Body shape variation is common across all vertebrates and has important consequences for an animal's ecology, locomotion,
Since Darwin, biologists have sought to understand the evolution and origins of phenotypic adaptations. The skull is particularly diverse due to intense natural selection on feeding biomechanics. We investigated the genetic and molecular origins of trophic adaptation using Lake Malawi cichlids, which have undergone an exemplary evolutionary radiation. We analyzed morphological differences in the lateral and ventral head shape among an insectivore that eats by suction feeding, an obligate biting herbivore, and their F2 hybrids. We identified variation in a series of morphological traits—including mandible width, mandible length, and buccal length—that directly affect feeding kinematics and function. Using quantitative trait loci (QTL) mapping, we found that many genes of small effects influence these craniofacial adaptations. Intervals for some traits were enriched in genes related to potassium transport and sensory systems, the latter suggesting co-evolution of feeding structures and sensory adaptations for foraging. Despite these indications of co-evolution of structures, morphological traits did not show covariation. Furthermore, phenotypes largely mapped to distinct genetic intervals, suggesting that a common genetic basis does not generate coordinated changes in shape. 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.
Divergence along the benthic-pelagic axis is one of the most widespread and repeated patterns of morphological variation in fishes, producing body shape diversity associated with ecology and swimming mechanics. This ecological shift is also the first stage of the explosive adaptive radiation of cichlid fishes in the East African Rift Lakes. We use two hybrid crosses of cichlids (Metriaclima sp. x Aulonocara sp. and Labidochromis sp. x Labeotropheus sp., >975 animals total) along the benthic-pelagic ecomorphological axis to determine the genetic basis of body shape diversification. Using a series of both linear and geometric shape measurements, we identify 55 quantitative trait loci (QTL) that underlie various aspects of body shape variation associated with benthic-pelagic divergence. These QTL are spread throughout the genome, each explain 3.0-7.2% of phenotypic variation, and are largely modular. Further, QTL are distinct both between these two crosses of Lake Malawi cichlids and compared to previously identified QTL for body shape in fishes such as sticklebacks. We find that body shape is controlled by many genes of small effects. In all, we find that convergent benthic and pelagic body phenotypes commonly observed across fish clades are most likely due to distinct genetic and molecular mechanisms.
Three-dimensional (3D) modeling techniques have been increasingly utilized across disciplines for the visualization and analysis of complex structures. We employ 3Ddigital photogrammetry for understanding the scaling of the body axis of 12 species of scincid lizards in the genus Brachymeles. These skinks represent a diverse radiation which shows tremendous variation in body size and degree of axial elongation. Because of the complex nature of the body axis, 3D-methods are important for understanding how the body axis evolves. 3D-digital photogrammetry presents a flexible, inexpensive, and portable system for the reconstruction of biological forms. As body size increased among species, the cross-sectional area and circumference of the head and other portions of the body axis increased isometrically, which indicates that species of differing sizes possess proportionally similar head and body shapes. These results suggest that there are no substantial head and body shape changes with body size among the sampled species, but further comparative studies with larger sample sizes and functional studies of size and morphology effects on burrowing or above-ground locomotion are needed.
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.
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