Data from: Hybridization and introgression in two ecologically dissimilar Fundulus hybrid zones

Hybridization and introgression appear more common in rapidly evolving groups, suggesting an important role in the evolutionary process. Detailed studies of how extrinsic or intrinsic forces regulate hybridization and introgression have the potential for broadening our understanding of mechanisms generating diversity. Species in the Fundulus notatus species complex have broad overlapping ranges and occur in replicated hybrid zones along predictable stream gradients. Typical hybrid zone structure has F. olivaceus in headwaters, F. notatus downstream, and hybrid zones near confluences or abrupt shifts in habitat. Rarely, the typical upstream-downstream orientation is reversed raising questions as to how hybrid zones are formed and maintained. We used next generation sequencing data to study hybridization and introgression in hybrid zones in neighboring drainages that differ in orientation (typical and reversed). We predicted extrinsic forces linked to stream gradients would result in noticeable differences between the two. Contrary to predictions, the data indicate the hybrid zones are remarkably similar. We used individual based simulations to explore the potential role of intrinsic and extrinsic forces in generating and maintaining typical and reversed hybrid zones. Simulation results were consistent with reversed hybrid zones being formed from stochastic processes combined with strong intrinsic forces and weak extrinsic forces.

杂交（hybridization）与渐渗（introgression）在快速演化类群中更为常见，这提示二者在演化进程中扮演着关键角色。针对外源性或内源性因素如何调控杂交与渐渗的深入研究，有望拓展我们对物种多样性产生机制的认知。底鳉（Fundulus notatus）物种复合体的类群拥有广泛重叠的分布范围，且沿可预测的河流梯度形成了多处重复出现的杂交带。典型杂交带的结构为：橄榄底鳉（F. olivaceus）栖息于上游源头河段，斑带底鳉（F. notatus）分布于下游区域，杂交带则多位于汇流处或生境发生突变转换的区域附近。极少数情况下，典型的上游-下游分布格局会发生反转，这引发了关于杂交带如何形成与维持的诸多疑问。本研究借助下一代测序（next generation sequencing）数据，对相邻流域中两类方向存在差异的杂交带（典型型与反转型）内的杂交与渐渗现象展开探究。我们曾预测，与河流梯度相关的外源性因素会导致两类杂交带出现显著差异，但实验数据与预测相悖，结果显示两类杂交带极为相似。随后我们通过基于个体的模拟（individual based simulations），探究了内源性与外源性因素在形成和维持典型型与反转型杂交带过程中的潜在作用。模拟结果表明，反转型杂交带的形成可能是随机过程与强内源性因素、弱外源性因素共同作用的结果。