Data from: The role of hermaphrodites in the experimental evolution of increased outcrossing rates in Caenorhabditis elegans

Background: Why most organisms reproduce via outcrossing rather than selfing is a central question in evolutionary biology. It has long ago been suggested that outcrossing is favoured when it facilitates adaptation to novel environments. We have previously shown that the experimental evolution of increased outcrossing rates in populations of the male-hermaphrodite nematode Caenorhabditis elegans were correlated with the experimental evolution of increased male fitness. However, it is unknown whether outcrossing led to adaptation, and if so, which fitness components can explain the observed increase in outcrossing rates. Results: Using experimental evolution in six populations with initially low standing levels of genetic diversity, we show with head-to-head competition assays that population-wide fitness improved during 100 generations. Since outcrossing rates increased during the same period, this result demonstrates that outcrossing is adaptive. We also show that there was little evolution of hermaphrodite fitness under conditions of selfing or under conditions of outcrossing with unrelated tester males. We nonetheless find a positive genetic correlation between hermaphrodite self-fitness and population-wide fitness, and a negative genetic correlation between hermaphrodite mating success and population-wide fitness. These results suggest that the several hermaphrodite traits measured are fitness components. Tradeoffs expressed in hermaphrodites, particularly noticed between self-fitness and mating success, may in turn explain their lack of change during experimental evolution. Conclusions: Our findings indicate that outcrossing facilitates adaptation to novel environments. They further indicate that the experimental evolution of increased outcrossing rates depended little on hermaphrodites because of fitness tradeoffs between selfing and outcrossing. Instead, the evolution of increased outcrossing rates appears to have resulted from unhindered selection on males.研究背景：绝大多数生物为何通过异交（outcrossing）而非自交（selfing）进行繁殖，是进化生物学领域的核心科学问题。长期以来学界提出，当异交有助于生物适应新环境时，异交更受自然青睐。此前我们的研究表明，在雄性-雌雄同体线虫秀丽隐杆线虫（Caenorhabditis elegans）的种群中，异交率升高的实验进化过程与雄性适合度（male fitness）提升的实验进化存在相关性。但目前仍不清楚异交是否推动了适应性进化，若确实如此，又是哪些适合度组分（fitness components）能够解释观测到的异交率升高现象。 研究结果：本研究针对6个初始遗传多样性水平较低的种群开展实验进化研究，通过直接竞争实验（head-to-head competition assays）证实，在100代的进化过程中，种群整体适合度得到了提升。由于同期异交率也出现升高，该结果证明异交具有适应性优势。此外我们还发现，在自交条件下，或是与无关测试雄性（unrelated tester males）进行异交的条件下，雌雄同体的适合度几乎未发生进化改变。尽管如此，我们仍观测到雌雄同体的自交适合度与种群整体适合度之间存在正向遗传相关，而雌雄同体的交配成功率与种群整体适合度之间存在负向遗传相关。上述结果表明，本研究测定的多项雌雄同体性状均属于适合度组分。雌雄同体个体中存在的适合度权衡（fitness tradeoffs），尤其是自交适合度与交配成功率之间的权衡，或许能够解释为何这些性状在实验进化过程中未发生明显改变。 研究结论：本研究结果表明，异交能够促进生物对新环境的适应。进一步研究发现，由于自交与异交之间存在适合度权衡，异交率升高的实验进化过程几乎不依赖雌雄同体个体。换言之，异交率的升高似乎源于对雄性个体不受制约的选择作用。