Abstract
BackgroundClassic population genetics theory predicts that mixed reproductive systems, where self reproduction (selfing) and outcrossing co-exist, should not be as common as they are in nature. One means of reconciling theory with observations is to recognize that sexual conflict between males and hermaphrodites and/or constraints in the allocation of resources towards sex functions in hermaphrodites can balance the fitness components of selfing and outcrossing.ResultsUsing experimental evolution in Caenorhabditis elegans, we test whether the adaptive maintenance of partial selfing is due to sexual conflict and/or to the evolution of sex allocation towards male function in hermaphrodites. For this, we characterized the reproductive schedule and longevity patterns in hermaphrodites under selfing and under outcrossing with naïve males that did not have the opportunity to evolve with them. A shift in reproductive schedule towards earlier reproduction would be indicative of adaptation in our imposed life-cycle, while longevity is expected to evolve as a response to the harm that males impinge on hermaphrodites upon mating. To determine adaptation in the absence of constraints in sex allocation, we also characterized the life history of females that reproduced during experimental evolution through obligate mating with males. As expected with adaptation, we find that after 100 generations of experimental evolution, selfing hermaphrodites and females showed improved reproduction at earlier ages. We did not observe similar reproductive shifts in outcrossed hermaphrodites. We further find increased longevity in outcrossed females after evolution but not in outcrossed hermaphrodites, a result that indicates that sexual conflicts were likely more prevalent under male–female evolution than under male-hermaphrodite evolution.ConclusionsTaken together, our findings suggest that the adaptive maintenance of partial selfing during C. elegans experimental evolution resulted from the evolution of sex allocation towards male function in hermaphrodites.
Highlights
Classic population genetics theory predicts that mixed reproductive systems, where self reproduction and outcrossing co-exist, should not be as common as they are in nature
One means of reconciling empirical observations with theory is to recognize that the fitness components expressed under selfing or outcrossing might tradeoff with one another, e.g., [11,12,13], and thereby add an additional layer of complexity to the population genetic dynamics
We found that the linear mixed model’s (LMM) residuals did not follow normality (Shapiro-Wilk W = 0.97, p = 0.012), though we confirmed the significance of evolutionary responses with generalized linear mixed models (GLMM) employing Poisson errors
Summary
Classic population genetics theory predicts that mixed reproductive systems, where self reproduction (selfing) and outcrossing co-exist, should not be as common as they are in nature. One means of reconciling theory with observations is to recognize that sexual conflict between males and hermaphrodites and/or constraints in the allocation of resources towards sex functions in hermaphrodites can balance the fitness components of selfing and outcrossing. One means of reconciling empirical observations with theory is to recognize that the fitness components expressed under selfing or outcrossing might tradeoff with one another, e.g., [11,12,13], and thereby add an additional layer of complexity to the population genetic dynamics. The hypothesis of a sexual conflict predicts negative genetic correlations between the fitness components of individuals of different sexes. Sexual conflict has largely been described for obligate outcrossers, e.g., [19,20,21], but could be important for partial selfers [22,23,24]
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