Monoicy, dioicy, and genetic structure in three species of <i>Sheathia</i> (Batrachospermales, Rhodophyta)

Sexual systems (i.e., separate vs. combined sexes) vary widely among eukaryotes and influence the evolution of reproductive systems, which shape genetic structure and evolutionary trajectories. In diploid-dominant angiosperms, combined (i.e., hermaphroditism) and separate sexes are expected to correlate with selfing and outcrossing, respectively. When sex is determined in the haploid phase, selfing is possible even when there are separate sexes. The freshwater red macroalgal genus Sheathia (Batrachospermales) displays sexual system variation within and among populations, but no prior data exist on the reproductive systems of these populations. We developed 16 polymorphic microsatellite loci to characterize the reproductive system and genetic structure of three Sheathia species. We observed cross-amplification of loci across the three targeted species, suggesting these markers may be useful in other Sheathia spp. We observed variation in monoicy (i.e., hermaphroditism) versus dioicy (i.e., separate sexes) in each species, including S. americana, which was previously believed to be obligately dioicous. Our data suggest that S. americana and S. involuta display more variation in their prevailing reproductive modes as compared to S. grandis. Generally, dioicy resulted in greater diversity in contrast to monoicy. We observed strong population structure that is likely driven by uniparental reproduction and limited dispersal; however, there is limited population connectivity that may be facilitated by long-distance dispersal events. Overall, these data contribute to our knowledge of the relationship between the sexual system, reproductive system, and population genetic structure in haploid-diploid taxa, thereby informing a broader understanding of the evolution of sex.