African cichlid fishes have repeatedly evolved highly specialized modes of feeding through adaptations of their oral jaws. Here, we explore the molecular genetic basis of the opening and closing lever mechanisms of the cichlid lower jaw, which have traditionally been used to describe the mechanics of feeding behavior in bony fishes. Quantitative genetic analyses demonstrate that the opening and closing mechanisms are genetically modular and therefore free to evolve independently. Bmp4 (bone morphogenetic protein 4) is one of two loci that segregate with the mechanical advantage of closing and that together account for >30% of the phenotypic variance in this trait. Species-specific differences in jaw shape are obvious early in cichlid larval development and are correlated with patterns of bmp4 expression in the mandibular primordium. When bmp4 is overexpressed in the obligate suction feeder Danio rerio, mandibular morphology exhibits specific transformations of opening and closing lever ratios. We conclude that patterns of morphological integration of the cichlid jaw reflect a balance among conflicting functional demands. Further, we demonstrate that bmp4 has the potential to alter mandibular morphology in a way that mimics adaptive variation among fish species.adaptive radiation ͉ bmp4 ͉ jaw shape ͉ morphological integration

East African cichlid fishes represent one of the most striking examples of rapid and convergent evolutionary radiation among vertebrates. Models of ecological speciation would suggest that functional divergence in feeding morphology has contributed to the origin and maintenance of cichlid species diversity. However, definitive evidence for the action of natural selection has been missing. Here we use quantitative genetics to identify regions of the cichlid genome responsible for functionally important shape differences in the oral jaw apparatus. The consistent direction of effects for individual quantitative trait loci suggest that cichlid jaws and teeth evolved in response to strong, divergent selection. Moreover, several chromosomal regions contain a disproportionate number of quantitative trait loci, indicating a prominent role for pleiotropy or genetic linkage in the divergence of this character complex. Of particular interest are genomic intervals with concerted effects on both the length and height of the lower jaw. Coordinated changes in this area of the oral jaw apparatus are predicted to have direct consequences for the speed and strength of jaw movement. Taken together, our results imply that the rapid and replicative nature of cichlid trophic evolution is the result of directional selection on chromosomal packages that encode functionally linked aspects of the craniofacial skeleton.

Biased left-right asymmetry is a fascinating and medically important phenomenon. We provide molecular genetic and physiological characterization of a novel, conserved, early, biophysical event that is crucial for correct asymmetry: H + flux. A pharmacological screen implicated the H + -pump H + -V-ATPase in Xenopus asymmetry, where it acts upstream of early asymmetric markers. Immunohistochemistry revealed an actin-dependent asymmetry of H + -V-ATPase subunits during the first three cleavages. H + -flux across plasma membranes is also asymmetric at the four-and eight-cell stages, and this asymmetry requires H + -V-ATPase activity. Abolishing the asymmetry in H + flux, using a dominant-negative subunit of the H + -V-ATPase or an ectopic H + pump, randomized embryonic situs without causing any other defects. To understand the mechanism of action of H + -V-ATPase, we isolated its two physiological functions, cytoplasmic pH and membrane voltage (V mem ) regulation. Varying either pH or V mem , independently of direct manipulation of H + -V-ATPase, caused disruptions of normal asymmetry, suggesting roles for both functions. V-ATPase inhibition also abolished the normal early localization of serotonin, functionally linking these two early asymmetry pathways. The involvement of H + -V-ATPase in asymmetry is conserved to chick and zebrafish. Inhibition of the H + -V-ATPase induces heterotaxia in both species; in chick, H + -V-ATPase activity is upstream of Shh; in fish, it is upstream of Kupffer's vesicle and Spaw expression. Our data implicate H + -V-ATPase activity in patterning the LR axis of vertebrates and reveal mechanisms upstream and downstream of its activity. We propose a pH-and V mem -dependent model of the early physiology of LR patterning. Development 133, 1657Development 133, -1671Development 133, (2006 DEVELOPMENT 1658 necessary to characterize the endogenous behavior of the relevant pumps in embryos and to place their function in the context of known LR patterning mechanisms. Here, we explore the properties of H + -V-ATPase function in several vertebrate embryos. Through endogenous localization of the H + -V-ATPase and gain-and loss-offunction experiments in chick, frog and zebrafish, we identify the H + -V-ATPase as a novel, conserved, obligate component of LR patterning upstream of asymmetric gene expression. KEY WORDS: Left-right asymmetry, H + -V-ATPase, V-ATPase, Xenopus, Chick, Zebrafish, Axial patterning, Cytoplasmic pH, Membrane voltage MATERIALS AND METHODS Animal husbandryXenopus embryos were collected according to standard protocols (Sive et al., 2000) in 0.1ϫ Modified Marc's Ringers (MMR) pH 7.8 + 0.1% Gentamicin. Xenopus embryos were staged according to Nieuwkoop and Faber (Nieuwkoop and Faber, 1967). Chick embryos from Charles River Laboratories, maintained at 38°C, were staged according to Hamburger and Hamilton (Hamburger and Hamilton, 1992). Zebrafish embryos (Westerfield, 1995) were maintained at 28.5°C in water containing 1 drop per gallon Methyl Blue. Assaying organ situsXenopus e...

BackgroundHow particular changes in functional morphology can repeatedly promote ecological diversification is an active area of evolutionary investigation. The African rift-lake cichlids offer a calibrated time series of the most dramatic adaptive radiations of vertebrate trophic morphology yet described, and the replicate nature of these events provides a unique opportunity to test whether common changes in functional morphology have repeatedly facilitated their ecological success.Methodology/Principal FindingsSpecimens from 87 genera of cichlid fishes endemic to Lakes Tanganyka, Malawi and Victoria were dissected in order to examine the functional morphology of cichlid feeding. We quantified shape using geometric morphometrics and compared patterns of morphological diversity using a series of analytical tests. The primary axes of divergence were conserved among all three radiations, and the most prevalent changes involved the size of the preorbital region of the skull. Even the fishes from the youngest of these lakes (Victoria), which exhibit the lowest amount of skull shape disparity, have undergone extensive preorbital evolution relative to other craniofacial traits. Such changes have large effects on feeding biomechanics, and can promote expansion into a wide array of niches along a bentho-pelagic ecomorphological axis.Conclusions/SignificanceHere we show that specific changes in trophic anatomy have evolved repeatedly in the African rift lakes, and our results suggest that simple morphological alterations that have large ecological consequences are likely to constitute critical components of adaptive radiations in functional morphology. Such shifts may precede more complex shape changes as lineages diversify into unoccupied niches. The data presented here, combined with observations of other fish lineages, suggest that the preorbital region represents an evolutionary module that can respond quickly to natural selection when fishes colonize new lakes. Characterizing the changes in cichlid trophic morphology that have contributed to their extraordinary adaptive radiations has broad evolutionary implications, and such studies are necessary for directing future investigations into the proximate mechanisms that have shaped these spectacular phenomena.