Genetic diversity of the largest African mole-rat genus, Bathyergus. One, two or four species?

Recent advances in sequencing technology and phylogenetic methods allow us to solve puzzling taxonomic questions using detailed analyses of genetic diversity of populations and gene flow between them. The genus of solitary-living dune mole-rat, Bathyergus, is quite unique among six genera of African mole-rats. The animals are by far the largest and the only scratch digging mole-rat genus possessing a skull less adapted to digging, grooved upper incisors, and more surface locomotor activity. Most authors recognize two species of dune mole-rats, B. suillus and B. janetta, but according to others, the genus is monotypic. In addition, recent molecular studies have revealed cryptic genetic diversity and suggested the existence of up to four species. In our study, we used mitochondrial and genome-wide nuclear data collected throughout the distribution of the genus to investigate the number of species. In agreement with previous studies, we found Bathyergus to be differentiated into several distinct lineages, but we also found evidence for a degree of gene flow between some of them. Furthermore, we confirmed that B. janetta is nested within B. suillus, making the latter paraphyletic and we documented an instance of local mitochondrial introgression between these two nominal species. Phylogeographic structure of the genus was found to be very shallow. Although traditionally dated to the Miocene, we found the first split within the genus to be much younger estimated to 0.82 Ma before present. Genealogical distinctiveness of some lineages was very low, and the coancestry matrix showed extensive sharing of closely related haplotypes throughout the genus. Accordingly, Infomap clustering on the matrix showed all populations to form a single cluster. Overall, our study tends to support the existence of only one species of Bathyergus namely, B. suillus. Environmental niche modelling confirmed its dependence on sandy soils and the preference for soils with relatively high carbon content. Bayesian skyline plots indicate recent population decline in the janetta lineage, probably related to global environmental change.

Species limits and phylogeographic structure in two genera of solitary African mole-rats Georychus and Heliophobius.

African mole-rats (Bathyergidae) are an intensively studied family of subterranean rodents including three highly social and three solitary genera. Although their phylogenetic interrelations are clear, genetic diversity and the number of species within each genus is much less certain. Among the solitary genera, Heliophobius and Georychus were for a long time considered as monotypic, but molecular studies demonstrated strong phylogeographic structure within each genus and proposed that they represent complexes of cryptic species. The present study re-evaluates their internal genetic/phylogenetic structure using a combination of methodological approaches. We generated datasets of one mitochondrial and six specifically selected nuclear markers as well as of a large number of double digest restriction site associated (ddRAD) loci and then applied species delimitation analyses based on the multispecies coalescent model or clustering on co-ancestry matrices. The population structure was largely congruent across all analyses, but the methods differed in their resolution scale when determining distinct gene pools. While the multispecies coalescent model distinguished five Georychus and between eleven to thirteen Heliophobius gene pools in both Sanger sequenced and ddRAD loci, two clustering algorithms revealed significantly finer or coarser structure in ddRAD based co-ancestry matrices. Tens of clusters were distinguished by fineRADstructure and one (in Georychus) or two clusters (in Heliophobius) by Infomap. The divergence dating of the bathyergid phylogeny estimated that diversification within both genera coincided with the onset of the Pleistocene and was likely driven by repeated large-scale climatic changes. Based on this updated genetic evidence, we suggest recognizing one species of Georychus and two species of Heliophobius, corresponding to a northern and southern major lineage, separated by the Eastern Arc Mountains. Yet, the final taxonomic revision should await integrated evidence stemming from e.g. morphological, ecological, or behavioral datasets.

Subspecies-specific response to ACTH challenge test in the house mouse (Mus musculus).

Long term changes in hormonal levels of small and rare animal species can be effectively monitored by non-invasive methods such as immunoenzymatic analysis. These methods generally analyze metabolites instead of the hormone itself and thus do not invade the organism. However, they can be influenced by many factors, so before they are used, they need to be validated. For this purpose we used the "ACTH challenge" test based on stimulating the animal's adrenocortical activity and measuring the consequent increase of the level of its glucocorticoid ("stress") hormones. Specifically, we measured concentrations of fecal glucocorticoid metabolites of two house mouse subspecies, Mus musculus musculus and M. m. domesticus. Using polyclonal antibody we investigated the mice's naturally occurring circadian fluctuation and their reaction to the adrenal stimulation. This study confirmed that the selected method is suitable for analysis of fecal corticosterone metabolites in the wild house mouse. More importantly, we revealed a subspecies-specific stress response at the level of corticosterone production: while the significant effect of ACTH was proved in both subspecies, a notable adrenocortical reaction was also elicited by injecting the saline solution in M. m. domesticus. Our results thus highlight the importance of considering potential cryptic variation within the species under study when designing projects on adrenal stress hormone assessments.

Shaking the myth: Body mass, aggression, steroid hormones, and social dominance in wild house mouse.

In social mammals, the position of a male in the group's hierarchy strongly affects his reproductive success. Since a high social rank is often gained through competition with other males, selection should favour bigger males over smaller ones. We may therefore predict faster growth and/or delayed sexual maturity in dominant males. Likewise, dominants should have higher levels of testosterone, hormone important in many aspects of male dominance. Less obvious is the relationship between dominance and levels of corticosterone but generally higher concentrations are expected in subordinate individuals. We studied body growth, sexual maturation and endocrinal changes in males of two house mouse subspecies, raised in fraternal pairs. Since Mus musculus domesticus is the subspecies which dominates mutual encounters with Mus musculus musculus we predicted higher growth rate, delayed puberty and aggression, and higher testosterone and corticosterone levels in domesticus males compared to musculus. In all comparisons, no differences were found between dominant and subordinate musculus brothers. On the other hand, in M. m. domesticus, dominant males revealed a different growth trajectory and lower corticosterone levels than subordinate males but not delayed puberty and higher testosterone concentrations, thus contradicting our predictions. In inter-subspecific comparisons, musculus males matured earlier but became aggressive at the same time as domesticus males. The musculus testosterone ontogeny suggests that social positions in this subspecies remain unfixed for an extended period and that the increasing levels probably reflect prolonged hierarchy contests. It appears that the ontogeny of behaviour and physiological traits diverge cryptically between the two subspecies.

There is no heterotic effect upon developmental stability in the ventral side of the skull within the house mouse hybrid zone.

Fluctuating asymmetry (FA) of a body is expected to be related to an organism's developmental instability. We studied patterns of FA in the ventral side of the skull along a transect across the central-European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. We found that FA was not significantly different between introgression classes relative to differences between localities within those classes and that the within-class differences were much larger than differences between individual-signed asymmetries within localities. However, if year was added to the same analysis as another factor, FA was not significantly different among localities within the same introgression class. When individual asymmetries were plotted against individual hybrid indices, hybrids appeared more asymmetric than individuals from outside of the zone. Thus contrary to previous studies, we did not find lower FA indicating heterotic effect in hybrids for the traits studied. It is suggested that the impact of hybridization on FA in the ventral side of the mouse skull is negligible or overwhelmed by other factors.