Temperature and mtDNA selection

Mitochondrial DNA (mtDNA) has traditionally been used in population genetic and biogeographic studies as a maternally-inherited and evolutionary-neutral genetic marker. However, it is now clear that polymorphisms within the mtDNA sequence are routinely non-neutral, and furthermore several studies have suggested that such mtDNA polymorphisms are also sensitive to thermal selection. 

A team of researchers from Japan, Australia, and the UK studied two naturally occurring mtDNA variants that are carried by fruit flies inhabiting the east coast of Australia. One of these variants is more common in the sub-tropical, northern part of the country, where temperatures are higher. The other fly variant is more common in the temperate, southern part, which tends to be colder. The colleagues collected flies from both sites and interbred them to get a series of populations with equally mixed genes. Each mixed population was subdivided into four subpopulations. Each of those wer maintained at different conditions, some at constant temperature(e.g. 19ºC and 25ºC respectively), some at fluctuating temperatures to simulate the thermal conditions at the two sites where the flies were collected. After three months, the researchers sequenced the mtDNA of flies from all these subpopulations.

In addition they also examined how the presence of bacteria such as Wolbachia, which commonly infects fruit flies, affects mtDNA selection. Some of the fly populations were treated with antibiotics to kill off any Wolbachia infections that they might be harboring.

The researchers found that in flies reared under warm laboratory conditions, one of the two mtDNA variants became more common than the other. The same mtDNA variant was also found to be widely prevalent in flies from the warmer northern parts of Australia. A similar pattern was observed with the other mtDNA variant in flies reared under cold laboratory conditions. However, researchers only observed this effect in populations where Wolbachia infections had been wiped out. Moreover, the variation patterns they observed in males didn't always match up with the ones in females.

The results show that temperature shapes how mtDNA varies in nature. They also suggest that additional factors, such as sex and infection with microbes, might also influence how the mitochondrial genome evolves in the wild. 

Our results strengthen the emerging view that intra-specific mtDNA variants are sensitive to selection, and suggest spatial distributions of mtDNA variants in natural populations of metazoans might reflect adaptation to climatic environments rather than within-population coalescence and diffusion of selectively-neutral haplotypes across populations.