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（hybrid zone）是探究近缘类群间基因流、物种分化与物种界限本质的天然实验场所。本研究基于采自65个样点的300条鳄类的遗传与形态学数据，评估了莫雷鳄（Crocodylus moreletii）与美洲鳄（Crocodylus acutus）之间的杂交水平与杂交范围。据我们所知，本研究是首个涵盖这两个物种完整历史分布区与同域分布区的遗传学研究。与预期相悖的是，贝叶斯遗传组分比例分析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。本研究仅发现少量地理隔离的亲本物种非杂交种群。杂交个体并非子一代（F1）或近期回交个体，更可能为多代杂交后代，这表明杂交现象已持续多代，且主要为自然过程所致。莫雷鳄并非美洲鳄的姊妹物种，这表明该杂交带源于二次接触而非初始分化。两个物种的非杂交个体可通过形态特征进行区分，而杂交个体则呈现复杂的形态镶嵌特征，这给野外物种鉴定带来了极大阻碍。野外现存的美洲鳄与莫雷鳄非杂交种群数量极少。因此，仅存的莫雷鳄非杂交种群已处于极危状态。事实上，不仅亲本物种，自然产生的杂交种群也应被纳入保护考量范畴，评估其潜在的保护价值。