping \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca\u0026nbsp;\u003c/em\u003eto outcompete \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eis genetic swamping: the relatively high abundance of hybrids and \u003cem\u003eT. latifolia,\u0026nbsp;\u003c/em\u003ecombined with their abilities to successfully fertilize \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003e(Pieper et al. 2017)\u003cem\u003e,\u0026nbsp;\u003c/em\u003ecould mean that the majority of offspring produced by \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eare hybrids. \u0026nbsp;Under this scenario, propagule pressure may be relatively high, which in turn could be facilitating the establishment and dispersal of \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca. \u0026nbsp;\u003c/em\u003eWe therefore tested the hypothesis that \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eis being genetically swamped, and is producing a high proportion of hybrid offspring. \u0026nbsp;Alternatively, the number of seeds produced may be less important than their competitive abilities. \u0026nbsp;Wetlands are dynamic environments, and \u003cem\u003eTypha\u0026nbsp;\u003c/em\u003estands often develop from seeds as opposed to clonal growth\u0026nbsp;(Pieper et al. 2020). \u0026nbsp;Repeated successful establishment of seedlings may therefore rely more on competitive ability, and the second hypothesis that we tested is that competition is more important for seedling establishment than propagule pressure. \u0026nbsp; These two hypotheses are not mutually exclusive, and together could help us to better understand the processes driving the wetland domination and range expansion by \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca.\u0026nbsp;\u003c/em\u003e\u003c/p\u003e"},{"header":"Methods","content":"\u003ch3\u003eFieldwork and taxonomic verification\u003c/h3\u003e\n\u003cp\u003eFrom June 21 to July 12, 2022, we located 12 sites\u003cem\u003e\u0026nbsp;\u003c/em\u003ein and around Peterborough, Ontario, Canada, in which morphological characteristics (leaf width, spike gap, spike width, and spike length\u0026nbsp;(Smith 1967; Grace and Harrison 1986)\u0026nbsp;reflected combinations of \u003cem\u003eT. angustifolia, T. latifolia,\u0026nbsp;\u003c/em\u003eand \u003cem\u003eT.\u0026nbsp;\u003c/em\u003ex \u003cem\u003eglauca\u003c/em\u003e. \u0026nbsp; We identified up to three target, putative \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eat each site, recorded their GPS locations using a Bad Elf Flex (Bad Elf, LLC), and labelled plants on their leaves with permanent markers. \u0026nbsp; Backcrossed and advanced-generation hybrids mean that morphological characteristics can overlap between hybrids and parental species\u0026nbsp;(Geddes et al. 2021; Tangen et al. 2022), and we therefore collected a leaf fragment from each focal, putative \u003cem\u003eT. angustifolia\u003c/em\u003e for genetic taxonomic verification, and placed these in coin envelopes within re-sealable plastic bags that contained Sorbead silica beads for desiccation. DNA was extracted from dried leaf samples and genotyped at five loci (one microsatellite locus and four PCR-RFLP loci) that harbour species-specific alleles, following the methods of\u0026nbsp;(Chambers et al. 2024). \u0026nbsp;We retained in our study 14 maternal plants from 12 sites, because these were homozygous for \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003ealleles at all five loci.\u0026nbsp;\u003c/p\u003e\n\u003ch3\u003eSeed germination, growth, and taxonomic identification\u003c/h3\u003e\n\u003cp\u003eOn September 14-15, 2022, we returned to the field sites and collected fruit from focal plants confirmed as \u003cem\u003eTypha angustifolia,\u003c/em\u003e and left fruit in paper bags to completely dry at room temperature. Once dried, we separated the seeds from the stem and processed these following the protocol of\u0026nbsp;(Ahee et al. 2014). \u0026nbsp;Processed seeds from each plant were transferred to individual petri dishes and left to germinate in the greenhouse for 7-10 days. \u0026nbsp;Seeds were then transferred into pots that provided two types of growing conditions for each maternal plant: singles, and groups. \u0026nbsp; Singles refers to germinated seedlings that were individually transplanted into the cells of a 200-cell plug tray (1 seed per cell; each cell 2.3 cm\u0026nbsp;\u0026times; 2.3 cm \u0026times; 4.5 cm;\u0026nbsp;T.O. Plastics, Clearwater MN), and which therefore grew without competition. \u0026nbsp; In contrast, groups were created by the transfer of multiple seedlings (approximately 40-80 seedlings in each pot) from the same maternal plant into a single 4\u0026rdquo; pot, and thus grew in competition with one another. \u0026nbsp;Both cells and pots were filled with Jiffy mix #1 soil (Jiffy Products of America, Lorain, Ohio, USA),\u0026nbsp;and rested in plastic trays filled with water. \u0026nbsp;Seedlings were fertilized after 45 days using 100 mL of 0.5 % water-soluble 20:20:20 N:P: K general-purpose fertilizer (Plant-Prod, Leamington, Ontario).\u003c/p\u003e\n\u003cp\u003eWhen seedlings had reached ~ 8 cm (after approximately 20-30 days for singles, 80-90 days for groups), 10 single seedlings and ten group seedlings per maternal plant were harvested using tweezers to remove the entire plant. \u0026nbsp;The leaves were detached from the roots using tweezers and the leaves were cleaned of any residual soil. The leaves were then placed in coin envelopes, dried in silica beads, and DNA was extracted following the same methods as described above. \u0026nbsp; Taxonomic identification also followed the same methods described above but with one difference: we genotyped each offspring at four loci with species-specific alleles, which is sufficient to differentiate parent species, F1 hybrids, and backcrossed hybrids\u0026nbsp;(Boecklen and Howard 1997). \u0026nbsp;Offspring were identified as \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eif they had only \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003ealleles at all loci; as F1 hybrids if they were heterozygous for \u003cem\u003eT. latifolia\u0026nbsp;\u003c/em\u003eand \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003ealleles at all loci; and as backcrossed hybrids if they had one or more loci homozygous for \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eplus one or more loci heterozygous for \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eand \u003cem\u003eT. latifiolia\u0026nbsp;\u003c/em\u003ealleles. Five samples did not amplify at all loci, and so the final data set includes marker-based taxonomic IDs from a total of 275 seedlings.\u003c/p\u003e\n\u003ch3\u003eData analysis\u003c/h3\u003e\n\u003cp\u003eWe compared the proportion of seedlings emerging in the two growth conditions (singles versus groups) using a generalized linear model with quasi-binomial errors to account for overdispersion using the \u003cem\u003eglm\u003c/em\u003e function in R (v. 4.4.1;\u0026nbsp;(R Core Team 2024). In the model, treatment (two levels: singles and groups) was the independent variable and the number of \u003cem\u003eT. angustifolia\u0026nbsp;\u003c/em\u003eand hybrid offspring was the (compound) response variable. Analysis of deviance was calculated using the \u003cem\u003eAnova\u003c/em\u003e function from the car package (Fox and Weisberg 2019). Data and R scripts are available at https://doi.org/10.6084/m9.figshare.26069629.v1\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eMore than three quarters of all seedlings screened were identified as \u003cem\u003eT. angustifolia\u003c/em\u003e (78% \u003cem\u003eT. angustifolia\u003c/em\u003e versus 22% hybrid offspring). However, the proportion of \u003cem\u003eT. angustifolia\u003c/em\u003e seedlings produced across sites depended on the conditions under which seeds were germinated. Among seedlings grown singly, 86% were identified as \u003cem\u003eT. angustifolia\u003c/em\u003e, but there were only 71% \u003cem\u003eT. angustifolia\u003c/em\u003e among seedlings sampled from group pots (one-way analysis of deviance: Likelihood ratio χ\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;4.28, \u003cem\u003edf\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe proportion of \u003cem\u003eTypha angustifolia\u003c/em\u003e seedlings that germinated and became established among the seeds produced by \u003cem\u003eT. angustifolia\u003c/em\u003e maternal parents depended on how the seeds were germinated. There was a higher proportion of \u003cem\u003eT. angustifolia\u003c/em\u003e seedlings among seeds grown singly within the cells of\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTreatment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eT. angustifolia\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eT. \u0026times; glauca\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTotal\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSingle\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e117\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e136\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGrouped\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e139\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eWhile much of the research on invasive hybrids has focussed on heterosis, the varied outcomes of hybridization, even when limited to intraspecific crosses (Johansen-Morris and Latta \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Hahn and Rieseberg \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Irimia et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), could mean that propagule pressure plays an important role in determining the success of novel hybrids. The establishment of invasive hybrids could therefore be explained by high propagule pressure, heterosis, or a combination of the two (Luquet et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). An example of the former was found in \u003cem\u003ePyrus calleryana\u003c/em\u003e, an ornamental tree species introduced from China into North America. Intraspecific hybridization between different cultivars has led to invasive populations that have recently began to spread across the USA, and this is at least partly attributable to high propagule pressure as a result of substantial seed set and germination rates (Hardiman and Culley \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). A constrasting situation occurs in southern Louisiana, where natural hybridization occurs among three Iris species (\u003cem\u003eIris hexagona, Iris fulva\u003c/em\u003e and \u003cem\u003eIris brevicaulis\u003c/em\u003e). In this hybrid zone, strong prezygotic isolation results in low propagule pressure, but the hybrids often show higher fitness measures than their parent species and thus can establish and persist despite being formed relatively infrequently (Carney et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Arnold \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Burke et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Martin et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Taylor et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this study we found that only\u0026thinsp;~\u0026thinsp;22% of \u003cem\u003eT. angustifolia\u003c/em\u003e seedlings that we genotyped were hybrids. This was an unexpected finding because our study was conducted in a region where \u003cem\u003eT. angustifolia\u003c/em\u003e is the least common \u003cem\u003eTypha\u003c/em\u003e taxon, and hand-pollination experiments have demonstrated comparable seed production by \u003cem\u003eT. angustifolia\u003c/em\u003e regardless of whether they are pollinated by \u003cem\u003eT. angustifolia, T. latifolia\u003c/em\u003e, or \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e (Pieper et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). This suggests alternative reproductive barriers to the production of hybrids such as differences in flowering phenology between \u003cem\u003eT. angustifolia\u003c/em\u003e and both \u003cem\u003eT. latifolia\u003c/em\u003e and \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e. Our data therefore show that propagule pressure is lower in \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e than in \u003cem\u003eT. angustifolia\u003c/em\u003e, and therefore unlikely to play an important role in the successful establishment of invasive \u003cem\u003eTypha\u003c/em\u003e hybrid populations. However, when \u003cem\u003eT. angustifolia\u003c/em\u003e seedlings were grown in competition, there was a significant increase in the proportion of hybrids, which demonstrates superior competitive ability in hybrid seedlings compared to \u003cem\u003eT. angustifolia\u003c/em\u003e seedlings. This evidence for heterosis at an early life stage adds to previous studies that reported heterosis in \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e at later life stages (Travis et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Bunbury-Blanchette et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zapfe and Freeland \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Szabo et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Collectively these studies show that heterosis in the form of superior growth and survival can explain the persistence and establishment of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e despite low propagule pressure.\u003c/p\u003e \u003cp\u003eOur finding that heterosis in hybrid seedlings is more important than propagule pressure for the establishment of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e populations has a number of important implications. Previous studies have suggested that \u003cem\u003eT. angustifolia\u003c/em\u003e could be a limiting factor for the long-term persistence of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e because of its relative scarcity across much of the hybrid zone; future creation of F1 hybrids could thus be limited by the availability of maternal plants combined with the breakdown of advanced-generation hybrids (Bhargav et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, we found no evidence to suggest that genetic swamping by \u003cem\u003eT. latifolia\u003c/em\u003e or \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e pollen is threatening the persistence of \u003cem\u003eT. angustifolia\u003c/em\u003e; furthermore, even if \u003cem\u003eT. angustifolia\u003c/em\u003e is relatively scarce it can reproduce following self-fertilization, which does not appear to lead to inbreeding depression (Whitehead et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The persistence of \u003cem\u003eT. angustifolia\u003c/em\u003e at low abundance and occupancy may have little to no impact on the continued establishment of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e populations. Recent studies have identified substantial areas of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e range expansion west of the Laurentian Great Lakes (Geddes et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Tangen et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Deb Joyee et al. (in review) identified a leading edge of this range expansion in the western Prairie Pothole Region of Canada, from where there are few to no historical reports of \u003cem\u003eT. angustifolia.\u003c/em\u003e In this region \u003cem\u003eT. angustifolia\u003c/em\u003e occurs in fewer sites, and across a smaller area, than \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e, despite the fact that \u0026ndash; similarly to this study - most of the genotyped seedlings (~\u0026thinsp;87%) from nine flowering \u003cem\u003eT. angustifolia\u003c/em\u003e were \u003cem\u003eT. angustifolia.\u003c/em\u003e Heterosis is therefore likely more important than propagule pressure within both established and expanding regions of this hybrid zone.\u003c/p\u003e \u003cp\u003eIn conclusion, this and other studies collectively suggest that the spread of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e is unlikely to be limited by the availability of its maternal plant \u003cem\u003eT. angustifolia. Typha latifolia\u003c/em\u003e produces very few seeds when pollinated by anything other than a conspecific (Pieper et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), and therefore should be producing almost entirely \u003cem\u003eT. latifolia\u003c/em\u003e seeds. The \u003cem\u003eTypha\u003c/em\u003e seed pool during the initial stages of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e establishment should therefore comprise primarily \u003cem\u003eT. latifolia\u003c/em\u003e and \u0026ndash; as identified in this study - \u003cem\u003eT. angustifolia\u003c/em\u003e seeds. Despite the relative scarcity of \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e in the total \u003cem\u003eTypha\u003c/em\u003e seed pool of central and eastern North America, this invasive hybrid now dominates wetlands across a vast geographic area and is continuing to expand its range (Travis et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Freeland et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Pieper et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Geddes et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Tangen et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This success can be attributed to heterosis, which will make future and ongoing management of invasive stands very challenging.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eFunding for this project was provided by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada awarded to J. Freeland (RGPIN-2023-03305) and M. Dorken (RGPIN-2018-04866).\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contributions\u003c/h2\u003e \u003cp\u003eStudy conception and design by Joanna Freeland and Marcel Dorken. Field work (sample collection) was performed by Olivia Kowalcyk, laboratory and greenhouse methods by Olivia Kowalcyk and Margaret Brennan (both students co-supervised by Joanna Freeland and Marcel Dorken). Data analysis was performed by Marcel Dorken. The first draft of the manuscript was written by Joanna Freeland and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eMany thanks to Avery Chambers, Braidy Chambers, and Heather Wilcox for assistance in the field and lab. Funding for this project was provided by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada awarded to J. Freeland (RGPIN-2023-03305) and M. Dorken (RGPIN-2018-04866).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAhee JE, Van Drunen WE, Dorken ME (2014) \u0026nbsp;Analysis of pollination neighbourhood size using spatial analysis of pollen and seed production in broadleaf cattail ( Typha latifolia ) . 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R Foundation for Statistical Computing, Vienna, Austria\u003c/li\u003e\n \u003cli\u003eS\u0026aelig;tre GP (2013) Hybridization is important in evolution, but is speciation? J. Evol. Biol. 26:\u0026nbsp;256\u0026ndash;258, https://doi.org/10.1111/jeb.12005\u003c/li\u003e\n \u003cli\u003eSchwenk K, Brede N, Streit B (2008) Introduction. Extent, processes and evolutionary impact of interspecific hybridization in animals. Philos. Trans. R. Soc. B Biol. Sci. 363:2805-2811. https://doi.org/10.1098/rstb.2008.0055\u003c/li\u003e\n \u003cli\u003eSimberloff D (2009) The role of propagule pressure in biological invasions. Annu Rev Ecol Evol Syst 40:81-102. https://doi.org/10.1146/annurev.ecolsys.110308.120304\u003c/li\u003e\n \u003cli\u003eSloan DB, Havird JC, Sharbrough J (2017) The on-again, off-again relationship between mitochondrial genomes and species boundaries. 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In press. \u0026nbsp;https://doi.org/https://doi.org/10.1007/s10682-024-10294-4\u003c/li\u003e\n \u003cli\u003eYakimowski SB, Rieseberg LH (2014) The role of homoploid hybridization in evolution: A century of studies synthesizing genetics and ecology. Am J Bot 101:\u0026nbsp;1247-1258. https://doi.org/10.3732/ajb.1400201\u003c/li\u003e\n \u003cli\u003eZapfe L, Freeland JR (2015) Heterosis in invasive F1 cattail hybrids (\u003cem\u003eTypha\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e). Aquat Bot 125:44\u0026ndash;47. https://doi.org/10.1016/j.aquabot.2015.05.004\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Typha, wetlands, hybrids, propagule pressure, competition","lastPublishedDoi":"10.21203/rs.3.rs-4632132/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4632132/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA notable proportion of invasive plant taxa are interspecific hybrids, and their success can be influenced by both the frequency with which hybrids are formed (propagule pressure) and their ability to outcompete their parent species. A vast cattail hybrid zone in central Canada and the USA comprises \u003cem\u003eT. latifolia, T. angustifolia\u003c/em\u003e, and their hybrid \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca.\u003c/em\u003e The maternal parent is \u003cem\u003eT. angustifolia\u003c/em\u003e, which in some regions is less common than \u003cem\u003eT. latifolia\u003c/em\u003e or \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e; whether this translates into low \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e propagule pressure will depend partly on whether \u003cem\u003eT. angustifolia\u003c/em\u003e produces a high proportion of hybrids. The success of hybrids also depends on seedling establishment, and although \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca\u003c/em\u003e exhibits heterosis at later life stages, little is known about its competitive ability at the seedling stage. We tested whether propagule pressure and/or competitive ability can help to explain the successful establishment of invasive \u003cem\u003eT.\u003c/em\u003e x \u003cem\u003eglauca.\u003c/em\u003e We collected fruit from 14 maternal \u003cem\u003eT. angustifolia\u003c/em\u003e plants across 12 sites in and around Peterborough, Ontario, Canada, and grew seedlings from each plant both singly (without competition) and in groups (with competition). We used genetic data to assign a subset of seedlings to taxon, and found that overall, most seedlings (78%) were \u003cem\u003eT. angustifolia\u003c/em\u003e, suggesting relatively low propagule pressure for hybrids. However, significantly more \u003cem\u003eT. angustifolia\u003c/em\u003e seedlings (86%) grew singly - and thus without competition - compared to those grown in a group, competitive environment (71%). \u003cem\u003eTypha\u003c/em\u003e hybrids dominate wetlands across a substantial area including the Laurentian Great Lakes and Prairie Pothole regions, and our data suggest that strong competitive ability is more important than propagule pressure for the establishment of these successful invaders.\u003c/p\u003e","manuscriptTitle":"Heterosis is more important than propagule pressure for the establishment of invasive hybrid cattail (Typha x glauca) populations","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-18 06:49:01","doi":"10.21203/rs.3.rs-4632132/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"83fe40ab-5f6f-4edf-830c-612b93bab29c","owner":[],"postedDate":"July 18th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-10-02T11:53:00+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-18 06:49:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4632132","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4632132","identity":"rs-4632132","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}