Male-biased triploidy in Populus tremuloides and genomic structure of its southern relict populations

PremiseIsolated populations at the southern range edge of aspen (Populus tremuloides Michx.) offer a unique opportunity to study ploidy, clonality and distribution of sex under arid, high-elevation conditions. We aimed to characterize such traits, including potential sex biases and associations with ploidy, investigating previously unexamined Texan populations in a range-wide genomic framework of genetic structure and demographic history of this keystone species. MethodsWe combined new genotypic data from western Texas (Davis Mountains, Big Bend, Guadalupe Mountains) with preexisting datasets, assessing range-wide genetic diversity, clonality, ploidy, and sex. We further conducted ADMIXTURE and phylogenetic analyses, reconstructed historical effective population sizes (Ne) with Stairway Plot 2. ResultsTexan stands revealed high clonality, with diploid and triploid genets. Most Davis and Guadalupe Mountain individuals clustered with a southwestern U.S. lineage of aspen, whereas Big Bend individuals grouped with a Mexican, demonstrating for the first time the continuous northward geographic distributions of allele frequencies in aspen coherent with phylogenetics. The significant male bias in sex ratios, particularly among triploids, suggests dimorphism in survival or reproduction. No effect of elevation on sex was identified. Demographic inference indicated an ancient bottleneck ([~]1-2 Mya) common to all six lineages, but more recent historical Ne trajectories differ between the northern and southern regions. ConclusionsThese findings shed light on how clonality, ploidy, and sex have interacted in shaping dioecious plant species, highlighting the importance of incorporating such information into projections of how climate change and habitat loss will affect their distribution, abundance, and the extinction risk of their marginal populations.