Data from: Repeated evolution and reversibility of self-fertilization in the volvocine green algae

Outcrossing and self-fertilization are fundamental strategies of sexual reproduction, each with different evolutionary costs and benefits. Self-fertilization is thought to be an evolutionary “dead-end” strategy, beneficial in the short term but costly in the long term, resulting in self-fertilizing species that occupy only the tips of phylogenetic trees. Here, we use volvocine green algae to investigate the evolution of self-fertilization. We use ancestral-state reconstructions to show that self-fertilization has repeatedly evolved from outcrossing ancestors and that multiple reversals from selfing to outcrossing have occurred. We use three phylogenetic metrics to show that self-fertilization is not restricted to the tips of the phylogenetic tree, a finding inconsistent with the view of self-fertilization as a dead-end strategy. We also find no evidence for higher extinction rates or lower speciation rates in selfing lineages. We find that self-fertilizing species have significantly larger colonies than outcrossing species, suggesting the benefits of selfing may counteract the costs of increased size. We speculate that our macroevolutionary results on self-fertilization (i.e. non-tippy distribution, no decreased diversification rates) may be explained by the haploid-dominant life cycle that occurs in volvocine algae, which may alter the costs and benefits of selfing.

异交与自交是有性生殖的两种核心策略，二者各自具备独特的进化成本与收益。自交曾被视为一种进化‘死胡同’策略：短期来看具有进化优势，但长期代价高昂，因此传统观点认为自交物种往往仅占据系统发育树的末端分支。本研究以团藻目绿藻（volvocine green algae）为研究对象，探究自交的进化历程。通过祖先性状重建分析，我们证实自交已多次从异交祖先类群中独立演化而来，且存在多起自交向异交的反向演化事件。我们采用三项系统发育指标开展分析，结果显示自交物种并非仅局限于系统发育树的末端分支，这一发现与‘自交为进化死胡同’的传统认知相悖。同时，我们未发现自交谱系具有更高灭绝速率或更低成种速率的证据。此外，我们观察到自交物种的藻落体积显著大于异交物种，这提示自交的进化优势或许能够抵消体型增大所带来的进化成本。我们推测，本研究中关于自交的宏观进化结果——即自交物种并非仅分布于系统发育树末端、自交谱系未表现出多样化速率降低——或可通过团藻目绿藻特有的单倍体主导型生活史加以解释：这一生活史特征可能会改变自交的进化成本与收益。