title: Developmental basis of phenotypic integration in two Lake Malawi cichlids.
abstract: BACKGROUND
  Cichlid fishes from the Rift Lakes of East Africa have undergone the most spectacular adaptive radiations in vertebrate history. Eco-morphological adaptations in lakes Victoria, Malawi and Tanganyika have resulted in a vast array of skull shapes and sizes, yet primary axes of morphological variation are conserved in all three radiations, prominently including the size of the preorbital region of the skull. This conserved pattern suggests that development may constrain the trajectories of cichlid head morphological evolution.
  
  
  RESULTS
  Here, we (1) present a comparative analysis of adult head morphology in two sand-dweller cichlids from Lake Malawi with preorbital size differences representative of the main axis of variation among the three lakes and (2) analyze the ontogeny of shape and size differences by focusing on known developmental modules throughout the head. We find that (1) developmental differences between the two species correlate with known developmental modules; (2) differences in embryonic cartilage development result in phenotypically integrated changes among all bones derived from a single cartilage, while differences in dermal bone development tend to influence isolated regions within a bone; and lastly (3) species-specific morphologies appear in the embryo as subtle differences, which become progressively amplified throughout ontogeny. We propose that this amplification takes place at skeletal growth zones, the locations and shapes of which are patterned during embryogenesis.
  
  
  CONCLUSIONS
  This study is the most anatomically comprehensive analysis of the developmental differences underlying cichlid skull evolution in the Rift Lakes of East Africa. The scale of our analysis reveals previously unnoticed correlations between developmental modules and patterns of phenotypic integration. We propose that the primary axes of morphological variation among East African cichlid adaptive radiations are constrained by the hierarchical modularity of the teleost head skeleton.
authors.affiliation: Department of Developmental and Cell Biology, 4109 Natural Sciences II, University of California, Irvine, CA 92697-2300 USA.
authors.forename: Pierre
authors.initials: P
authors.lastname: Le Pabic
authors.affiliation: School of Biological Sciences, Washington State University Tri-cities, 2710 Crimson Way, Richland, WA 99354 USA.
authors.forename: W James
authors.initials: WJ
authors.lastname: Cooper
authors.affiliation: Department of Developmental and Cell Biology, 4109 Natural Sciences II, University of California, Irvine, CA 92697-2300 USA.
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Schilling
keywords: Cichlid evolution; Developmental constraint; Developmental module; Phenotypic integration; Skeletal development