Data from: Genetic and morphological evidence of a geographically widespread hybrid zone between two crocodile species, Crocodylus acutus and Crocodylus 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场所。本研究基于来自65个采样点的300条鳄鱼的遗传与形态学数据，评估了莫瑞雷鳄（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及同域分布带的遗传学研究。与预期相悖的是，贝叶斯混血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混血个体，且该杂交带在地理分布上极为广泛，远超二者历史上的同域分布区域。本研究还发现了少量地理隔离且未发生混血的双亲本物种种群。杂交个体并非F1代或近期回交个体，更大概率为多代混血个体，这表明杂交过程已持续数代乃至数十代，且主要由自然过程驱动。莫瑞雷鳄并非美洲鳄的姊妹物种，这表明该杂交带源自二次接触而非初期分化。两个物种的未混血个体可通过形态特征加以区分，而杂交个体则呈现复杂的形态嵌合特征，这使得野外个体鉴定难度大幅提升。野外现存的未混血美洲鳄与莫瑞雷鳄种群数量极少。因此，现存的最后一批未混血莫瑞雷鳄种群已处于极危状态。实际上，不仅双亲本物种，自然产生的杂交个体也应被纳入保护考量范围，以评估其潜在的保护价值。