Adaptive sequence evolution in a color gene involved in the formation of the characteristic egg-dummies of male haplochromine cichlid fishes.
BACKGROUND
The exceptionally diverse species flocks of cichlid fishes in East Africa are prime examples of parallel adaptive radiations. About 80% of East Africa's more than 1 800 endemic cichlid species, and all species of the flocks of Lakes Victoria and Malawi, belong to a particularly rapidly evolving lineage, the haplochromines. One characteristic feature of the haplochromines is their possession of egg-dummies on the males' anal fins. These egg-spots mimic real eggs and play an important role in the mating system of these maternal mouthbrooding fish.


RESULTS
Here, we show 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 reveal 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. We also find that csf1ra is expressed in the egg-dummies of a distantly related cichlid species, the ectodine cichlid Ophthalmotilapia ventralis, in which markings with similar functions evolved on the pelvic fin in convergence to those of the haplochromines.


CONCLUSIONS
We conclude that modifications of existing signal transduction mechanisms might have evolved in the haplochromine lineage in association with the origination of anal fin egg-dummies. That positive selection has acted during the evolution of a color gene that seems to be involved in the morphogenesis of a sexually selected trait, the egg-dummies, highlights the importance of further investigations of the comparative genomic basis of the phenotypic diversification of cichlid fishes.
Salzburger
Walter
W
Lehrstuhl für Zoologie und Evolutionsbiologie, Department of Biology, University Konstanz, 78457 Konstanz, Germany. walter.salzburger@unibas.ch
Braasch
Ingo
I
Meyer
Axel
A
D000222:Adaptation, Physiological; D000818:Animals; D002856:Chromatophores / Q000502:physiology*; D023681:Cichlids / Q000235:genetics* / Q000502:physiology; D019143:Evolution, Molecular; D015870:Gene Expression; D017403:In Situ Hybridization; D016173:Macrophage Colony-Stimulating Factor / Q000235:genetics; D008297:Male; D010063:Ovum; D010802:Phylogeny; D010858:Pigmentation / Q000235:genetics*; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics; D016184:Receptors, Colony-Stimulating Factor / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D012727:Sex Characteristics
D016184:Receptors, Colony-Stimulating Factor; D016173:Macrophage Colony-Stimulating Factor; D020794:Receptor Protein-Tyrosine Kinases