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.
Highlights
Whole genome duplication (WGD) is an important driving force in the evolutionary process of flowering plants[1,2]
paralogous genes resulting from salicoid duplication (PGRS) in poplar and willow A total of 39,514 and 24,931 coding sequences contained in 19 chromosomal reconstructions were extracted from the genomic database of P. trichocarpa and S. suchowensis, respectively, and these genes were used to detect the PGRS in poplar and willow
Plotting the average 4DTV values for the paralogous genes contained on each syntenic segment revealed two peaks both in poplar and willow (Fig. 1a, b), and the highest peak was recognized to result from salicoid duplication according to Tuskan et al.’s study[14]
Summary
Whole genome duplication (WGD) is an important driving force in the evolutionary process of flowering plants[1,2]. Wholesale gene loss after WGDs can drastically shrink genome size and gene content, which has long been viewed as a critical driving force in the evolution of higher plants[10,11,12]. The two lineages are important fiber resources with rapid growth rates, ease of vegetative propagation, predisposition to hybridization, and high productivity of wood[13]. These characteristics, together with their relatively small genomes and the rapidly growing research resources, have led to Salicaceae species emerging as the model system
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