Evolution of fragmented mitochondrial ribosomal RNA genes in Chlamydomonas

Abstract. The fragmented mitochondrial ribosomal RNAs (rRNAs) of the green algae Chlamydomonas eu-gametos and Chlamydomonas reinhardtii are discontinu-ously encoded in subgenic modules that are scrambled in order and interspersed with protein coding and tRNA genes. The mitochondrial rRNA genes of these two algae differ, however, in both the distribution and organization of rRNA coding information within their respective ge-nomes. The objectives of this study were (1) to examine the phylogenetic relationships between the mitochondrial rRNA gene sequences of C. eugametos and C. rein-hardtii and those of the conventional mitochondrial rRNA genes of the green alga, Prototheca wickerhamii, and land plants and (2) to attempt to deduce the evolu-tionary pathways that gave rise to the unusual mitochon-drial rRNA gene structures in the genus Chlamydomo-nas. Although phylogenetic analysis revealed an affilia-tion between the mitochondrial rRNA gene sequences of the two Chlamydomonas taxa to the exclusion of all other mitochondrial rRNA gene sequences tested, no specific affiliation was noted between the Chlamydomo-nas sequences and P. wickerhamii or land plants. Calcu-lations of the minimal number of transpositions required to convert hypothetical ancestral rRNA gene organiza-tions to the arrangements observed for C. eugametos and C. reinhardtii mitochondrial rRNA genes, as well as a limited survey of the size of mitochondrial rRNAs in other members of the genus, lead us to propose that the last common ancestor of Chlamydomonas algae con-tained fragmented mitochondrial rRNA genes that were nearly co-linear with conventional rRNA genes