Where have all the ‘N-mt’s gone?

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This week I would like to highlight a recent scientific publication by one of our very own contributor’s here at Nothing in Biology Makes Sense! – Devin Drown. Drown and colleagues recently published an article in Genome Biology and Evolution that investigates how nuclear genes that interact with the mitochondria (N-mt genes) are distributed within the genome. They show that when it comes to the location of genes within the genome, all is not equal, and suggest that conflict between males and females may influence where our genes are located.

The article focuses on understanding how unique features of the mitochondria may influence genomic architecture. Unlike our other organelles, mitochondria carry their own separate, circular genome. Furthermore, mitochondria are maternally inherited via the cytoplasm of the egg. This means that unlike the rest of the genome, the mitochondrial genome is inherited only from mom. Interestingly, over time, some of the genes that are important for the function of the mitochondria have moved from the mitochondrial genome to the nuclear genome. This gene movement has lead to variation in the mode of inheritance of the genes that must work together to produce properly functioning mitochondria. As a result, favorable nuclear-mitochondrial gene combinations are less likely to be co-transmitted to offspring.

However, sex chromosomes present an interesting exception to the general pattern of equal bi-parental inheritance of the nuclear genome. In particular, females possess two copies of the X chromosome in comparison to the single copy carried by males. As a result, the X chromosome is inherited two thirds of the time from mom and only one third of the time from dad. If N-mt genes are located on the X chromosome, it is more likely that beneficial nuclear-mitochondrial gene combinations will be co-transmitted. Thus, we may expect that N-mt genes are found at higher frequencies on the X chromosome.

Drown and colleagues set out to test this hypothesis by looking at the frequency of N-mt genes across the genome of 14 mammalian species and two avian species. Surprisingly, they observed a significant under-representation of N-mt genes on the X chromosome in the mammalian genomes. Interestingly, there was neither an over- or under-representation of N-mt chromosomes on the avian Z chromosome.

The results of their study set forth a new puzzle: Why are nuclear genes that interact with mitochondria less likely to be found on the X chromosome than the rest of the genome? The authors propose that sexual conflict may be behind it all. When N-mt genes are on the X chromosome, the maternally-biased inheritance can allow mutations that are beneficial to females, but deleterious to males to accumulate over time. The movement of N-mt genes off the of the X chromosome would allow for the resolution of this sexual conflict and provide an explanation for the observed pattern. However, for now, we will have to stay tuned to see if evidence emerges that supports their hypothesis or whether we will need to identify alternative explanations for this interesting pattern.

Drown, D.M., K.M. Preuss, and M.J. Wade (2012) Evidence of a Paucity of Genes that Interact with the Mitochondrion on the X in Mammals. Genome Biol. Evol. 4(8):875-880. doi: 10.1093/gbe/evs064

Image credit: Wikipedia Commons,  Louisa Howard