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This Week in PNAS: Oct 16, 2012

Recombination-rich regions of the bee genome with higher-than-usual levels of guanine and cytosine nucleotides contain many of the genes that are preferentially expressed by worker honeybees, according to York University researchers reporting in the early, online version of the Proceedings of the National Academy of Sciences this week. By comparing the honeybee, Apis mellifera, genome sequence with sequences from other Apis species, the team got a glimpse at ways in which derived allele frequency patterns differ across the bee genome, which contains clusters of GC-poor and GC-rich sequences. The analysis suggests that GC-rich parts of the genome are maintained through processes such as recombination and biased gene conversion, plumping up the genetic diversity in fast-evolving regions that harbor a slew of genes expressed by honeybees in the worker caste. "Taken together," the researchers say, "these findings suggest that recombination acts to maintain a genetically diverse and dynamic part of the genome where genes underlying worker behavior evolve more quickly."

Max Planck Institute for Ornithology researcher Kamran Safi and collaborators from Germany and the UK explore the influence of selection on "relative brain size" in animals — the size of an animal's brain in relation to its body size. The researchers brought together brain, body mass, and phylogenetic data for mammals from bat, primate, and carnivoran orders. Although the patterns differed somewhat depending on the animals considered, investigators saw that brain-to-body size ratios within the mammalian orders considered had often been more strongly influenced by selective pressure on the size of the body than that of the brain. "It is clear that comparative correlations involving relative brain size cannot be interpreted as selection on neuronal capacity alone," Safi and co-authors say. "Relative brain size is the compromise of two traits taking potentially different evolutionary pathways involving different combinations of brain-body adaptations."

For another study slated to appear online in PNAS this week, an Indiana University group refines mutation rate estimates for two unicellular organisms: the bacteria Mesoplasma florum and the unicellular green algae Chlamydomonas reinhardtii. The prokaryote (M. florum) has an unusually small genome, shy of 800,000 bases, the team notes, while the eukaryote (C. reinhardtii) has a massive genome. Through whole-genome sequencing on dozens of M. florum lines and four C. reinhardtii lines developed over a known number of cell divisions, the researchers determined mutation rates for each organism — looking at how mutation types and locations corresponded with factors such as genome size and effective population size. "As a consequence of an expansion in genome size," it says, "some microbial eukaryotes with large [effective population size] appear to have evolved mutation rates that are lower than those known to occur in prokaryotes, but multicellular eukaryotes have experience elevations in the genome-wide deleterious mutation rate because of substantial reductions in [effective population size]."

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