Linking phenotypic variation to patterns of genetic isolation along a speciation continuum

Investigating taxa at varying stages of divergence can shed light on the evolutionary forces that lead to reproductive isolation and eventual speciation. The forces promoting isolation vary in space and time, which makes it difficult to reconstruct the trajectory that resulted in the divergence observed among species today. The red macroalgal genus Plocamium is known worldwide for its cryptic genetic and chemical diversity. Previous work on the genus Plocamium in Antarctica observed two haplotypes along the western Antarctic Peninsula that have been treated as the same species. Using 10 microsatellite loci, we observed that these two haplotypes correspond to two highly divergent, co-occurring genetic entities in Antarctic Plocamium, which are located within close vicinity of each other at the same sites. The morphology of the reproductive structures, a feature commonly used to identify cryptic species in Plocamium, as well as the timing of reproduction, differed significantly between the two genetic entities. Altogether, this suggests that two Antarctic Plocamium species exist on the western Antarctic Peninsula. We observed evidence for high levels of selfing in both genetic entities, which likely exacerbated the lack of gene flow between them. In addition, we identified concomitant chemodiversity that generates compelling evidence of early evolutionary divergence within one of these entities. This chemodiversity has ecological consequences for its main grazer, which alludes to one putative evolutionary driver of divergence. Antarctic Plocamium spp. provide a promising model system for investigating the eco-evolutionary forces that initiate and maintain species boundaries.