The egg-dummies of haplochromine cichlids

The cichlid assemblages of the East African Great Lakes feature an unrivalled rate of speciation and are thus renowned as a prime model system in evolutionary biology. The majority of East African cichlid species are members of a single tribe, the Haplochromini, comprising most riverine East African cichlid species and the species flocks of Lakes Malawi and Victoria. An important feature shared by haplochromines is their special breeding behaviour, in the form of maternal mouthbrooding with females incubating the eggs in their buccal cavities. Mouthbrooding evolved several times during cichlid evolution, but only haplochromines show a characteristic polygynous or polygynandrous maternal mouthbrooding system with males carrying conspicuous egg-spots on the anal fin. These yellow to orange egg-like spots have an important function in the courtship and spawning behaviour of haplochromines, as they serve as intraspecific sexual advertisment to attract females and to maximize breeding success. The number, arrangement and morphology of haplochromine egg-spots differ between species (and to some degree also within species) and some haplochromines even lack egg-spots entirely, either due to their ancestral phylogenetic branching or because they lost their dummies secondarily. The function of egg spots has been under experimental investigation and various hypotheses have been formulated. So far, behavioural studies corroborate the idea that egg spots are an evolutionary important feature of haplochromine cichlids. Recently, it was shown that the egg-spots of haplochromines are made up of yellow pigment cells, xanthophores, and that a gene coding for a type III receptor tyrosine kinase, colony-stimulating factor 1 receptor a (csf1ra), is expressed in egg-spot tissue. Molecular evolutionary analyses revealed that the extracellular ligand-binding and receptor-interacting domain of csf1ra underwent adaptive sequence evolution in the ancestral lineage of the haplochromines, coinciding with the emergence of egg dummies. This proposal aims to reveal the evolutionary significance of egg spots - a complex etho-morphological trait, applying behavioral experiments and uncovering its genetic and developmental basis using molecular techniques. Behavioural experiments will involve mate choice trials with five haplochromine species, ranging from a riverine generalist (lacking true egg spots) to highly specialised lake dwelling haplochromines. To this end, females will be given a choice between four males with differently manipulated egg-spots to determine wether there is a difference in mating success and/or fertilization rates for males displaying more and/or enlarged egg-spots. Additionally, it will be tested wether there is a selective advantage for males displaying more and/or larger egg-spots over males with unaltered egg-spots in turbid water conditions. The genetic part of the project will involve cloning and RT-PCR of candidate genes and subsequent in situ hybridisation experiments. Once it is shown that a gene is expressed in the egg dummy of male haplochromines, molecular evolutionary analyses will be applied to test for positive selection in the coding region of the respective gene.