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This Week in PNAS: Nov 24, 2015

Editor's Note: Some of the articles described below are not yet available at the PNAS site, but they are scheduled to be posted some time this week.

In the early, online edition of the Proceedings of the National Academy of Sciences, the University of Copenhagen's Ludovic Orlando and colleagues report on findings from a genome sequencing study of Yakutian horses — animals adapted to the extreme cold of winters in Siberia. By sequencing nine horses from present-day Yakutian horse populations, one 19th century horse sample, and a roughly 5,200-year-old horse sample from the same region, the team determined that the Yakutian horses are related to domestic horses and were likely introduced to the region by Yakut individuals migrating to the region in around the 13th century, either coming after or completely replacing the ancient, highly diverged horses living in the region thousands of years ago.

University of Toronto researchers have used association mapping to search for clues to evolutionary forces that reinforce genetic variation in gene expression. Using a model plant called Capsella grandiflora, the researchers explored the possibility that gene expression variation is subject to purifying selection with the help of expression quantitative trait loci and allele-specific expression variation in the plant. From the patterns they found, the authors of the study argue that "the distribution of frequencies and effect sizes of the loci responsible for local expression variation within a single outcrossing population are consistent with the effects of purifying selection."

Finally, a University of North Carolina at Chapel Hill-led team describes the slew of sequences acquired through horizontal gene transfer in the tardigrade, or water bear, genome. After putting together a draft genome assembly for the hardy microscopic creature, the researchers annotated some 38,145 predicted protein-coding tardigrade genes. The set included most core eukaryotic reference genes, the study's authors note, but it also included more than 6,000 genes originating in bacteria, fungi, plants, or other organisms. Although the functions of these horizontally acquired genes need to be explored further, they suspect that the genes may participate in processes that help water bears survive extreme cold, heat, pressure, desiccation, or other forbidding conditions. GenomeWeb has more on the study, here.