Further, we determine experimentally that the mechanism underlying reduced fecundity at extreme temperatures differs for low versus high temperature extremes, but that the total number of sperm produced by the gonad is unaffected by rearing temperature. Low rearing temperatures result in facultatively reduced oocyte production by hermaphrodites, whereas extreme high temperatures experienced during development induce permanent defects in sperm fertility. Available and emerging genetic tools for this organism will permit the characterization of the evolutionary genetic basis to this putative example of adaptation in latitudinally separated populations.K E Y W O R D S : Adaptation, biogeography, fecundity, population structure, reproductive strategies.

A fundamental problem in genome biology is to elucidate the evolutionary forces responsible for generating nonrandom patterns of genome organization. As the first metazoan to benefit from full-genome sequencing, Caenorhabditis elegans has been at the forefront of research in this area. Studies of genomic patterns, and their evolutionary underpinnings, continue to be augmented by the recent push to obtain additional full-genome sequences of related Caenorhabditis taxa. In the near future, we expect to see major advances with the onset of whole-genome resequencing of multiple wild individuals of the same species. In this review, we synthesize many of the important insights to date in our understanding of genome organization and function that derive from the evolutionary principles made explicit by theoretical population genetics and molecular evolution and highlight fertile areas for future research on unanswered questions in C. elegans genome evolution. We call attention to the need for C. elegans researchers to generate and critically assess nonadaptive hypotheses for genomic and developmental patterns, in addition to adaptive scenarios. We also emphasize the potential importance of evolution in the gonochoristic (female and male) ancestors of the androdioecious (hermaphrodite and male) C. elegans as the source for many of its genomic and developmental patterns.

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.

SUMMARYSperm count evolution is driven by sexual selection, with an added role of selection on gamete resource allocation for hermaphrodite spermatogenesis. However, self-fertilization by hermaphrodites retards sexual selection and results in the evolution of reduced investment in sperm or pollen. In contrast to reproduction limited by female gametes (Bateman's Principle), self-fertilizing Caenorhabditis elegans hermaphrodites exhibit sperm-limited reproduction. Caenorhabditis elegans hermaphrodites are thought to experience a fitness trade-off between lifetime fecundity and generation time: longer sperm production decreases the risk of self-sperm depletion, but at the same time delays the onset of selfing and thus increases egg-toegg generation time. Theory predicts that shorter larval development will favor lower sperm counts and longer development will favor more sperm. To investigate how developmental trajectories affect the evolution of sperm production, we performed experimental evolution by directly competing alleles controlling hermaphrodite sperm count, conducted under different environmental conditions that alter development time. Results are partially consistent with theory: rapid larval development generally favored alleles encoding production of few sperm. However, we identify some previously unrecognized simplifications of the theory and its application to our experimental system. In addition, we evaluated the generality of sperm limitation in C. elegans. Although optimal growth conditions yield sperm limitation, non-optimal conditions induce oocyte limitation, suggesting that this species might conform to Bateman's Principle under many natural settings. These findings demonstrate how developmental trajectories can shape the fitness landscape for the evolution of reproduction and sperm traits, even without sexual selection. Supplementary material available online at