Asymmetrical hybridization and environmental factors influence the spatial genetic structure of a killifish hybrid zone

Hybridization offers insight into speciation and the forces that maintain
barriers to reproduction, and hybrid zones provide excellent opportunities
to test how environment shapes barriers to reproduction and hybrid
fitness. A hybrid zone between the killifish, Fundulus heteroclitus and F.
grandis, had been identified in northeastern Florida, although the spatial
structure and parameters that affect the distribution of the two species
remain unknown. The present study aimed to determine the fine-scale
spatial genetic patterns of the hybrid zone to test the hypothesis that
species ranges are influenced by changes in dominant vegetation, and to
determine how differences in reproductive barriers between the two species
influence the observed patterns. The area of overlap between the two
species spanned ~37 km and showed a mosaic pattern of hybridization,
suggesting the spatial structure of the hybrid zone is largely influenced
by the environment. Environmental association analysis, however, suggested
that while dominant vegetation had a significant influence on the spatial
structure of the hybrid zone, a combination of environmental factors was
driving the observed patterns. Hybridization tended to be rare at sites
where F. heteroclitus was the more abundant species, suggesting that
differences in preference for conspecifics can lead to differences in
rates of introgression into parental taxa and likely result in a
range-shift as opposed to adaptation in the face of climate change.

杂交研究可为物种形成以及维持生殖隔离的演化动力提供重要见解，而杂交带为探究环境如何塑造生殖隔离与杂交后代适合度提供了绝佳的实验体系。此前在美国佛罗里达州东北部已发现斑节底鳉（Fundulus heteroclitus）与大底鳉（F. grandis）之间的杂交带，但调控这两个物种种群分布的空间结构与相关参数仍不明确。本研究旨在解析该杂交带的精细空间遗传格局，以验证“物种种群分布受优势植被变化调控”这一假说，并探究两个物种间生殖隔离的差异如何影响观测到的空间格局。两个物种的重叠分布区域跨度约37km，呈现出镶嵌式的杂交格局，提示该杂交带的空间结构在很大程度上受环境因子调控。然而环境关联分析结果显示，尽管优势植被对杂交带的空间结构存在显著影响，但观测到的格局实则由多种环境因子共同驱动。当斑节底鳉为优势物种的采样位点中，杂交事件往往较为罕见，这提示物种对同种个体的择偶偏好差异可导致向亲本类群的基因渐渗速率出现分化，并最终可能引发种群分布范围迁移，而非在气候变化背景下发生适应性演化。