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Abstract 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 genusSheathia(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 threeSheathiaspecies. We observed cross‐amplification of loci across the three targeted species, suggesting these markers may be useful in otherSheathiaspp. We observed variation in monoicy (i.e., hermaphroditism) versus dioicy (i.e., separate sexes) in each species, includingS. americana, which was previously believed to be obligately dioicous. Our data suggest thatS. americanaandS. involutadisplay more variation in their prevailing reproductive modes as compared toS. 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. 

Abstract The freshwater red algaBatrachospermum gelatinosumhas a well‐documented distribution spanning historically glaciated and unglaciated eastern North America. This alga has no known desiccation‐resistant propagule; thus, long‐distance dispersal events are likely rare. We predicted strong genetic structure among drainage basins and admixture among sites within basins. We predicted greater genetic diversity at lower latitude sites because they likely serve as refugia and the origin of northward, post‐Pleistocene range expansion. We used 10 microsatellite loci to investigate genetic diversity from 311 gametophytes from 18 sites in five major drainage basins: South Atlantic Gulf, Mid‐Atlantic, Ohio River, Great Lakes, and Northeast. Our data showed strong genetic partitioning among drainage basins and among sites within basins, yet no isolation by distance was detected. Genetic diversity varied widely among sites and was not strictly related to latitude as predicted. The results fromB. gelatinosumprovide strong support that each stream site contributes to the unique genetic variation within the species, potentially due to limited dispersal and the prevailing reproductive mode of intragametophytic selfing. Simulations of migration suggested post‐Pleistocene dispersal from the Mid‐Atlantic.Batrachospermum gelatinosumpotentially persisted in refugia that were just south of the ice margins rather than in the southernmost part of its range. Research of other taxa with similar ranges could determine whether these results are generally applicable for freshwater red algae. Nevertheless, these results fromB. gelatinosumadd to the growing literature focused on the patterns and genetic consequences of post‐Pleistocene range expansion by eastern North American biota.