Rapid generation of ecologically relevant behavioural novelty in experimental cichlid hybrids

The East African cichlid radiations are characterised by repeated and rapid diversification into many distinct species with different ecological specialisations and by a history of hybridization events between non-sister species. Such hybridization might provide important fuel for adaptive radiation. Interspecific hybrids can have extreme trait values or novel trait combinations and such transgressive phenotypes may allow some hybrids to explore ecological niches neither of the parental species could tap into. Here, we investigate the potential of second-generation (F2) hybrids between two generalist cichlid species from Lake Malawi to exploit a resource neither parental species is specialised on: feeding by sifting sand. Some of the F2 hybrids phenotypically resembled fish of species that are specialised on sand sifting. We combined experimental behavioural and morphometric approaches to test whether the F2 hybrids are transgressive in both morphology and behaviour related to sand sifting. We then performed a quantitative trait loci (QTL) analysis using RADseq markers to investigate the genetic architecture of morphological and behavioural traits. We show that transgression is present in several morphological traits, that novel trait combinations occur, and we observe transgressive trait values in sand sifting behaviour in some of the F2 hybrids. Moreover, we find QTLs for morphology and for sand sifting behaviour, suggesting the existence of some loci with moderate to large effects. We demonstrate that hybridization has the potential to rapidly generate novel and ecologically relevant phenotypes that may be suited to a niche neither of the parental species occupies. Interspecific hybridization may thereby contribute to the rapid generation of ecological diversity in cichlid radiations.