title: Constraint and diversification of developmental trajectories in cichlid facial morphologies.
abstract: BACKGROUND
  A major goal of evolutionary biology is to understand the origins of phenotypic diversity. Changes in development, for instance heterochrony, can be a potent source of phenotypic variation. On the other hand, development can also constrain the spectrum of phenotypes that can be produced. In order to understand these dual roles of development in evolution, we examined the developmental trajectory of a trait central to the extensive adaptive radiation of East African cichlid fishes: craniofacial adaptations that allow optimal exploitation of ecological niches. Specifically, we use geometric morphometric analysis to compare morphological ontogenies among six species of Lake Malawi cichlids (n > 500 individuals) that span a major ecomorphological axis. We further evaluate how modulation of Wnt signaling impacts the long-term developmental trajectory of facial development.
  
  
  RESULTS
  We find that, despite drastic differences in adult craniofacial morphologies, there are general similarities in the path of craniofacial ontogeny among species, suggesting that natural selection is working within a conserved developmental program. However, we also detect species-specific differences in the timing, direction, and/or duration of particular developmental trajectories, including evidence of heterochrony. Previous work in cichlids and other systems suggests that species-specific differences in adult morphology are due to changes in molecular signaling pathways that regulate early craniofacial development. In support of this, we demonstrate that modulation of Wnt signaling at early stages can shift a developmental trajectory into morphospace normally occupied by another species. However, without sustained modulation, craniofacial shape can recover by juvenile stages. This underscores the idea that craniofacial development is robust and that adult head shapes are the product of many molecular changes acting over extended periods of development.
  
  
  CONCLUSIONS
  Our results are consistent with the hypothesis that development acts to both constrain and promote morphological diversity. They also illustrate the modular nature of the craniofacial skeleton and hence the ability of selection to act upon distinct anatomical features in an independent manner. We propose that trophic diversity among cichlids has been achieved via shifts in both specific (e.g., stage-specific changes in gene expression) and global (e.g., heterochrony) ontogenetic processes acting within a conserved developmental program.
authors.affiliation: Department of Biology, 221 Morrill Science Center South, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 USA.
authors.forename: Kara E
authors.identifier: 0000-0001-7415-4262
authors.initials: KE
authors.lastname: Powder
authors.affiliation: Department of Biology, 221 Morrill Science Center South, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 USA.
authors.forename: Kayla
authors.initials: K
authors.lastname: Milch
authors.affiliation: Department of Biology, 221 Morrill Science Center South, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 USA.
authors.forename: Garrett
authors.initials: G
authors.lastname: Asselin
authors.affiliation: Department of Biology, 221 Morrill Science Center South, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 USA.
authors.forename: R Craig
authors.initials: RC
authors.lastname: Albertson
keywords: Cichlid; Craniofacial; Developmental trajectory; Geometric morphometrics; Ontogeny; Phenotypic variation