Data from: Strong natural selection on juveniles maintains a narrow adult hybrid zone in a broadcast spawner

Natural selection can maintain and help form species across different habitats, even when dispersal is high. Selection against inferior migrants (immigrant inviability) acts when locally adapted populations suffer high mortality on dispersal to unsuitable habitats. Habitat-specific populations undergoing divergent selection via immigrant inviability should thus show (1) a change in the ratio of adapted to nonadapted individuals among age/size classes and (2) a cline (defined by the environmental gradient) as selection counterbalances migration. Here we examine the frequencies of two depth-segregated lineages in juveniles and adults of a Caribbean octocoral, Eunicea flexuosa. Distributions of the two lineages in both shallow and deep environments were more distinct when inferred from adults than juveniles. Despite broad larval dispersal, we also found an extremely narrow hybrid zone (<100 m), with coincident clines for molecular and morphological characters of the host coral and its algal symbiont. Effective dispersal estimates derived from the hybrid zone are remarkably small (<20 m) for a broadcast spawner. The large selection coefficient against mismatched genotypes derived from cohort data is consistent with that from field transplant experiments. Narrow hybrid zones and limited effective dispersal may be a common outcome of long periods of postsettlement, prereproductive selection across steep ecological gradients. Strong diversifying selection provides a mechanism to explain the prevalence of depth-segregated sibling species in the sea.

自然选择（natural selection）可在不同生境中维持并促进物种形成，即便物种扩散（dispersal）能力较强时亦是如此。当本地适应种群扩散至不适生境并遭遇高死亡率时，便会产生针对劣质外来个体的选择（immigrant inviability）。经由该选择作用发生歧化选择（diversifying selection）的生境特化种群，应呈现出两类特征：其一，不同年龄/体型组中适应型与非适应型个体的比例发生改变；其二，随着选择与扩散的动态平衡，出现以环境梯度为界定的渐变群（cline）。本研究以加勒比海八放珊瑚（octocoral）弯曲艾氏珊瑚（Eunicea flexuosa）的幼体和成体为研究对象，分析了两个按深度分隔的演化支的频率分布。相较于幼体，基于成体推断得到的两个演化支在浅海与深海生境中的分布差异更为显著。尽管幼体扩散范围广泛，我们仍发现了一条宽度不足100米的极窄杂交带（hybrid zone），宿主珊瑚及其藻类共生体（algal symbiont）的分子与形态特征均呈现出重合的渐变群分布。对于体外产卵物种（broadcast spawner）而言，基于该杂交带估算得到的有效扩散距离极小，仅不足20米。基于同龄群数据（cohort data）得到的针对不匹配基因型（mismatched genotypes）的强选择系数（selection coefficient），与野外移植实验（field transplant experiments）所得结果一致。狭窄杂交带与有限的有效扩散，可能是跨陡峭生态梯度（ecological gradients）的长期定居后（postsettlement）、生殖前（prereproductive）选择的常见结果。强烈的歧化选择为海洋中按深度分隔的姊妹种（sibling species）的普遍存在提供了合理的解释机制。