Department of Fish Ecology, Academy of Sciences of the Czech Republic, Institute of Vertebrate Biology, Brno, Czech Republic.
PLoS One. 2011;6(8):e22684. doi: 10.1371/journal.pone.0022684. Epub 2011 Aug 5.
Interspecific reproductive isolation is typically achieved by a combination of intrinsic and extrinsic barriers. Behavioural isolating barriers between sympatric, closely related species are often of primary importance and frequently aided by extrinsic factors causing spatial and temporal interspecific separation. Study systems with a severely limited role of extrinsic factors on reproductive isolation may provide valuable insights into how reproductive isolation between sympatric species is maintained. We used no-choice experimental set-up to study reproductive barriers between two closely related sympatric African killifish species, Nothobranchius furzeri and Nothobranchius orthonotus. These fish live in small temporary savannah pools and have complete spatial and temporal overlap in reproductive activities and share a similar ecology.
We found that the two species display largely incomplete and asymmetric reproductive isolation. Mating between N. furzeri males and N. orthonotus females was absent under standard experimental conditions and eggs were not viable when fish were forced to mate in a modified experimental setup. In contrast, male N. orthonotus indiscriminately mated with N. furzeri females, the eggs were viable, and offspring successfully hatched. Most spawnings, however, were achieved by male coercion and egg production and embryo survival were low. Behavioural asymmetry was likely facilitated by mating coercion from larger males of N. orthonotus and at relatively low cost to females. Interestingly, the direction of asymmetry was positively associated with asymmetry in post-mating reproductive barriers.
We showed that, in fish species with a promiscuous mating system and multiple matings each day, selection for strong mate preferences was relaxed. This effect was likely due to the small proportion of resources allocated to each single mating and the high potential cost to females from mating refusal. We highlight and discuss the fact that males of rarer species may often coercively mate with females of a related, more abundant species.
种间生殖隔离通常是由内在和外在障碍共同作用形成的。在同域、亲缘关系密切的物种中，行为隔离障碍通常是首要的，并且常常受到导致种间空间和时间分离的外在因素的辅助。生殖隔离中外在因素作用严重受限的研究系统可能为了解同域物种之间生殖隔离是如何维持的提供有价值的见解。我们使用非选择实验设置来研究两种密切相关的非洲食蚊鱼，即 Nothobranchius furzeri 和 Nothobranchius orthonotus 之间的生殖障碍。这些鱼生活在小而临时的热带稀树草原池塘中，其生殖活动在空间和时间上完全重叠，并且具有相似的生态。
我们发现这两个物种的生殖隔离很大程度上是不完全和不对称的。在标准实验条件下，N. furzeri 雄性和 N. orthonotus 雌性之间的交配不存在，并且当鱼被迫在改良的实验设置中交配时，卵是不可育的。相比之下，雄性 N. orthonotus 可以与 N. furzeri 雌性随机交配，卵是可育的，后代成功孵化。然而，大多数产卵都是通过雄性胁迫实现的，卵子的产量和胚胎的存活率都很低。行为不对称可能是由较大的 N. orthonotus 雄性的交配胁迫以及雌性相对较低的成本促成的。有趣的是，不对称的方向与交配后生殖障碍的不对称性呈正相关。
我们表明，在具有滥交交配系统和每天多次交配的鱼类物种中，对强烈的配偶偏好的选择放宽了。这种效应可能是由于每次交配分配的资源比例较小，以及雌性拒绝交配的潜在高代价所致。我们强调并讨论了这样一个事实，即稀有物种的雄性可能经常胁迫与相关的、更丰富的物种的雌性交配。
