Abstract
BackgroundSperm competition is a driving force in the evolution of male sperm characteristics in many species. In the nematode Caenorhabditis elegans, larger male sperm evolve under experimentally increased sperm competition and larger male sperm outcompete smaller hermaphrodite sperm for fertilization within the hermaphrodite reproductive tract. To further elucidate the relative importance of sperm-related traits that contribute to differential reproductive success among males, we quantified within- and among-strain variation in sperm traits (size, rate of production, number transferred, competitive ability) for seven male genetic backgrounds known previously to differ with respect to some sperm traits. We also quantified male mating ability in assays for rates of courtship and successful copulation, and then assessed the roles of these pre- and post-mating traits in first- and second-male fertilization success.ResultsWe document significant variation in courtship ability, mating ability, sperm size and sperm production rate. Sperm size and production rate were strong indicators of early fertilization success for males that mated second, but male genetic backgrounds conferring faster sperm production make smaller sperm, despite virgin males of all genetic backgrounds transferring indistinguishable numbers of sperm to mating partners.ConclusionsWe have demonstrated that sperm size and the rate of sperm production represent dominant factors in determining male fertilization success and that C. elegans harbors substantial heritable variation for traits contributing to male reproductive success. C. elegans provides a powerful, tractable system for studying sexual selection and for dissecting the genetic basis and evolution of reproduction-related traits.
Highlights
Sperm competition is a driving force in the evolution of male sperm characteristics in many species
We confirm that sperm size is an important indicator of fertilization success in male-male sperm competition and that large sperm come at a cost because they take longer to produce [44] - we demonstrate that heritable differences in the rate of sperm production is the strongest correlate of second-male sperm precedence
The factors contributing to sperm precedence and male reproductive success The C. elegans literature shows that, in mated hermaphrodites, male sperm outcompete self-sperm [23,24] and the larger size of male sperm likely contributes to their superior competitive ability [44]
Summary
Sperm competition is a driving force in the evolution of male sperm characteristics in many species. To further elucidate the relative importance of sperm-related traits that contribute to differential reproductive success among males, we quantified within- and among-strain variation in sperm traits (size, rate of production, number transferred, competitive ability) for seven male genetic backgrounds known previously to differ with respect to some sperm traits. When two or more males compete for the fertilization of oocytes in a multiplymated female, it is often true that the male that produces the most sperm will procure the greatest fertilization success [1] This type of sperm competition (a ‘fair raffle’) can lead to selection for more, and further miniaturized, male gametes as limited resources are allocated to create more individual gametes [2,3]. We investigate male-male sperm competitive ability in the nematode model organism Caenorhabditis elegans to better understand the relative importance of mating and sperm traits for heritable variation in male reproductive success
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