Carleton Karen L, Spady Tyrone C, Streelman J Todd, Kidd Michael R, McFarland William N, Loew Ellis R
Department of Biology, University of Maryland, College Park, MD 20742, USA.
BMC Biol. 2008 May 23;6:22. doi: 10.1186/1741-7007-6-22.
Cichlid fishes have radiated into hundreds of species in the Great Lakes of Africa. Brightly colored males display on leks and vie to be chosen by females as mates. Strong discrimination by females causes differential male mating success, rapid evolution of male color patterns and, possibly, speciation. In addition to differences in color pattern, Lake Malawi cichlids also show some of the largest known shifts in visual sensitivity among closely related species. These shifts result from modulated expression of seven cone opsin genes. However, the mechanisms for this modulated expression are unknown.
In this work, we ask whether these differences might result from changes in developmental patterning of cone opsin genes. To test this, we compared the developmental pattern of cone opsin gene expression of the Nile tilapia, Oreochromis niloticus, with that of several cichlid species from Lake Malawi. In tilapia, quantitative polymerase chain reaction showed that opsin gene expression changes dynamically from a larval gene set through a juvenile set to a final adult set. In contrast, Lake Malawi species showed one of two developmental patterns. In some species, the expressed gene set changes slowly, either retaining the larval pattern or progressing only from larval to juvenile gene sets (neoteny). In the other species, the same genes are expressed in both larvae and adults but correspond to the tilapia adult genes (direct development).
Differences in visual sensitivities among species of Lake Malawi cichlids arise through heterochronic shifts relative to the ontogenetic pattern of the tilapia outgroup. Heterochrony has previously been shown to be a powerful mechanism for change in morphological evolution. We found that altering developmental expression patterns is also an important mechanism for altering sensory systems. These resulting sensory shifts will have major impacts on visual communication and could help drive cichlid speciation.
丽鱼科鱼类在非洲大湖地区已辐射演化出数百个物种。色彩鲜艳的雄鱼在求偶场进行展示，竞相被雌鱼选为配偶。雌鱼的强烈偏好导致雄鱼交配成功率出现差异，进而促使雄鱼体色模式快速进化，甚至可能导致物种形成。除了体色模式的差异，马拉维湖丽鱼科鱼类在视觉敏感度上的变化也是近缘物种中已知最大的。这些变化源于七个视锥视蛋白基因的表达调控。然而，这种调控表达的机制尚不清楚。
在本研究中，我们探究这些差异是否可能源于视锥视蛋白基因发育模式的变化。为验证这一点，我们将尼罗罗非鱼（Oreochromis niloticus）视锥视蛋白基因表达的发育模式与几种马拉维湖丽鱼科鱼类的进行了比较。在罗非鱼中，定量聚合酶链反应显示视蛋白基因表达从幼虫基因组合动态变化为幼鱼基因组合，最终形成成年基因组合。相比之下，马拉维湖的丽鱼科鱼类呈现出两种发育模式之一。在某些物种中，表达的基因组合变化缓慢，要么保留幼虫模式，要么仅从幼虫基因组合发展为幼鱼基因组合（幼态持续）。在其他物种中，相同的基因在幼虫和成虫中均有表达，但与罗非鱼的成年基因相对应（直接发育）。
马拉维湖丽鱼科鱼类物种间视觉敏感度的差异是相对于罗非鱼外类群个体发育模式的异时性变化导致的。异时性此前已被证明是形态进化变化的强大机制。我们发现，改变发育表达模式也是改变感觉系统的重要机制。这些由此产生的感觉变化将对视觉通讯产生重大影响，并可能有助于推动丽鱼科鱼类的物种形成。
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