Entwicklungsgen-Evolution und Adaptation im Cichliden-Kopf

The remarkably species-rich assemblages of cichlid fishes in the three Great East African Lakes represent a natural “mutant model system” for the study of the factors contributing to the formation of multiple new species within short time periods. The evolutionary success of cichlid fishes was explained by their “key-innovation” of two sets of jaws, the oral and the pharyngeal jaws, allowing them to rapidly evolve diverse feeding specializations.
The theory of genetic assimilation and the “Flexible Stem Hypothesis” suggest that (1) a mutation or environmental change triggers the expression of one or more novel, heritable, phenotypic variant(s) within a population; (2) the initially rare variant(s) start to spread (in the case of an environmentally induced change, due to the consistent recurrence of the environmental factor), each creating a differentiated subpopulation expressing one alternative fit variant; and (3) selection on existing genetic variation for the regulation or form of the trait under
selection causes it to become genetically fixed in individuals of a subpopulation. Such subpopulations ultimately evolve to new species adapted to alternative ecological niches, at the expense of losing the ancestral phenotypic
plasticity.
We plan to utilize the great comparative potential of the different evolutionary ages of the radiations in the three Great East African Lakes Victoria, Malawi, and Tanganyika to tackle the validity of the Flexible Stem Hypothesis as a central mechanism of cichlid fish diversification. This concept proposes that the broad range of phenotypic plasticity of ancestral "generalist" species, which allows survival in varied environmental conditions, is progressively reduced to be replaced and expanded by genetically based, specialized phenotypes. Since trophicniche differentiation and the decoupling of oral and pharyngeal jaws in cichlids are considered a key factor facilitating rapid radiation, we will compare two outgroup riverine species with known adaptive jaw phenotypic plasticity to three monophyletic sets of eco-morphologically equivalent model species from three independent lake radiations with increasing evolutionary age. We study the genomic basis of oral and pharyngeal jaw evolution by combining geometric morphometric and genomic analyses. Thereby, we intend to find genetic and
gene regulatory correlates to divergent eco-morphologies after they have been shaped during embryogenesis. We assess the scope of phenotypic plasticity for each study species in relation to the divergence time from the riverine generalists by comparing the head-morphology of corresponding wild and captivity-bred individuals via micro-computed tomography as a standardized measure to assess phenotypic plasticity. This allows us to test if the genetic architecture of eco-morphological divergence corresponds to the pattern predicted by the concept of genetic assimilation in accordance with the Flexible Stem Hypothesis. We address this at four levels: non-coding RNAs, the epigenetic regulation via CpG methylation, gene expression, and divergence in coding regions (protein evolution, gene duplications).