GRENIER, J.K.: Minor Phyla Fill the Gaps: Priapulida and Onychophora

The advent of comparative molecular databases has generated new hypotheses about animal phylogeny that are reshaping our understanding of animal evolution. For example, the bilaterian phyla are now often grouped into three major clades, the deuterostomes, the lophotrochozoans, and the ecdysozoans. The Ecdysozoa include several phyla that were once classified as pseudocoelomates (nematodes, priapulids) in addition to arthropods and onychophorans. The proposed Ecdysozoan clade is supported by molecular evidence including ribosomal RNA sequence and the presence of characteristic Hox genes. One primary value of a redesigned (and hopefully more accurate)animal phylogeny, including the placement of phylogenetically cryptic minor phyla within the animal tree, is the implication for evolutionary comparisons and reconstructions. The so-called minor phyla are critical for establishing ancestral conditions, both molecular and morphological, at interesting nodes in animal phylogeny. Further, these phyla contribute to the analysis of evolutionary changes in gene function during animal evolution. For example, priapulids and onychophorans fill in the gaps in evolutionary comparisons between the model organisms Drososphila (Arthropoda), C. elegans (Nematoda), and the mouse (Chordata). These and other minor phyla help us to interpret the ongoing experiment that is animal evolution.