Parallel loss of sex in field populations of a brown alga sheds light on the mechanisms underlying the emergence of asexuality

A bstract Sexual reproduction is widespread among eukaryotes, but asexual lineages have repeatedly arisen from sexual ancestors across a wide range of taxa. Despite extensive research on the evolution of asexuality from sexual ancestors, the molecular changes underpinning the switch to asexual reproduction remain elusive, particularly in organisms with haploid sexual systems such as bryophytes, and red and brown algae in which males and females are haploid and multicellular. Here, we investigate independent events in which asexuality has emerged from sexual ancestor lineages in species of the brown algal Scytosiphon , we examine the proximate and evolutionary mechanisms involved, and test the importance of sexual conflict on gene expression changes following loss of sex. We find that individuals from asexual, female-only (‘Amazon’) populations lose their ability to produce and sex pheromone and, consequently, are unable to attract and fuse with male gametes, whereas they gain the ability to trigger parthenogenic (asexual) development from large, unfertilized eggs. This independent and convergent decline in pheromone production and optimization of asexual traits is accompanied by convergent changes in gene expression, including de-feminization and masculinization of the Amazon gamete transcriptomes. These data are consistent with the idea that decay of female functions, rather than relaxation of sexual antagonism, is the dominant force at play during the emergence of asexuality in haploid sexual systems. Moreover, we identify a locus on an autosomal protein-coding gene that is associated with the switch to asexuality. We propose that the sex chromosome, together with this autosomal locus, may underlie the switch to obligate asexuality in the Amazon populations.