Eggspot Number and Sexual Selection in the Cichlid Fish Astatotilapia burtoni

Sexual selection on male coloration is one of the main mechanisms proposed to explain the explosive speciation rates in East African cichlid fish. True eggspots are color patterns characteristic of the most species-rich lineage of cichlids, the Haplochromini, and have been suggested to be causally related to the speciation processes. Eggspots are thought to have originated by sensory exploitation and subsequently gained several roles in sexual advertisement. However, for most of these functions the evidence is equivocal. In addition, the genetic architecture of this trait still is largely unknown. We conducted bidirectional selective breeding experiments for eggspot numbers in the model cichlid, Astatotilapia burtoni. After two generations, low lines responded significantly, whereas the high lines did not. Body size was both phenotypically and genotypically correlated with eggspot number and showed correlated response to selection. Males with higher numbers of eggspots were found to sire larger offspring. Despite the potential to act as honest indicators of fitness, the behavioral experiments showed no evidence of a role in either intra- or inter-sexual selection. Visual-based female preference was instead explained by courtship intensity. The evolution of this trait has been interpreted in light of adaptive theories of sexual selection, however the present and published results suggest the influence of non-adaptive factors such as sensory exploitation, environmental constraints and sexual antagonism.

雄性体色所经受的性选择，是学界用以解释东非慈鲷（East African cichlid fish）爆发式物种形成速率的核心机制之一。真卵斑是物种最为丰富的慈鲷支系——哈普洛慈鲷族（Haplochromini）所特有的体色图案，有假说认为其与物种形成过程存在因果关联。一般认为，卵斑起源于感官利用（sensory exploitation），随后在性信号展示中获得了多重功能。然而，针对这些功能中的绝大多数，现有证据仍模棱两可。此外，该性状的遗传架构在很大程度上仍未明确。我们针对模式慈鲷物种伯氏妊丽鱼（Astatotilapia burtoni）的卵斑数量开展了双向选择性繁殖实验。历经两代选育后，低选育系呈现出显著的响应，而高选育系则未出现明显响应。体型大小与卵斑数量在表型和基因型层面均存在相关性，且随选育出现了相关响应。卵斑数量更多的雄性，其后代体型更大。尽管卵斑具备作为适合度诚实指示物的潜力，但行为实验并未发现其在性内或性间选择中发挥作用的证据。基于视觉的雌性择偶偏好，实则可由求偶强度予以解释。此前学界多以性选择的适应性理论解读该性状的演化，但本研究与已发表的相关结果均表明，感官利用、环境限制以及性拮抗等非适应性因素或对其演化产生了影响。