Uneven selection pressure accelerating divergence of Populus and Salix

Abstract

Populus (poplars) and Salix (willows) are sister genera in the Salicaceae family that arise from a common tetraploid ancestor. The karyotypes of these two lineages are distinguished by two major interchromosomal and some minor intrachromosomal rearrangements, but which one is evolutionarily more primitive remains debatable. In this study, we compare the selection pressure acting on the paralogous genes resulting from salicoid duplication (PGRS) within and between the genomes of the two lineages. Purifying selection was determined to act more strongly on the PGRS in willow than on those in poplar, which would cause a faster loss of paralogous duplicates in willow. Therefore, Salix species are supposed to evolve faster than Populus species, which is consistent with the observation that the former are taxonomically and morphologically more diverse than the latter. In these two lineages, different autosomes were found to have been evolving into sex chromosomes. Examining the ω ratio and the PGRS in the sex determination regions in willow and poplar revealed higher convergent selection pressure and a faster loss of PGRS in the sex determination regions of both lineages. At the chromosome level, the sex chromosome in poplar is characterized by the lowest gene density among all chromosome members, while this feature is not observed on the sex chromosome in willow, suggesting that Populus species may inherit the more incipient sex chromosome from their progenitor. Taken together, Salix is supposed to be the nascent lineage arising from the additional round of genome reorganization that distinguishes the karyotypes of the two sister genera. In this study, assessment of ω ratios also detected a list of paralogous genes under unusual selection pressure, which could have special consequences for the adaptive evolution of Salicaceae species. In conclusion, the results of this study provide unique information for better understanding the genetic mechanism accelerating the divergence of these two closely related lineages.Genomics: Salicaceae\xa0family members under different selection pressuresDifferent selection pressure on duplicate genes explains how willow and poplar diverged from a common progenitor with four copies of their genome. A study by Tongming Yin and colleagues at Nanjing Forestry University in China compared the genomes of these two fast-growing trees to shed light on how they lost ancestral genes to retain only two copies of their genome. They found an uneven selection pressure on different genomic regions and a faster loss of duplicated genes in willow. This could explain why willow evolved more nascent and gave rise to over 300 species, whereas there are only about 29 species of poplar. Differences in gene density in the sex chromosomes are also key to the divergent evolution of these two sister lineages.