Heterochrony and Repurposing in the Evolution of Gymnosperm Seed Dispersal Units

Abstract Background: Plant dispersal units, or diaspores, allow the colonization of new environments, expanding geographic range and promoting gene flow. Two broad categories of diaspores found in seed plants are dry and fleshy, in connection to abiotic and biotic dispersal agents, respectively. Our understanding of the anatomical and developmental genetics of fleshy angiosperm fruits is fairly advanced, yet there is a knowledge gap for the analogous fleshy structures found in gymnosperm diaspores. Improved understanding of the structural basis of modified accessory organs that aid in seed dispersal will not only inform the evolution of this important trait in specific lineages but will also enable future work on the underlying genetics, contributing to hypotheses on the origin of angiosperm fruits.To generate a structural framework for the development and evolution of gymnosperm fleshy diaspores, we studied the anatomy and histochemistry of Ephedra (Gnetales) seed cone bracts, the modified leaves surrounding the reproductive organs. We took an ontogenetic approach, comparing and contrasting the anatomy and histology of fleshy and papery-winged seed cone bracts in four closely related species in the South American clade, their pollen cone bracts, and leaves. Results: Seed bract fleshiness in Ephedra derives from mucilage accumulated in chlorenchyma cells, also found in their reduced young leaves before they reach their mature, dry stage. Cellulosic fibers, an infrequent cell type in gymnosperms, were found in leaves and most bracts where they presumably function as a source of supplementary apoplastic water in fleshy seed cone bracts. Papery bract development more closely resembles that of leaves, which lack mucilage in this species, with further extension of hyaline margins into “wings”.Conclusions: We propose an evolutionary developmental model whereby fleshy seed cone bracts result from a novel differentiation program repurposed from anatomical elements and compounds present in young leaves, whereas papery bracts derive from an extension of the mature leaf program. This model for the evolution of cone bract morphology in South American Ephedra hence involves changes in the timing of leaf differentiation, or heterochrony, that can further be tested in other gymnosperms with fleshy diaspores.