Uneven selection pressure accelerating divergence of Populus and Salix

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 theωratio 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 ofωratios 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.

Identification and Validation of Single Nucleotide Polymorphisms in Poplar Using Publicly Expressed Sequence Tags

By using assembled expressed sequence tags （ESTs） from 14 different eDNA libraries that contain 84 132 sequences reads, 556 Populus candidate single nucleotide polymorphisms （SNPs） were identified. Because traces were not available from dbEST （http：//www.ncbi.nlm.nih.gov/dbEST/index.html）, stringent filters were used to identify reliable candidate SNPs. Sequences analysis indicated that the main types of substitutions among candidate SNPs were A/G and T/C transitions, which accounted for 22.0% and 30.8%, respectively. One hundred and ten candidate SNPs were tested. As a result, 38 candidate SNPs were confirmed by directed sequencing of PCR products amplified from six different individuals. Thirteen new SNPs in intron regions were found and multiple SNPs were found to be located in both intron and exon regions of four contigs. Heterozygosis was found in all 47 candidate sites and five SNP sites were heterozygous in all six samples. This is the first report of SNP identification in a tree species which reveals that assembled ESTs from multiple libraries of the public database may provide a rich source of comparative sequences for an SNP search in the poplar genome.