Natural selection and intrinsic barriers play important roles in
speciation of two closely related Populus (Salicaceae) species
Abstract
Despite the growing number of recent studies on genome-wide divergence
during speciation, the genetic basis and mechanisms of genomic
divergence are still incompletely understood. In most species, natural
selection plays a key role in heterogeneous genomic divergence.
Additionally, intrinsic barriers, such as chromosomal rearrangements or
gene incompatibilities, can also cause genomic heterogeneity. Based on
whole genome re-sequencing data from 27 Populus alba and 28 P. adenopoda
individuals, we explored the reasons for heterogeneous genomic
divergence of these two closely related species. The results showed that
the two species diverged ~5-10 million years ago (Mya),
when the Qinghai-Tibet Plateau reached a certain height and the inland
climate of the Asian continent became arid, which is associated with the
fact that the two species begin to diverge and eventually led to
speciation. In highly differentiated regions, the absolute divergence
(dxy) was significantly higher than genomic background, and relative and
absolute divergence were highly correlated, which indicates that
intrinsic barriers played an important role in maintaining genomic
heterogeneous divergence. Additionally, θπ and shared polymorphisms
decreased while fixed differences increased in highly differentiated
regions, which are characteristics of natural selection. The above
description indicates that the combination of intrinsic barriers and
natural selection result in heterogeneous genomic divergence and
reproductive isolation. We further found some genes that are related to
reproduction may be involved in explaining the reproductive isolation of
the two species.