Reproduction, ageing, and longevity in two species of laboratory rodents with different life histories

Numerous insertions of mitochondrial DNA in the genome of the northern mole vole, Ellobius talpinus

BACKGROUND

Ellobius talpinus is a subterranean rodent representing an attractive model in population ecology studies due to its highly special lifestyle and sociality. In such studies, mitochondrial DNA (mtDNA) is widely used. However, if nuclear copies of mtDNA, aka NUMTs, are present, they may co-amplify with the target mtDNA fragment, generating misleading results. The aim of this study was to determine whether NUMTs are present in E. talpinus.

METHODS AND RESULTS

PCR amplification of the putative mtDNA CytB-D-loop fragment using 'universal' primers from 56 E. talpinus samples produced multiple double peaks in 90% of the sequencing chromatograms. To reveal NUMTs, molecular cloning and sequencing of PCR products of three specimens was conducted, followed by phylogenetic analysis. The pseudogene nature of three out of the seven detected haplotypes was confirmed by their basal positions in relation to other Ellobius haplotypes in the phylogenetic tree. Additionally, 'haplotype B' was basal in relation to other E. talpinus haplotypes and found present in very distant sampling sites. BLASTN search revealed 195 NUMTs in the E. talpinus nuclear genome, including fragments of all four PCR amplified pseudogenes. Although the majority of the NUMTs studied were short, the entire mtDNA had copies in the nuclear genome. The most numerous NUMTs were found for rrnL, COXI, and D-loop.

CONCLUSIONS

Numerous NUMTs are present in E. talpinus and can be difficult to discriminate against mtDNA sequences. Thus, in future population or phylogenetic studies in E. talpinus, the possibility of cryptic NUMTs amplification should always be taken into account.

Noninvasive age estimation in rodents by measuring incisors width, with the Zaisan mole vole (Ellobius tancrei) as an example

Non-invasive age estimation is critically important for many field studies but often it represents a significant challenge. In rodents, upper incisors represent a part of a cranium which can be readily measured in an alive animal. We propose an age estimation method based on the measurements of both upper incisors’ width (IW) for a subterranean rodent, Ellobius tancrei. The IW measurements for 77 laboratory-born females and 81 males were fitted, separately for the sexes, by sigmoidal Gompertz growth functions to obtain the asymptotic values. Based on the data set encompassing pre-asymptotic ages, we then derived a polynomial regression model with log-transformed age as а predictor. The analysis revealed no statistically significant sex difference in growth patterns. Derived model explained 91% of IW variation and was used to obtain predicted values of age and their 95% prediction intervals through inverse calculations. We conclude that IW is a potentially useful age indicator for mole voles and possibly other rodents in cases when invasive or time-consuming methods are non-appropriate.

Reproduction and mortality rates in ecologically distinct species of murid rodents

The trade-off between reproduction and somatic maintenance is one of the most studied concepts of modern evolutionary ecology. This theory predicts a negative relationship between maximum species longevity and total reproductive output. However, studies performed on natural animal populations have found contradictory results, probably due to the unlikelihood of wild animals gaining both maximum longevity and maximum potential fecundity. A comparison of the mortality rates and reproductive output of four ecologically distinct rodent species of Cricetidae family that were maintained in the laboratory in controlled conditions revealed the different life-history tactics of subterranean social mole voles and three related aboveground species: hydrophilic water voles, arid dwarf hamsters and steppe lemmings. Regardless of the relatively higher mortality rates at early ages in mole voles, this species has considerably higher maximum species longevity and smaller litter sizes that do not depend on calendar age, whereas in dwarf hamsters and water voles clear negative correlations between female age and litter size were detected. Steppe lemmings, as a semi-social arid species, shared some life-history tactics with both mole voles and aboveground non-social rodents.