Data from: Male courtship preferences demonstrate discrimination against allopatric colour morphs in a cichlid fish

Whether premating isolation is achieved by male-specific, female-specific or sex-independent assortative preferences often depends on the underlying evolutionary processes. Here we test mate preferences of males presented with females of different allopatric colour variants of the cichlid fish Tropheus, a Lake Tanganyika endemic with rich geographic colour pattern variation, in which the strength of sexual isolation varies between populations. We conducted two-way mate choice experiments to compare behaviour of males of a red-bodied morph (population Moliro) towards females from their own population with behaviour towards females from four allopatric populations at different stages of phylogenetic and phenotypic divergence. Males courted same-population females significantly more intensely than females of other populations, and reduced their heteromorphic courtship efforts both with increasing genetic and increasing phenotypic distinctness of the females. In particular, females of a closely related red-bodied population received significantly more courtship than either genetically distinct, similarly coloured females (“Kirschfleck” morph) or genetically related, differently coloured females (“yellow-blotch” morph), both of which were courted similarly. Genetically and phenotypically distinct females (T. polli) were not courted at all. Consistent with previous female-choice experiments, female courtship activity also decreased with increasing genetic distance from the males’ population. Given successful experimental and natural introgression between colour morphs and the pervasive allopatry of related variants, we consider it unlikely that assortative preferences of both sexes were driven by direct selection during periods of secondary contact or, in turn, drove colour pattern differentiation in allopatry. Rather, we suggest that sexual isolation evolved as by-product of allopatric divergence.

交配前生殖隔离（premating isolation）究竟由雄性特异性、雌性特异性还是性别非依赖性的选型交配偏好（assortative preferences）所介导，往往取决于其背后的进化进程。本研究聚焦坦噶尼喀湖（Lake Tanganyika）特有、具备丰富地理色斑变异的慈鲷鱼属饰颊丽鱼（Tropheus）——该类群不同种群间的性隔离强度存在显著差异——开展雄性择偶偏好实验。我们以红色体色的莫利罗（Moliro）种群雄性为受试对象，通过双向择偶实验，对比其对同种群雌性与来自4个处于不同系统发育与表型分化阶段的异域种群雌性的行为表现。实验结果表明，雄性对同种群雌性的求偶强度显著高于异种群雌性，且针对异源雌性的求偶投入随雌性遗传与表型分化程度的升高而降低。具体而言，亲缘关系较近的红色色型种群雌性所获得的求偶次数显著多于两类雌性：一类是遗传分化显著但体色相似的“Kirschfleck”色型雌性，另一类是遗传关系相近但体色不同的“黄斑（yellow-blotch）”色型雌性，且后两类雌性的受求偶程度并无显著差异。而遗传与表型均高度分化的波氏饰颊丽鱼（T. polli）雌性则完全未获得雄性求偶。与此前的雌性择偶实验结果一致，雌性的求偶活跃度同样随与雄性种群遗传距离的增加而下降。鉴于不同色型间已被证实存在实验性与自然的基因渐渗（introgression），且相关类群广泛呈异域分布，我们认为，两性的选型交配偏好不太可能是在次生接触时期由直接选择所驱动，亦不会反过来推动异域种群间的色斑分化。相反，我们提出，性隔离的演化是异域分化的副产物。