Evolutionary Genetics of Coloration in Cichlids

iv asymmetrically to selection. There was also a correlated response of body size. Furthermore, no evidence was found for the previously proposed sexual advertisement functions. All of this is inconsistent with the view that such trait is under strong directional sexual selection. It seems more likely that either non-adaptive processes currently shape the distribution of eggspot numbers or that selection acting on a correlated trait does. The second case-study concerns the gold/normal polychromatism in the Midas cichlid species of the genus Amphilophus. Gold or normal colored adults exist in several different populations and species in Nicaragua. All fish start their lives with a grayish (normal) color pattern and most individuals maintain their melanophores (dark pigment cells) throughout their lives. Others undergo morphological color change due to a process of melanophore cell death and exhibit a distinctive white-to-gold coloration in adulthood. It was shown that the color morphs mate assortatively and have diverged significantly in at least two lakes. In the present thesis, the genetic architecture of the gold phenotype is shown to be a simple one. This incipient speciation phenotype is determined by a dominant allele at a single gene. It was found that the gold locus might also control mating preference. In order to identify the causative gene, a number of approaches were taken, including forward-genetics (familyand population-based mapping), candidate-gene and transcriptomic experiments. The genomic region harboring the gold locus contains only a few positional candidate-genes out of which a single one has a known pigmentation function. Significant genotype-phenotype association was found in this region in an independent, field-collected population sample. Known color-genes that determine similar phenotypes in other organisms were either unrelated to or appear to be downstream targets of the gold locus. Interestingly, the genes that showed an expression pattern consistent with a role in the process of morphological color change are also related to human pigmentation disorders (e.g. skin cancer, psoriasis). One curious and unexpected result was that heterozygous gold fish undergo color change later in life than homozygous fish. This observation has implications for the evolutionary dynamics of this allele in the wild. In summary, the present results contribute towards filling the knowledge gaps of cichlid pigmentation and speciation genetics. By questioning previous conceptions, confirming theoretical expectations and generating new and exciting hypotheses, the present thesis illustrates the importance of understanding the genetic bases of ecologically-relevant phenotypes.