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核心策略，二者各自具备独特的进化成本与收益。自交被认为是一种进化‘死端’策略：短期来看具有适应性优势，但长期会带来演化代价，最终导致自交物种仅局限于系统发育树的顶端分支。本研究以团藻目绿藻（Volvocine green algae）为研究对象，探究自交的演化历程。我们通过祖先状态重建（Ancestral-state reconstructions）分析发现，自交曾多次从异交祖先类群中独立演化而来，且发生过多次自交向异交的演化逆转事件。我们采用三类系统发育指标（Phylogenetic metrics）证实，自交物种并非仅分布于系统发育树顶端，这一发现与‘自交为进化死端’的传统观点相悖。此外，我们未发现自交谱系存在更高灭绝速率或更低物种形成速率的证据。我们还发现，自交物种的集落（Colonies）体积显著大于异交物种，这表明自交的适应性优势或许能够抵消体积增大带来的演化代价。我们推测，本研究中关于自交的宏观演化结果（即自交物种并非局限于系统发育树顶端、未表现出多样化速率降低），或许可以用团藻目绿藻所特有的单倍体主导型生活周期（Haploid-dominant life cycle）来解释——该生活史可能会改变自交的演化成本与收益。