The effects of ancestral plasticity and genetic assimilation on convergence and the diversification of East African cichlid fishes

Different environments mean different selective regimes and are thereby expected to affect fitness. The ability to respond adaptively with developmental plasticity to such environmental challenges would be expected to increase an organism’s chances to persist. Theoretical and empirical work suggests that phenotypic plasticity itself can be the target of selection and thus evolve. Phenotypic plasticity has been documented in many ecologically relevant traits and should buffer organisms from natural selection. But its role in speciation and during rapid adaptive radiations is not well understood and controversial. The "flexible stem model", proposes that phenotypic plasticity can boost a lineage’s diversification. An adaptive radiation initiated by a plastic ancestor would lead to a sudden increase in phenotypic diversity followed by a gradual canalization into potentially different formerly plastic phenotypes and thus lead to several lineages with then distinct and less plastic phenotypes. This might happen through mechanisms that reduce environmental sensitivity such as genetic assimilation/canalization. When an initially plastically expressed phenotype undergoes genetic assimilation, its plasticity would be concomitantly reduced, the trait’s phenotype becomes genetically fixed and its heritability increased. Selection would be expected act on these phenotypes faster, potentially promoting phenotypic divergence and speeding up speciation. Cichlid fishes are a promising evolutionary model systems in which to investigate the potential contributions of a flexible stem and genetic assimilation to lineage diversification and rapid adaptive radiations. One most interesting key innovation of cichlids that is thought to have facilitated their degree of trophic specializations and thereby permitting their un-rivalled diversification is their highly modified pharyngeal jaw apparatus (PJA, a functional second jaw with different phenotypically plastic dentition that gets replaced continuously). The phenotype of the PJA and the size and shape of the teeth are phenotypically plastic and a particular PJA morphology reflects the trophic niche of the species. In this study, we focus on the most crucial and most controversially debated assumption of the flexible stem: the period when divergent populations start to undergo genetic assimilation and thus genetic fixation of a phenotype. We will investigate differences in inducible plasticity (through split-brood diet treatments in common garden experiments) between riverine (ancestral/source) and lacustrine (specialized/derived) populations in several cichlids. We will investigate (through RNAseq, qPCR and in situ hybridization) the potential role of plasticity candidate genes, some of which we already obtained. For them will characterize the expression dynamics and functional relevance during the experimentally induced plastic response to the PJA morphology.

Publications

• Gunter HM, Schneider RF, Karner I, Sturmbauer C, Meyer A (2017) Molecular investigation of genetic assimilation during the rapid adaptive radiations of East African cichlid fishes. Molecular Ecology 26(23):6634-6653. DOI: 10.1111/mec.14405
  
  
• Ralf F. Schneider and Axel Meyer (2017) How plasticity, genetic assimilation and cryptic genetic variation may contribute to adaptive radiations. Molecular Ecology 26:330-350. DOI: 10.1111/mec.13880