Genetic architecture of trophic adaptations in cichlid fishes
Since Darwin, biologists have sought to understand the evolution and origins of phenotypic adaptations. The skull is particularly diverse due to intense natural selection such as feeding biomechanics. We investigate the genetic and molecular origins of trophic adaptation using Lake Malawi cichlids, which have undergone an exemplary evolutionary radiation. We analyze morphological differences in the lateral and ventral head among an insectivore that eats by suction feeding, an obligate biting herbivore, and their F2 hybrids. We identify variation in a series of morphologies including mandible width, mandible length, and buccal length that directly affect feeding kinematics and function. Using quantitative trait loci (QTL) mapping, we find that many genes of small effects influence these craniofacial adaptations. Intervals for some traits are enriched in genes related to potassium transport and sensory systems, the latter suggesting correlation between feeding structures and sensory adaptations for foraging. Craniofacial phenotypes largely map to distinct genetic intervals, and morphologies in the head do not correlate. Together, these suggest that craniofacial traits are mostly inherited as separate modules, which confers a high potential for the evolution of morphological diversity. Though these traits are not restricted by genetic pleiotropy, functional demands of feeding and sensory structures likely introduce constraints on variation. In all, we provide insights into the quantitative genetic basis of trophic adaptation, identify mechanisms that influence the direction of morphological evolution, and provide molecular inroads to craniofacial variation.