Conservation genomics of Dioon holmgrenii (Zamiaceae) reveals a history of range expansion, fragmentation, and isolation of populations

Abstract

Many Cycad species may not survive the current extinction crisis, despite belonging to the oldest living seed plant lineage. Conservation of endangered and threatened species will require a combination of in situ and ex situ programs, both of which will benefit from better knowledge of species’ population genetic structure, as will assessments of threatened status. Here we develop a cost-effective method of obtaining population-level genomic data from across the range of the Mexican cycad, Dioon holmgreni, and use these data to characterize the genetic structure and diversity of the species. We also reconstruct aspects of the demographic history of the species and evaluate the taxonomic cohesion of populations across the range using genomic and morphological data. We find that D. holmgrenii harbors moderate genetic diversity across genetically and geographically isolated populations that each possess a substantial percentage of private alleles. We further find that the history of this species likely includes a widespread range expansion followed by fragmentation due to population contraction. These results argue for conservation of all populations and their unique alleles but also suggest an unexpected ability of this species to maintain genetic diversity despite dramatic reductions in population size.