Tuesday, March 13, 2007

Imprinted gene undergoing accelerated evolution in humans?   posted by Razib @ 3/13/2007 10:38:00 AM

From PLOS Genetics Identification of the Imprinted KLF14 Transcription Factor Undergoing Human-Specific Accelerated Evolution:
Imprinted genes are expressed in a parent-of-origin manner and are located in clusters throughout the genome...By sequence analysis of numerous species, we place the timing of this event after the divergence of Marsupialia, yet prior to the divergence of the Xenarthra superclade. We identify a large number of sequence variants in KLF14 and, using several measures of diversity, we determine that there is greater variability in the human-lineage with a significantly increased number of non-synonymous changes, suggesting human-specific accelerated evolution. Thus, KLF14 may be the first example of an imprinted transcript undergoing accelerated evolution in the human lineage.

Genomic imprinting has evolutionary implications, and could suggest a recent change in hominid mating systems.

Update: I was in a hurry when I posetd so I didn't clarify some issues. First:
Under this hypothesis, the maternally expressed KLF14 is predicted to suppress embryonic growth. This, together with its predicted function as a transcriptional repressor, suggests that the protein may suppress the expression of genes which enhance fetal or placental development.

Why would a female want to restrain the resource consumption of the fetus? Because it reduces her own fitness, and therefore the chance of her reproducing in the future. In a polygynous species where the male will likely not inseminate the same female twice this is acceptable for the "paternal" allele, as future paternal alleles will be unrelated. In contrast, the "maternal" allele is going to be related to the future offspring (coefficient 0.5, since females contribute one of two alleles). So you haev intralocus conflict. The confusing sexual dynamics are alluded too in the earlier link.

Also, this from the discussion might interest some:
Due to KLF14's increased expression in neuronal cells, as well as the accelerated evolution observed in the human lineage, this gene may have played a role in the acquisition of human-specific traits. Such a function would agree with our hypothesis postulated above, describing the role of imprinting in the variability of the gene. Despite being imprinted in ancestral mammals, we observe that the evolutionary aspects of KLF14 are unique to the human lineage. This observation could be due to selective pressures unique to this species, and possibly unique to demographic populations within the human population, which have accelerated the evolution of the gene. Consequently, the variations seen in KLF14 may be beneficial to humans or subpopulations of the human species, particularly the variations that are fixed or are going towards fixation. However, further studies are required to determine the gene's function in the brain, particularly in neurons, in order to assess its contribution to human speciation and its putative role in cognitive disease.

Finally, please be cautious, as much of the paper is littered with "not statistically significant" as they are looking for signs of positive selection, not rejecting neutral evolution and suggesting a possible role for relaxation of purifying selection. Kind of messy, but they looks like they might have been the first to snatch at some new fruit....

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