Frisky mice produce fertile sons

Mice with many sexual partners produce more fertile sons than do monogamous mice, providing a biological benefit of promiscuity, according to research published online today (January 20) in BMC Evolutionary Biology.

Image: Wikimedia commons

When a female mates with multiple partners -- a scenario called polyandry, which is common in mice -- sperm from rivals must face off to fertilize her eggs. This so-called "sperm competition" has been linked to the evolution of testes size, as well as sperm form and function, and promiscuity is believed to have evolved partly as a way for females to select genes for the highest quality sperm to pass onto her sons. But this is the first experimental evidence in mammals showing that promiscuity can affect the offspring's fertility. "I think [the new results are] an important confirmation of the evolutionary power of polyandry and the sperm competition selection that arises from polyandry," said linkurl:Tommaso Pizzari,;https://www.zoo.ox.ac.uk/staff/academics/pizzari_t.htm an evolutionary biologist from the University of Oxford in the UK, who was not involved with the study. The experimental approach used in the new study is much more powerful than earlier comparative studies, and that untangles the causality of the relationship, he added, namely that promiscuity really does seem to be driving the evolution of sperm quality, rather than the other way around. In the study, evolutionary biologist linkurl:Leigh Simmons;https://www.ceb.uwa.edu.au/our_people/leigh_simmons and postdoctoral fellow Renée Firman from the University of Western Australia bred house mice by giving them access to either one or several partners. Within the first few generations, females from polygamous lines started having larger litters than females from the monogamous lines. "We knew something was going on," Firman said. By eighth generation the litter size differences were sizeable, so the researchers decided to look at the sperm. Sure enough, the scientists noticed differences in the quality of sperm between the two groups: Male mice from the polygamous lines had greater numbers of sperm and better sperm motility compared to those from the monogamous lines. The females from both groups were releasing the same number of eggs per cycle, but the higher sperm quality in males from polygamous lines "could account for the increase in litter size because perhaps more of the ova were being fertilized and implanting," Firman said."It is not too surprising that better quality sperm are favored over lower quality sperm," noted biologist linkurl:Michael Ferkin,;https://www.memphis.edu/biology/faculty/michael_ferkin.php biologist from the University of Memphis in Tennessee who was not involved with the research, in an email. But it's fascinating that the mating system affects the quality of sperm, he added. The differences in sperm quality between the two groups appeared surprisingly quickly, noted Pizzari. This could be due to genetic reasons, he said, if selection is strong, or genetic variability is high, or both -- necessary ingredients for rapid evolution. Alternatively, mothers may contribute some epigenetic factors in utero, such as higher testosterone levels, that could influence the fertility of sons. The scientists also challenged monogamous and polygamous male mice of the 12^th generation to a mating competition with a female from either group. Near the time of ovulation, each female mated with one monogamous and one polygamous partner in succession. Fourteen days later, researchers investigated the embryos of the pregnant mice, and found that males from the polygamous lines fathered more offspring compared with controls. In particular, polygamous mice sired 33 percent of litters exclusively, whereas those from monogamous lines fathered only 14 percent. The rest of the litters were mixed, though still biased toward the males from polygamous lines. (Whether the female came from the polygamous or monogamous group made no difference to the results.)Simmons and Firman plan to conduct sperm competition experiments in vitro to see whether there's still a bias toward polygamous males during fertilization.It would be interesting to see whether same findings occur in other promiscuous species of rodents, such as montane voles, or in rodents with more monogamous mating systems, such as California mice, Ferkin said. Although the experimental evolution in the new study did not seem to affect female fertility, females are an important consideration, said Pizzari. Some studies in insects, for example, have suggested that having multiple male partners can be detrimental to the reproductive success of females by causing transmission of parasites or diseases, among other costs. "We shouldn't forget that females are not a passive arena for sperm competition" Pizzari added, "but they are an actively co-evolving environment in which the ejaculates of different males compete."
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