Abstract
BackgroundInterspecific 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.Principal FindingsWe 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.SignificanceWe 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.
Highlights
The basis to defining a species is contingent on its delimitation as a unit whose reproduction is separated from that of other such units
The rate of male chasing was affected by parental combination when females were separated for 24 hours (GLLM with quasipoisson error structure, F3,36 = 5.16, P = 0.005), but no significant effect was observed after 72 hours of separation (F3,36 = 2.04, P = 0.126), probably due to increased interest in N. orthonotus females by N. furzeri males after separation (Fig. 2a)
The number of consent spawnings was highest in conspecific treatments (GLLM with quasi-poisson error structure, 24 h: F3,36 = 5.21, P = 0.004; 72 h: F3,36 = 17.52, P,0.001) and more consent spawnings were observed after a 72 h separation compared to a 24 h separation period, except for N. furzeri males paired with N. orthonotus females where only a single consent spawning was observed (Fig. 2b)
Summary
The basis to defining a species is contingent on its delimitation as a unit whose reproduction is separated from that of other such units. Interspecific gene flow in sympatric species is usually restricted by a combination of intrinsic and extrinsic reproductive isolation 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. 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
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