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This Week in PNAS: Aug 19, 2014

The pygmy phenotype — an adaptation shared by multiple human populations living in different tropical rainforest environments — may stem from convergent changes involving multiple genes, according to a study in the early, online edition of the Proceedings of the National Academy of Sciences. Starting with genome-wide admixture mapping on individuals from Uganda's Batwa pygmy population, a team from Pennsylvania State University, University of Montreal, and elsewhere identified more than a dozen sites in the genome that coincided with small body size. Following comparisons with agricultural and hunter-gatherer populations living near the Batwa or in other parts of Africa, the researchers concluded that the pygmy phenotype provides a selective advantage for rainforest populations with a hunter-gatherer lifestyle, potentially appearing multiple times as a consequence of natural selection.

A Stanford University team outlines an optogenetic approach for selectively spurring the activity of certain neurons in the mouse brain in an effort to boost recovery after a stroke. Using transgenic mice expressing light-activated microbial proteins such as channelrhodopsin 2, the researchers demonstrated that they could effectively stimulate neurons in a mouse brain region known as the ipsilesional primary motor cortex with light, prompting better cerebral blood flow and enhanced stroke recovery in the mice. "Understanding the cell type and mechanisms driving recovery may identify potential drug targets for stroke treatment," the study's authors say, "as well as ultimately help develop precise brain stimulation techniques for stroke therapy."

Finally, the University of Massachusetts' Michael Green and colleagues demonstrate the feasibility of awakening inactivated X chromosomes in mammalian female cells. The team used tens of thousands of short, hairpin RNAs to perform an RNA interference screen to characterize some of the components required for successful X chromosome inactivation in mouse cells — a search that highlighted the importance of cell signaling, transcription, and methyltransferase pathways in this process. From there, the researchers considered the types of chemicals that would mute the activity of such factors, as well as the gene expression consequences of reactivating previously silenced X chromosomes.