Alpha-tubulin from early-diverging eukaryotic lineages and the evolution of the tubulin family.

The tubulin gene family, which includes alpha-,beta-, and gamma-tubulin subfamilies, is composed of highly conserved proteins which are the principle structural and functional components of eukaryotic microtubules. We are interested in (1) establishing when in eukaryotic evolution the duplications leading to paralogous alpha, beta, and gamma subfamilies occurred and (2) the possible utility of tubulin sequences in reconstructing organismal phylogeny. To broaden the taxonomic representation of alpha-tubulins so that it roughly equals that of beta-tubulins, alpha-tubulin genes from three Microsporidia (Encephalitozoon hellem, Nosema locustae, and Spraguea lophii), two Parabasalia (Monocercomonas sp. and Trichomitus batrachorum), and one Heterolobosean (Acrasis rosea) were sequenced. With these new genes, phylogenetic trees of alpha- and beta-tubulins were constructed and compared. Trees were congruent with each other, but incongruent with other molecular phylogenies. The agreement between alpha- and beta-tubulin trees could arise by the co-adaptation of one molecule to variants of the other as a result of their intimate steric association in microtubules. Thus, these trees may not be providing independent support for the phylogenetic results. However, one of these unexpected results, that microsporidia cluster with fungi, is supported by other circumstantial evidence, and may therefore reflect a real relationship despite the basal position usually assigned to microsporidia. Relationships between the three tubulins were also examined by constructing trees of all three types. These trees were found to be of limited value for determining the position of the root within each subfamily because of the great interfamily distances, but they do confirm the classification of all known genes into three monophyletic subfamilies. Divergent genes from Caenorhabditis elegans and Saccharomyces cerevisiae that have been proposed to represent the novel classes delta- and epsilon-tubulin were found to be specifically related to gamma-tubulins from animals and fungi respectively, and therefore are best seen as rapidly evolving orthologues of gamma-tubulin.