Data from: Genetic and morphological evidence of a geographically widespread hybrid zone between two crocodile species, Crocodylus acutus and C. moreletii

Hybrid zones represent natural laboratories to study gene flow, divergence and the nature of species boundaries between closely related taxa. We evaluated the level and extent of hybridization between Crocodylus moreletii and C. acutus using genetic and morphological data on 300 crocodiles from 65 localities. To our knowledge, this is the first genetic study that includes the entire historic range and sympatric zone of the two species. Contrary to expectations, Bayesian admixture proportions and maximum likelihood estimates of hybrid indexes revealed that most sampled crocodiles were admixed and that the hybrid zone is geographically extensive, extending well beyond their historical region of sympatry. We identified a few geographically isolated, non-admixed populations of both parental species. Hybrids do not appear to be F1s or recent backcrosses, but rather are more likely later-generation hybrids, suggesting that hybridization has been going on for several to many generations and is mostly the result of natural processes. C. moreletii is not the sister species of C. acutus, suggesting that the hybrid zone formed from secondary contact rather than primary divergence. Non-admixed individuals from the two species were distinguishable based on morphological characters, whereas hybrids had a complex mosaic of morphological characters that hinders identification in the wild. Very few non-admixed C. acutus and C. moreletii populations exist in the wild. Consequently, the last non-admixed C. moreletii populations have become critically endangered. Indeed, not only the parental species but also the naturally occurring hybrids should be considered for their potential conservation value.

杂交带是研究近缘类群间基因流、物种分化以及物种边界本质的天然实验体系。本研究依托采自65个采样点的300条鳄鱼的遗传与形态学数据，对莫雷鳄（Crocodylus moreletii）与美洲鳄（Crocodylus acutus）之间的杂交水平及杂交范围展开了评估。据我们所知，本研究是首个覆盖这两个物种完整历史分布区与同域分布带的遗传学研究。与既有预期相悖的是，贝叶斯遗传混合比例估算与杂交指数的最大似然估计结果显示，绝大多数采样个体均为杂交个体，且该杂交带的地理分布范围极广，远超二者历史上的同域分布区域。我们还识别出少数地理隔离且未发生遗传混合的双亲本物种种群。本次研究发现，杂交个体并非F1代（子一代）或近期回交个体，更大概率为多代杂交后代，这表明杂交现象已持续多代，且主要由自然过程主导。莫雷鳄并非美洲鳄的姊妹物种，这意味着该杂交带源自二次接触而非初始分化。两个物种的未混合个体可通过形态特征进行区分，而杂交个体则呈现出复杂的形态镶嵌特征，这极大阻碍了其野外识别工作。目前野外现存的未发生遗传混合的美洲鳄与莫雷鳄种群数量极少。因此，最后残存的未混合莫雷鳄种群已处于极危状态。事实上，不仅双亲本物种，自然形成的杂交种群也应被纳入保护评估范畴，考量其潜在的保护价值。