GURALNICK, R.P.; JEFFREY, J; University of Colorado; Leiden University: Re-evaluating phylogenetic hypotheses generated from cell-lineage data using event-pairing analysis

In all animals in the super-phylum Eutrochozoa (containing phyla such as Annelida, Mollusca), the cell formation events that occur from formation of zygote to gastrulation are relatively stereotyped. The major visible differences in early development are timing of formation of cells. These timing changes mean that different taxa with the same number of cells may have different cell lineages in different juxtapositions. We have built a multi-taxon database of cell formation timing information. Recent meta-analysis of cell lineage timing data for Eutrochozoans has shown that cell-timing information potentially contains phyletic information. However, those analyses used simplistic measures of cell timing differences to reconstruct relationships, and better methods for such analyses are now available. Here, we re-examine the cell lineage timing dataset using event-pairing analysis, an approach more suitable than previous analyses. For each of the 64 species, we used the event-pair method to encode the relative time of appearance of the 53 cells producing 1378 event-pair characters. A phylogenetic analysis in PAUP* recovered 11 most parsimonious trees (MPTs). These trees were similar, differing largely in the relationships of only five species. The trees show some familiar grouping and some that are unlikely. In some cases, there are obvious explanations for discrepancies. For example, some leeches have major accelerations in some cell lineages associated with direct development while other lineages fail to form at all. Given lack of information in other cell-lineages, these leeches nestle within the Mollusca where direct development has also led to major speed-ups of similar lineages. With proper analysis, we argue that the cell-timing information is useful in understanding Eutrochozoa evolution and development.