Micronuclear and macronuclear forms of beta-tubulin genes in the ciliate Chilodonella uncinata reveal insights into genome processing and protein evolution

Abstract

Chilodonella uncinata, like all ciliates, contains two distinct nuclei in every cell: a germline micronucleus and a somatic macronucleus. During development of the macronucleus from a zygotic nucleus, the genome is processed in several ways, including elimination of internal sequences. In this study, we analyze micronuclear and macronuclear copies of beta-tubulin in C. uncinata and find at least four divergent paralogs of beta-tubulin in the macronucleus. We characterize the micronuclear version of one paralog and compare its internally eliminated sequences (IESs) with previously described IESs in this species. These comparisons reveal the presence of a conserved sequence motif within IESs. In addition, we compare the sequences of beta-tubulin from C. uncinata with other ciliates and to other alveolates in order to test the hypothesis that the mode of molecular evolution in ciliates obscures phylogenetic signal in protein-coding genes. We find that heterogeneous rates of substitution in beta-tubulin across ciliates result in unstable genealogies that are inconsistent with phylogenies based on small subunit rDNA genes and on ultrastructure. We discuss the implications of our findings for genome processing and protein evolution in ciliates.