STRUCTURE OF THE LARGE SUBUNIT RDNA FROM A DIATOM, AND COMPARISON BETWEEN SMALL AND LARGE SUBUNIT RIBOSOMAL-RNA FOR STUDYING STRAMENOPILE EVOLUTION

The aim of this study was to compare the usefulness of complete small and large subunit rRNA, and a combination of both molecules, for recon structing stramenopile evolution. To this end, phylogenies from specie s of which both sequences are known were constructed with the neighbor -joining, maximum parsimony, and maximum likelihood methods. Also the use of structural features of the rRNAs was evaluated. The large subun it rRNA from the diatom Skeletonema pseudocostatum was sequenced in or der to have a more complete taxon sampling, and a group I intron was i dentified. Our results indicated that heterokont algae are monophyleti c, with diatoms diverging first. However, as the analysis was restrict ed to a particular data set containing merely six taxa, the outcome ha s limited value for elucidating stramenopile relationships. On the oth er hand, this approach permits comparison of the performance of both r RNA molecules without interference from other factors, such as a diffe rent species selection for each molecule. For the taxa used, the large subunit rRNA clearly contained more phylogenetic information than the small subunit rRNA. Although this result can definitely not be genera lized and depends on the phylogeny to be studied, in some cases determ ining complete large subunit rRNA sequences certainly seems worthwhile .