Abstracts (first author)
The molecular basis of a novel pigment trait in cichlid fishes
How novel traits emerge and are modified is one of the many unresolved questions in evolutionary biology. The spectacularly diverse adaptive radiations of cichlid fishes in the East Africa Great Lakes provide an ideal system to study the molecular basis of evolutionary novelties in the context of adaptation and explosive speciation. One characteristic innovation of the most species-rich lineage of cichlids, the haplochromines, is a set of brightly pigmented spots on male anal fins, known as “egg-spots”. Egg-spots are a diverse trait (number, colour and shape), and this trait plays a key role in the territorial and breeding behaviour of about 1,500 species of cichlids. Here we report the identification of several egg-spot candidate genes by quantitative next generation sequencing of RNA from egg-spot tissue in the haplochromine cichlid Astatotilapia burtoni. We confirmed these results in other haplochromine species through quantitative gene expression analysis (qPCR), and narrowed down our study to one gene – an androgen receptor (AR) cofactor. A comparative genomic analysis between haplochromines and egg-spot-less non-haplochromine species reveals that the coding region of this gene cannot explain the origin and diversity of this trait. However, the upstream regulatory region of the AR cofactor differs between these groups: haplochromines bear a unique transposable element insertion in the proximity of the transcription initiation site of the AR cofactor. We designed constructs containing this AR-cofactor transposable element region upstream of a GFP (green fluorescent protein) coding sequence and tested them in zebrafish. GFP expression was detected in iridophores (pigment cells) and in the fin rays. We thus propose that this transposable element insertion might have changed the expression pattern of AR-cofactor, thereby initiating the morphogenesis of a key evolutionary innovation in one of the most species-rich lineages of vertebrates.