Data from: Fitness and morphological outcomes of many generations of hybridization in the copepod Tigriopus californicus

Hybridization between genetically divergent populations is an important evolutionary process, with an outcome that is difficult to predict. We used controlled crosses and freely mating hybrid swarms, followed for up to 30 generations, to examine the morphological and fitness consequences of interpopulation hybridization in the copepod Tigriopus californicus. Patterns of fitness in two generations of controlled crosses were partly predictive of long-term trajectories in hybrid swarms. For one pair of populations, controlled crosses revealed neutral or beneficial effects of hybridization, and hybrid swarm fitness always equalled or exceeded that of the midparent. For a second pair, controlled crosses showed F2 hybrid breakdown, but elevated fitness in backcrosses, and hybrid swarm fitness deviated both above and below that of the parentals. Nevertheless, individual swarm replicates exhibited divergent fitness trajectories over time that were not related in a simple manner to their hybrid genetic composition, and fixation of fitter hybrid phenotypes was not observed. Hybridization did not generally increase overall morphological variation, and underlying genetic changes may have been masked by phenotypic plasticity. Nevertheless, one type of hybrid swarm exhibited a repeatable pattern of transgressively large eggsacs, suggesting a positive effect of hybridization on individual fecundity. Additionally both parental and hybrid swarms exhibited common phenotypic trends over time, indicating common selective pressures in the laboratory environment. Our results suggest that, in a system where much work has focused on F2 hybrid breakdown, the long-term fitness consequences of interpopulation hybridization are surprisingly benign.

遗传分化种群间的杂交是一类关键的进化过程，其最终结果往往难以预判。本研究以桡足类（copepod）加州虎斑猛水蚤（Tigriopus californicus）为研究对象，通过受控杂交与自由交配的杂交群（追踪时长可达30代），探究了种群间杂交对其形态特征与适合度的影响。两代受控杂交的适合度模式，可部分预测杂交群的长期演化轨迹。针对一对种群，受控杂交实验显示杂交具有中性或有益效应，且杂交群的适合度始终等于或高于双亲的中间值。针对另一对种群，受控杂交实验观测到F2代杂交衰退（F2 hybrid breakdown），但回交后代的适合度有所提升；该组的杂交群适合度既高于也低于亲本种群。不过，各杂交群重复组的适合度轨迹随时间呈现分化，且与杂交遗传组成并无简单线性关联，同时未观测到更优杂交表型的固定。杂交总体上并未提升整体形态变异水平，潜在的遗传变化可能被表型可塑性（phenotypic plasticity）所掩盖。但有一类杂交群呈现出可重复的跨亲本表型超大卵囊模式，表明杂交对个体繁殖力存在正向效应。此外，亲本与杂交群均随时间表现出共同的表型趋势，提示实验室环境中存在共通的选择压力。本研究结果表明，在大量研究聚焦于F2代杂交衰退的生物体系中，种群间杂交的长期适合度后果出人意料地温和。