Evolution and role of colour polymorphism in cichlid fishes

Intraspecific variation in the distribution and hue of integument coloration is a widespread phenomenon in Lake Tanganyika`s cichlid species, and is most strikingly realised in the 120 colour variants within the genus Tropheus. The occurrence of a unique colour morph at virtually every isolated location is difficult to explain in the light of the past population admixtures triggered by lake level fluctuations and the short history of the extant distribution since the last rise in water level less than 20,000 years ago. The proposed study addresses the forces behind the creation of the extant plethora of colour variants in Tropheus by investigating the role of colour variation in mating behaviour, and the influence of lake level fluctuations on the current distribution of colour morphs and genetic lineages. To this aim, mate choice experiments will compare the importance of specific factors of male nuptial coloration such as hue and area of colouration for assortative mate choice by females, and look for correlations between observed assortative mating tendencies and actual genetic distances between populations. Phylogeographic analyses will be conducted at two levels. AFLP studies shall shed light on the phylogenetic relationships among populations all around Lake Tanganyika, while the mitochondrial and nuclear (SNP) variation at selected locations along the southern shoreline will be analysed to reconstruct movements of populations triggered by lake level changes and to gauge the extent of genetic exchange between populations that potentially merged versus that of populations whose isolation was enforced with changing habitats. We will also continue to monitor the consequences of a man-made admixture of five distinct populations released four years ago at one location in Zambia to assess the level of interbreeding under natural conditions. Finally, polymorphism in colour visual capabilities of Tropheus variants will be examined by opsin gene sequence comparisons, gene expression analysis and studies of spectral sensibilities of retinal cones. The proposed study will contribute to the identification of the evolutionary forces at work in one of Lake Tanganyika`s most remarkable species complexes, and augment the basis for a comparison of the species radiations in the old lake and the much younger Lakes Malawi and Victoria. The recognition of parallels and differences between the three lake systems will eventually teach us to which degree the current status of each lake can be interpreted as a temporal sample in the reconstruction of the architecture of an explosive lacustrine radiation.

Intraspecific variation in the distribution and hue of integument coloration is a widespread phenomenon in Lake Tanganyika`s cichlid species, and is most strikingly realised in the 120 colour variants within the genus Tropheus. The occurrence of a unique colour morph at virtually every isolated location is difficult to explain in the light of the past population admixtures triggered by lake level fluctuations and the short history of the extant distribution since the last rise in water level less than 20,000 years ago. The proposed study addresses the forces behind the creation of the extant plethora of colour variants in Tropheus by investigating the role of colour variation in mating behaviour, and the influence of lake level fluctuations on the current distribution of colour morphs and genetic lineages. To this aim, mate choice experiments will compare the importance of specific factors of male nuptial coloration such as hue and area of colouration for assortative mate choice by females, and look for correlations between observed assortative mating tendencies and actual genetic distances between populations. Phylogeographic analyses will be conducted at two levels. AFLP studies shall shed light on the phylogenetic relationships among populations all around Lake Tanganyika, while the mitochondrial and nuclear (SNP) variation at selected locations along the southern shoreline will be analysed to reconstruct movements of populations triggered by lake level changes and to gauge the extent of genetic exchange between populations that potentially merged versus that of populations whose isolation was enforced with changing habitats. We will also continue to monitor the consequences of a man-made admixture of five distinct populations released four years ago at one location in Zambia to assess the level of interbreeding under natural conditions. Finally, polymorphism in colour visual capabilities of Tropheus variants will be examined by opsin gene sequence comparisons, gene expression analysis and studies of spectral sensibilities of retinal cones. The proposed study will contribute to the identification of the evolutionary forces at work in one of Lake Tanganyika`s most remarkable species complexes, and augment the basis for a comparison of the species radiations in the old lake and the much younger Lakes Malawi and Victoria. The recognition of parallels and differences between the three lake systems will eventually teach us to which degree the current status of each lake can be interpreted as a temporal sample in the reconstruction of the architecture of an explosive lacustrine radiation.