Genetic and developmental analysis of trophic differentiation in two Lake Malawi cichlid fishes and their hybrids
The cichlid fishes of the East African Great Lakes represent a classic case of adaptive radiation. Lake Malawi alone contains over 500 endemic cichlid species in 49 genera. Like their counterparts among Galapagos finches and Hawaiian honeycreepers, cichlid taxa differ primarily in trophic architecture. While significant effort has been directed towards elucidating selective forces responsible for the sorting of phenotypic variation in populations, little data are available on intrinsic processes that account for the origination of this variation. This study considers aspects of the genetic and developmental control of trophic morphology in two Lake Malawi taxa and their hybrids.
The inheritance of thirteen oral jaw characters in experimentally produced F1 hybrids between Pseudotropheus zebra and Labeotropheus fuelleborni was quantitatively examined. Hybrids are morphologically distinct from both parental taxa; all three groups can be discriminated using discriminant function and canonical variates analysis. Three oral jaw characters are intermediate in, and thus diagnostic of, hybrids. Hybrids are neomorphic for five characters. It appears from these data that hybrids display a novel multivariate phenotype, suggesting that (1) the genetic control of trophic morphology may involve a significant component of dominance and (2) hybrid populations (if produced in situ) may lie along an ecomorphological trajectory distinct from that separating parents.
Shape differences associated with trophic differentiation between P. zebra and L. fuelleborni were quantified using landmark-based morphometrics. L. fuelleborni is characterized by (1) an expansion of the neurocranium, (2) rostrodorsal translation of the hyoid, and (3) ventrad rotation of the oral jaws relative to P. zebra. Neurocranial expansion appears driven by relative enlargement of the preorbital neurocranium; hyoid displacement may reflect a change in the position of the hyoid-suspensorium articulation. Both of these features of shape change induce correlated effects across the form and may represent common patterns underlying trophic differentiation among African cichlid taxa. Modification of the preorbital neurocranium and displacement of the hyoid may thus constitute key morphological features in the morphological evolution of African cichlids.
The results of this study have implications for the nature of morphological evolution
in cichlids. First, interspecific hybridization (if historically important) could result
in the production of novel phenotypes with unique evolutionary potentials. More generally,
the existence of significant dominance in the genetic control of morphology implies that
marked phenotypic shifts could occur with limited genetic change. Further, since changes
in a few key morphological elements can have correlated epigenetic consequences,
integrated morphological evolution could proceed rapidly and discontinuously. These data
emphasize the importance of considering intrinsic determinants of biological design.