By studying the relationship between sperm viability and reproductive success in a cricket species, researchers have come closer to understanding the contribution of sperm quality to reproductive success, at least in insects.
A major advance in evolutionary theory has been the realization that sexual selection continues after mating; selection includes not only the competition among males to mate with females but also, in many species, the contest among the males' ejaculates to fertilize eggs (sperm competition). After more than three decades of studying sperm competition, researchers have clearly proved that this phenomenon has been a powerful selective force that has shaped many aspects of sexual reproduction.
The most obvious adaptation to sperm competition is the selection in males for increased sperm number; producing more sperm is adaptive when sperm competition is based on sheer numbers of gametes. It is also known that in vertebrates sperm quality plays an important role in predicting a male's fertilization ability--and, more importantly, in predicting a male's ultimate fertilization success in mating contexts that involve sperm competition. However, although sperm competition is particularly common among insects, there is a paucity of insect studies examining the selective pressures acting on sperm quality.
In the new work, researchers Francisco Garcia-Gonzalez and Leigh Simmons of the University of Western Australia tested the hypothesis that sperm viability influences insect paternity success by conducting sperm-competition trials involving pre-screened males that differed in the viability of their sperm. Using the Australian field cricket as the study's subject, the researchers showed that the proportion of live sperm in a male's ejaculate determines that male's paternity success. Furthermore, when two males competed for fertilizing the ova of a female, the researchers were able to predict the patterns of paternity based on the relative representation of a male's viable sperm in the female's sperm storage organ. These findings provide the first experimental evidence for the theory that sperm competition selects for higher sperm quality in insects and indicate that the understanding of post-copulatory sexual selection generally would gain from taking into account variation in sperm quality among males.
Francisco García-González and Leigh W. Simmons: "Sperm Viability Matters in Insect Sperm Competition"
The study was supported by grants from the Secretaria de Estado de Educacion y Universidades and the European Social Fund to F.G.-G. and from the Australian Research Council to L.W.S.
Publishing in Current Biology, Volume 15, Number 3, February 8, 2005, pages 271–275. http://www.current-biology.com
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