CELL LINEAGE DATA AND SPIRALIAN EVOLUTION: A REPLY TO NIELSEN AND MEIER

Nielsen and Meier (2002) are skeptical of our claim of phyletic information being present in the cell division patterns of spiralian taxa. They conclude that cell division patterns must be known in greater detail, and that coding methods for cell lineage data should be better refined before phyletic information can be identified. As noted by Nielsen and Meier (2002), earlier studies had focused almost exclusively on molluscan taxa. In addition, only one or two cell lineages had been typically considered, and none of these previous studies had used an explicit cladistic methodology to investigate potential phyletic information. Our analyses (as well as Nielsen and Meier's reanalysis of our data [2002, Fig. 1]) produce several groupings that were congruent with both molecular and morphological phylogenies. For example, we recovered an Oligochaete-leech clade, the sister relationship of the two Echiura species, and the overall structure of the gastropod mollusc clade. We also noted that the bivalve mollusc and annelid groupings in our tree were not monophyletic. However, the vast majority of the annelid taxa (Echiura + Polycheata + Pogonophora + Oligochaete + Leech) formed a grade outside the Polyplacophora + Gastropoda. Members of these groups were not randomly interdigitated within other taxa across the tree as would be expected if no phylogenetic information were present (as in the case of some polychaete worms and bivalve molluscs). Polyphyletic polychaetes, bivalves, and other worm groups have also been included in analyses of some molecular datasets so we were not that surprised by this result (e.g., Winnepenninckx et al. 1995). We reported the polyphyly in these and other groups (relative to traditional spiralian classifications) as part of our assessment of the degree of phylogenetic information remaining in cell lineage data. Like all characters, the cell lineage characters provided better resolution and more congruent information in some clades than they did in others. Nielsen and Meier (2002) suggest that our "puzzling phylogenetic results" might be partially explained by the coding of the cell division data into character states for cladistic analysis. Determining a metric by which to score cell lineage data is problematic. Absolute time from fertilization can vary greatly depending on temperature, and cleavage cycle is invariant with cell lineage and therefore noninformative. For our analysis we chose embryonic cell number, a standard comparative metric for spiralian embryos that dates from the late 1890s. More recently, cell number has been used for almost 20 years to argue acceleration and retardation of molluscan cell lineages during early development (e.g., Verdonk and Van den Biggelaar 1983; Freeman and Lundelius 1992; Haszprunar 1993; Van den Biggelaar 1993, 1996; Van den Biggelaar and Haszprunar 1996). For our cladistic analysis these same data were transformed into discrete character states by dividing the cell number by three. This was done to transform the cell number into a character state within PAUP's allowable range for ordered characters (31 steps; Swofford 1998). At that time we considered this to be a relatively assumption-free transformation as well as conservative because small variation in cell numbers would be grouped into a single state by rounding error (e.g., 23, 24, 25 cells would all be scored as state 8) rather than each given a unique state (e.g., Nielsen and Meier 2002, Fig. 2b). We do not doubt that homoplasy was present in the data, but this can be discovered only after the analysis, not a priori. The scenario constructed by Nielsen and Meier (2002) in their Figure 2 presents a similar problem for all studies using embryonic cell number as a metric, not just those that use a cladistic methodology. For example, contrary to Nielsen and Meier's (2002) interpretation, Van den Biggelaar and Haszprunar (1996) did not map developmental characters onto existing phylogenies. Instead, these latter two authors actually constructed alternative trees based on two characters presence or absence of the polar lobe and cell number at time of 4d formation, and we would suggest that Nielsen and Meier's (2002) scenario is much more problematic for these limited datasets than it is for our larger dataset in which all available data were included in the analysis. And, as with molecular data, it was anticipated that any robust phylogenetic signal would have to emerge from the background noise of homoplasy. Although we appreciate Nielsen and Meier's (2002) suggestion to instead map this data onto an existing tree, we wonder why they would accept our coding for mapping the characters but not for using them in the analysis itself. We consider Nielsen and Meier's suggested coding prohibitively restrictive in that differences early in cell lineages are propagated to later lineages. Their argument is that any upstream change in other connected lineages prohibits the downstream coding of similarity. For Nielsen and Meier (2002), although two cells may form at 32 cells, they should only be coded as similar (e.g., putatively homologous) if the