GENOMIC INSIGHTS INTO EVOLUTIONARY RELATIONSHIPS AMONG HETEROKONT LINEAGES EMPHASIZING THE PHAEOPHYCEAE(1)

Abstract

Heterokonts comprise a large and diverse group of organisms unified by the heterokont biflagellate condition. Monophyly of many of these lineages is well established, but evolutionary relationships among the various lineages remain elusive. Among these lineages, the brown algae (Phaeophyceae) are a monophyletic, taxonomically diverse, and ecologically critical group common to marine environments. Despite their biological and scientific importance, consensus regarding brown algal phylogeny and taxonomic relationships is missing. Our long-term research goal is to produce a well-resolved taxon-rich phylogeny of the class to assess evolutionary patterns and taxonomic relationships among brown algal lineages and their relationship to other closely related heterokont groups. To accomplish this goal and augment existing loci for phaeophycean-wide systematic studies, we generated expressed sequence tags (ESTs) from several major brown algal lineages and from the heterokont lineage representing the closest sister group to brown algae. To date, we have successfully constructed cDNA libraries for two lineages (Choristocarpus tenellus Zanardini and Schizocladia ischiensis E. C. Henry, Okuda et H. Kawai) and in the library test phase obtained up to 1,600 ESTs per organism. Annotation results showed a gene discovery rate of 45%-50% for each library revealing 500-700 unique genes from each organism. We have identified several potential genes for phylogenetic inference and used these loci for preliminary molecular clock analyses. Our molecular clock analysis suggests that the basal divergence in brown algae occurred around the time of the pennate-centric diatom divergence. Here we report this analysis and other uses of ESTs in brown algal phylogenomics and the utility of these data for resolving the phylogeny of this group.

Keywords: Choristocarpus; Schizocladia; expressed sequence tags (ESTs); molecular clock; stramenopiles; unigenes.