Effects of the Mitochondrial and Nuclear Genomes on Nonshivering Thermogenesis in a Wild Derived Rodent.

A key adaptation of mammals to their environment is their ability to maintain a constant high body temperature, even at rest, under a wide range of ambient temperatures. In cold climates, this is achieved by an adaptive production of endogenous heat, known as nonshivering thermogenesis (NST), in the brown adipose tissue (BAT). This organ, unique to mammals, contains a very high density of mitochondria, and BAT correct functioning relies on the correct functioning of its mitochondria. Mitochondria enclose proteins encoded both in the maternally inherited mitochondrial genome and in the biparentally inherited nuclear genome, and one overlooked hypothesis is that both genomes and their interaction may shape NST. By housing under standardized conditions wild-derived common voles (Microtus arvalis) from two distinct evolutionary lineages (Western [W] and Central [C]), we show that W voles had greater NST than C voles. By introgressing those two lineages over at least nine generations, we then experimentally tested the influence of the nuclear and mitochondrial genomes on NST and related phenotypic traits. We found that between-lineage variation in NST and BAT size were significantly influenced by the mitochondrial and nuclear genomes, respectively, with the W mitochondrial genotype being associated with higher NST and the W nuclear genotype with a larger BAT. There were significant mito-nuclear interactions on whole animal body weight and resting metabolic rate (RMR). Hybrid voles were lighter and had higher RMR. Overall, our findings turn new light on the influence of the mitochondrial and nuclear genomes on thermogenesis and building adaptation to the environment in mammals.

Short- and long-term effects of litter size manipulation in a small wild-derived rodent.

Iteroparous organisms maximize their overall fitness by optimizing their reproductive effort over multiple reproductive events. Hence, changes in reproductive effort are expected to have both short- and long-term consequences on parents and their offspring. In laboratory rodents, manipulation of reproductive efforts during lactation has however revealed few short-term reproductive adjustments, suggesting that female laboratory rodents express maximal rather than optimal levels of reproductive investment as observed in semelparous organisms. Using a litter size manipulation (LSM) experiment in a small wild-derived rodent (the common vole; Microtus arvalis), we show that females altered their reproductive efforts in response to LSM, with females having higher metabolic rates and showing alternative body mass dynamics when rearing an enlarged rather than reduced litter. Those differences in female reproductive effort were nonetheless insufficient to fully match their pups' energy demand, pups being lighter at weaning in enlarged litters. Interestingly, female reproductive effort changes had long-term consequences, with females that had previously reared an enlarged litter being lighter at the birth of their subsequent litter and producing lower quality pups. We discuss the significance of using wild-derived animals in studies of reproductive effort optimization.