Stable species boundaries despite ten million years of hybridization in tropical eels

Genomic evidence is increasingly underpinning that hybridization between taxa is commonplace, challenging our views on the mechanisms that maintain their boundaries. Here, we focus on seven catadromous eel species (genus Anguilla), and use genome-wide sequence data from more than 450 individuals sampled across the tropical Indo-Pacific, morphological information, and three newly assembled draft genomes to compare contemporary patterns of hybridization with signatures of past gene flow across a time-calibrated phylogeny. We show that the seven species have remained distinct entities for up to 10 million years, despite a dynamic scenario of incomplete isolation whereby the current frequencies of hybridization across species pairs (over 5% of all individuals were either F1 hybrids or backcrosses) contrast remarkably with patterns of past introgression. Based on near-complete asymmetry in the directionality of hybridization and decreasing frequencies of later-generation hybrids, we identify...基因组学证据日益证实，不同分类群（taxa）间的杂交现象极为普遍，这对学界关于维持物种边界的机制的传统认知提出了挑战。本研究聚焦7种降海洄游型鳗鲡属（Anguilla）鱼类，利用采集自热带印度洋-太平洋区域的450余尾个体的全基因组序列数据（genome-wide sequence data）、形态学信息，以及3条新组装的基因组草图（draft genomes），对比当代杂交模式与时间校准的系统发育树（time-calibrated phylogeny）中过往基因流的特征。研究表明，尽管存在动态的不完全生殖隔离情景——当前物种种间杂交频率（所有个体中超过5%为F1杂交子代（F1 hybrids）或回交个体（backcrosses））与过往基因渐渗（introgression）模式存在显著差异，但这7个物种在长达1000万年的演化历程中始终保持独立的类群属性。基于杂交方向性近乎完全不对称以及后续世代杂交个体频率递减的特征，本研究鉴定出……