Comparative survey of early embryogenesis of Secernentea (Nematoda), with phylogenetic implications

Insight into the evolution of class Secernentea (Nematoda) for the purpose of providing a phylogenetic context for the model Caenorhabditis elegans is being gained from the use of molecular character sets. Such phylogenies pr ovide a framework for mapping the evolution of diversity in some early-deve lopment characters for 70 species and 19 families of Secernentea. These cha racters include (i) whether AB and P1 blastomeres initially develop at the same (synchronous) or different (asynchronous) rates, (ii) whether AB and p i are initially aligned along the linear axis of the embryo (tandem pattern ) or obliquely (rhomboidal pattern), and (iii) whether the founder germ cel l, P4, is established early, i.e., by the sixth cleavage, or later. Evoluti onary polarity of characters was evaluated through outgroup comparisons. Fr om our data the following inferences are made. The derived character, late establishment of P4, evolved primarily in the ancestor of the monophyletic groups Diplogastrina, Rhabditina, and Panagrolaimidae. Asynchronous develop ment is convergent, defining one clade of Tylenchina as well as Cephalobina , and also arising independently in Aphelenchina. The rhomboidal embryo is ancestral to the tandem-pattern embryo that defines a second clade of Tylen china. Early-embryo characters are congruent with the polyphyly of Cephalob ina and Aphelenchina, as has been demonstrated by molecular phylogenies. Ma ny aspects of early embryogenesis, rather than being highly conserved, evol ve at a rate appropriate to defining taxa within Secernentea.