Are differences in variation and allometry in testicular size of two sibling species of the genus Mus (Mammalia, Rodentia) caused by female promiscuity?

Bioenergetic changes in response to sperm capacitation and two-way metabolic compensation in a new murine model

The acquisition of fertilizing ability by mammalian spermatozoa, known as "capacitation," includes processes that depend on particular metabolic pathways. This has led to the hypothesis that ATP demands might differ between capacitated and non-capacitated cells. Mouse sperm can produce ATP via OXPHOS and aerobic glycolysis, an advantageous characteristic considering that these cells have to function in the complex and variable environment of the female reproductive tract. Nonetheless, despite evidence showing that both metabolic pathways play a role in events associated with mouse sperm capacitation, there is contradictory evidence regarding changes promoted by capacitation in this species. In addition, the vast majority of studies regarding murine sperm metabolism use Mus musculus laboratory strains as model, thus neglecting the wide diversity of sperm traits of other species of Mus. Focus on closely related species with distinct evolutionary histories, which may be the result of different selective pressures, could shed light on diversity of metabolic processes. Here, we analyzed variations in sperm bioenergetics associated with capacitation in spermatozoa of the steppe mouse, Mus spicilegus, a species with high sperm performance. Furthermore, we compared sperm metabolic traits of this species with similar traits previously characterized in M. musculus. We found that the metabolism of M. spicilegus sperm responded to capacitation in a manner similar to that of M. musculus sperm. However, M. spicilegus sperm showed distinct metabolic features, including the ability to perform cross-pathway metabolic compensation in response to either respiratory or glycolytic inhibition, thus revealing a delicate fine-tuning of its metabolic capacities.

Impact of low sperm competition on male reproductive trait allometries in a bush-cricket

Background

Studying reproductive trait allometries can help to understand optimal male investment strategies under sexual selection. In promiscuous mating systems, studies across several taxa suggest that testes allometry is usually positive, presumably due to strong selection on sperm numbers through intense sperm competition. Here, we investigated testes allometry in a bush-cricket species, Metaplastes ornatus, in which females mate promiscuously, but where sperm removal behaviour by males likely drastically reduces realised sperm competition level.

Results

As hypothesised, we found evidence for negative testes allometry and hence a fundamentally different male investment strategy compared to species under intense sperm competition. In addition, the mean relative testes size of M. ornatus was small compared to other species of bush-crickets. Surprisingly, the spermatophore gland, a potential alternative trait that males could invest in instead of testes, also did not show positive allometry, but was approximately isometric. We further observed the expected pattern of negative allometry for the male morphological structure responsible for sperm removal in this species, the subgenital plate, supporting the one-size-fits-all hypothesis for intromittent genitalia.

Conclusion

Our findings suggest that the evolution of sperm removal behaviour in M. ornatus was a key adaptation for avoiding sperm competition, with important consequences for reproductive trait allometries. Nevertheless, they also imply that it does not pay for larger males to invest disproportionately in nuptial gift production in this species.