Accelerated Evolution of the surface Amino Acids in the WD-Repeat Domain Encoded by the hagoromo Gene in an Explosively Speciated Lineage of East African Cichlid Fishes

摘要

Lakes Victoria, Malawi, and Tanganyika in the East African rift Valley harbor approximately 200, 400, and 170 endemic species of cichlid fishes, respectively (Fryer and Iles 1972; Greenwood 1984). These fishes provide a spectacular example of the explosive adaptive radiation of living vertebrates (Fryer and Iles 1972; Greenwood 1984). they exploit almost all resources that are available to freshwater fishes in general (Fryer and Iles 1972; greenwood 1984) and are extremely diverse, both ecologically and morphologically, despite having evolved during a very short evolutionary period (Meyer et al. 1990; Johnson et al. 1996). In cichlids, species are sexually isolated as a consequence of mate choice (Crapon De Caprona 1996; Seehausen, Van Alphen, and Witt 1997), which is based on coloration. Assortative mating among individuals with various colorations can rapidly lead to sexual isolation of color morphs (Seehausen, Van Alphen, and Witte 1997). therefore, it is reasonable to postulate that genes that control formation of pigment patterns that are responsible for cichlid speciation must have changed at an accelerated rate in parallel with the diversification of the pigment patterns of species. In an attempt to identify such genes, we have focused our attention on genes responsible for the formation of pigment patterns in cichlids. Mechanisms underlying patterns of pigmentation remain, however, totally unknown. In zebrafish, various mutations affecting pigmentation have been described (Johnson et al. 1995; Haffer et al. 1996), and some relevant genes have been cloned and characterized (e.g., sparse, nacre, and hagoromo hag; Lister et al. 1999; Parichy et al. 1999; Kawakami et al. 2000). The genes that have been identified in zebrafish should help us to analyze pigmentation in other species of fishes, including cichlids. As a first step toward an understanding of the molecular basis for the divergence of pigment patterns and speciation in cichlids, we cloned and characterized a cichlid homolog of the zebrafish hag gene.