Dioecious Poa species diversification parallels dune formation in Atlantic coastal ecosystems

Abstract The southern Atlantic coastal dunes of South America formed ca. 6000–4000 years ago. Within this dynamic landscape, three dioecious species of Poa occur: P. lanuginosa, a widely distributed species, and two dune endemics, P. bergii and P. schizantha, occupying contrasting microhabitats from foredunes to interdunal slacks and inland grasslands. We investigated the evolutionary relationships, population structure, and extent of introgression among these dune-adapted grasses, conducting a population-level phylogenomic analysis using genotyping-by-sequencing (GBS). Genome-wide data provided robust phylogenetic resolution, whereas NeighborNet analyses revealed reticulate relationships and admixture among taxa. We tested introgression and significant signals of gene flow were detected across major groups. Bayesian and DAPC analyses identified distinct genetic lineages within both dune endemics. Poa bergii comprised three geographically structured genetic groups, and P. schizantha showed clear internal differentiation despite its restricted distribution. Whereas P. lanuginosa was inferred as a widespread paraphyletic lineage consistent with an ancestral role in dune species. These distinct patterns of divergence and population structure indicate species-specific evolutionary responses to a recent and dynamic dune landscape, in which habitat heterogeneity and differential environmental tolerance have driven lineage diversification. Together, our findings highlight the role of coastal dune dynamics in recent adaptation and speciation. Ongoing gene flow alongside clear lineage structure is consistent with recent and still incomplete speciation among these dune species. Poa schizantha emerged as the most differentiated lineage, combining morphological distinctiveness, ecological specialization, and strong genomic structure. Our results emphasize the importance of integrating population genetics into conservation strategies to preserve both endemic taxa and ongoing evolutionary processes in threatened coastal environments.