Data from: The cost of reinforcement in Drosophila yakuba

When the ranges of two species overlap and the species can hybridize, some
individuals may waste gametes on inviable or infertile hybrids. In these
cases, enhanced reproductive isolation may evolve as a byproduct of
selection against maladaptive hybridization in a process called
reinforcement. On the slopes of the African island of São Tomé, Drosophila
yakuba and its endemic sister species D. santomea have a well-demarcated
hybrid zone. D. yakuba females from within this zone, but not from outside
it, show an increase in gametic isolation from males of D. santomea. To
understand why reinforced gametic isolation does not spread to the whole
geographic range and stays confined to areas of secondary contact, we
studied the associated costs of reinforced gametic isolation in D. yakuba
by using a combination of natural collections and experimental evolution.
We found that D. yakuba males from sympatric populations sire fewer
progeny than allopatric males when the female involved in the mating is an
allopatric female. The results here shown suggest that the advantageous
nature of reinforcement in D. yakuba is local, as its associated costs
(i.e., reduced male fertility) might prevent its dispersal outside the
hybrid zone.

当两个物种的分布范围发生重叠且二者能够杂交时，部分个体可能会将配子浪费在无法存活或不育的杂交后代上。在此类情形中，针对有害杂交的选择会附带演化出增强的生殖隔离，这一过程被称为生殖强化（reinforcement）。在非洲圣多美岛的山坡地带，雅库巴果蝇（Drosophila yakuba）与其特有姊妹物种圣多美果蝇（D. santomea）存在界限清晰的杂交带。该杂交带内的雅库巴果蝇雌性（而非杂交带外的雌性），与圣多美果蝇雄性交配时，配子隔离（gametic isolation）程度显著提升。为阐明为何经强化产生的配子隔离未能扩散至整个地理分布范围，而仅局限于次生接触区，我们结合自然采集与实验演化的研究手段，对雅库巴果蝇中强化配子隔离的相关成本展开了探究。研究发现，当交配对象为异域雌性（allopatric female）时，同域种群（sympatric population）的雅库巴果蝇雄性所产生的后代数量显著少于异域雄性（allopatric male）。本研究结果表明，雅库巴果蝇中生殖强化的优势具有局域性，其伴随的相关成本（即雄性生育力下降）或可阻碍该性状在杂交带外的扩散。