Slow sperm prevent inbreeding

Female guppies have evolved a clever way to avoid having inbred offspring: Slow down the sperm of closely-related males, a new study in linkurl:The Proceedings of the Royal Society B;http://rspb.royalsocietypublishing.org/ reports.

Male and female guppies (Poecilia reticulata)
linkurl:Image: Wikimedia Commons;http://commons.wikimedia.org/wiki/File:Guppy_coppia_gialla.jpg

The results are the first direct evidence for a post-mating but pre-zygotic mechanism to avoid producing inbred offspring. It supports the hypothesis that females mate promiscuously as a way to reduce their chances of reproducing with closely related males, the authors report. "It's new and interesting," said linkurl:Bob Montgomerie,;http://www.queensu.ca/biology/people/faculty/montgomerie.html an evolutionary biologist at Queen's University in Ontario, Canada, who was not involved in the research. "But it's not unexpected," he added: If females can prevent fertilization by a close relative, they aren't wasting energy forming a zygote that will likely die due to the accumulation of harmful mutations. Researchers have long suspected that polyandry -- females taking multiple mates -- evolved in some species as a strategy to reduce breeding with relatives, but lack of evidence for how the females bias paternity after mating has caused the hypothesis to remain simply that. To confirm that polyandrous female guppies do, somehow, give an edge to sperm from non-related males, linkurl:Andrea Pilastro;http://dept.bio.unipd.it/main/?eng and Clelia Gasparini at the University of Padova in Italy artificially inseminated 28 virgin females with sperm from either an unrelated male or from a brother. They found that the related males fertilized 10 percent fewer eggs than unrelated males.To then investigate the source of the advantage, the team looked at the interaction between sperm and ovarian fluid, which past studies have suggested might affect fertilization success. In an in vitro analysis, Pilastro and Gasparini found that a male guppy's sperm travels 5-10 percent slower in the ovarian fluid of his sister than that of an unrelated female."There's an interesting coincidence between the effect we measure in sperm velocity and the effect we found in paternity," said Pilastro. Still, it may not be the only explanation for the biased paternity, he added. "It's an interesting paper because it will alert people to this possibility" of post-copulatory, pre-zygotic sexual selection, said Montgomerie. He, like Pilastro, believes it may be a common mode of sexual selection among other species.How the guppies' genetics influence the interaction of sperm and ovarian fluid is still unknown, through signaling peptides on the surface of sperm is one possibility, suggested Pilastro. His team is now examining how ovarian fluid affects sperm velocity during different times in a female guppy's breeding cycle, when a female may be more or less sexually available. Gasparini, C., and Pilastro A., "Cryptic female preference for genetically unrelated males is mediated by ovarian fluid in the guppy," Proc. R. Soc. B, doi:10.1098/rspb.2010.2369
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[5th May 2006]