title: Trends, stasis, and drift in the evolution of nematode vulva development.
abstract: BACKGROUND
  A surprising amount of developmental variation has been observed for otherwise highly conserved features, a phenomenon known as developmental system drift. Either stochastic processes (e.g., drift and absence of selection-independent constraints) or deterministic processes (e.g., selection or constraints) could be the predominate mechanism for the evolution of such variation. We tested whether evolutionary patterns of change were unbiased or biased, as predicted by the stochastic or deterministic hypotheses, respectively. As a model, we used the nematode vulva, a highly conserved, essential organ, the development of which has been intensively studied in the model systems Caenorhabditis elegans and Pristionchus pacificus.
  
  
  RESULTS
  For 51 rhabditid species, we analyzed more than 40 characteristics of vulva development, including cell fates, fate induction, cell competence, division patterns, morphogenesis, and related aspects of gonad development. We then defined individual characters and plotted their evolution on a phylogeny inferred for 65 species from three nuclear gene sequences. This taxon-dense phylogeny provides for the first time a highly resolved picture of rhabditid evolution and allows the reconstruction of the number and directionality of changes in the vulva development characters. We found an astonishing amount of variation and an even larger number of evolutionary changes, suggesting a high degree of homoplasy (convergences and reversals). Surprisingly, only two characters showed unbiased evolution. Evolution of all other characters was biased.
  
  
  CONCLUSIONS
  We propose that developmental evolution is primarily governed by selection and/or selection-independent constraints, not stochastic processes such as drift in unconstrained phenotypic space.
authors.affiliation: Department of Biology, New York University, Main Building, Room 1009, 100 Washington Square East, New York, New York 10003, USA. kk52@nyu.edu
authors.forename: Karin
authors.initials: K
authors.lastname: Kiontke
authors.forename: Antoine
authors.initials: A
authors.lastname: Barrière
authors.forename: Irina
authors.initials: I
authors.lastname: Kolotuev
authors.forename: Benjamin
authors.initials: B
authors.lastname: Podbilewicz
authors.forename: Ralf
authors.initials: R
authors.lastname: Sommer
authors.forename: David H A
authors.initials: DH
authors.lastname: Fitch
authors.forename: Marie-Anne
authors.initials: MA
authors.lastname: Félix
mesh_terms: D000818:Animals; D005075:Biological Evolution; D017173:Caenorhabditis elegans / Q000033:anatomy & histology / Q000145:classification / Q000196:embryology* / Q000502:physiology; D005260:Female; D040961:Genetic Drift; D010802:Phylogeny; D014844:Vulva / Q000033:anatomy & histology / Q000196:embryology* / Q000502:physiology