Genomic Insights into the Historical Population Dynamics and Spatial Differentiation of an Endangered Island Endemic Palm,Brahea edulis

Abstract

Premise of research. Brahea edulis is an endangered palm endemic to the oceanic island of Guadalupe, Mexico, with a small population of just about 4000 individuals; it was chronically grazed by goats for about two centuries. The evolutionary specificities of insular biotas provide unique scenarios for studying the roles of geography, demography, and climate in driving population divergence. A comprehensive comparison between insular and mainland sister species contributes to a much better understanding of the species-specific responses to the ecological drivers of diversity, divergence, and speciation. Methodology. Here we used a comparative approach coupled with genomic (genotyping by sequencing; single-nucleotide polymorphisms) data to analyze samples from island and mainland Brahea palm species and elucidate their diversity, population divergence, and demographic history. Pivotal results. Genetic diversity analyses revealed higher diversity in insular palms compared with their mainland sister species. Genetic relationships among palm individuals on Guadalupe Island resulted in two strongly diverged clusters that reflected their geographic distributions. Divergence among B. edulis populations was mostly driven by the joint effect of low migration and the climatic heterogeneity of the island’s conditions. Using approximate Bayesian analysis, we further evaluated the demographic history of B. edulis. We found the genetic signatures of demographic fluctuations, inferring an effective population size (N_e) reduction that occurred in the late Quaternary and relatively low current N_e. Conclusions. These findings contribute to the ever-growing body of evidence that low genetic diversity in island populations cannot always be generalized. Besides, genetic diversity and differentiation on islands are more likely to be species specific and are strongly influenced by population size and historical factors such as the time since the population was established, ecological characteristics of the island, and anthropogenic impact. Our results also represent groundwork for future conservation and restoration programs for this endangered insular palm species.