WHAT DOES COMBINING MOLECULAR DATA SETS TELL US ABOUT DIATOM ORIGINS AND PHYLOGENY?

Abstract

The past several years have seen an abundance of molecular sequence data gathered on heterokont algae and other stramenopiles with the goal of resolving phylogenetic relationships among major groups. The original focus was on SSU rDNA sequence, but lately a significant number of sequences of plastid and mitochondrial encoded genes (specifically rbcL and coxI) have been made available. Of particular interest to us has been the origin of diatoms and the relationship of diatoms to other stramenopiles. According to most claims based on morphological data, typically viewed from a non‐rigorous evolutionary taxonomy standpoint (i.e. not with explicit cladistic or phylogenetic systematic methodology), diatoms are closely related to silica‐scaled golden brown algae (chrysophytes or synurophytes). SSU rDNA sequence data, however, often place diatoms at the base of the heterokont alga tree, and chryso/synurophytes at the tip with eustigmatophytes, for example, as the chryso/synurophyte sister group. More recent analysis of rbcL sequences, however, supports the traditional classification. It is not automatically to be assumed that there is incongruence between the sequences, however. Taxon sampling is different in the different analyses, methods of analysis are often different, assumptions used to “filter” data are different, etc. Moreover, the relative strength of signal appears to be different in the data sets. We will present an analysis of combined SSU, rbcL and coxI data, an analysis of taxon‐sampling issues, and review underlying assumptions and methodologies in an attempt to a) better understand the results of prior studies and b) reconcile the different hypotheses.