Fragmented and scrambled mitochondrial ribosomal RNA coding regions among green algae: a model for their origin and evolution

Mitochondrial ribosomal RNA coding regions in the only three green algaltaxa investigated to date are fundamentally different in that they arecontinuous in Prototheca wickerhamii, but highly fragmented and scrambledin Chlamydomonas reinhardtii and Chlamydomonas eugametos. To gain moreinsight into the mode of evolution of fragmented and scrambledmitochondrial ribosomal RNA (rRNA) genes within the green algal group, thiswork (1) provides additional information on fragmentation patterns ofmitochondrial small- and large-subunit (SSU and LSU) rRNAs that stronglysupports the concept of a gradual increase in the extent of discontinuityof mitochondrial rRNAs among chlorophycean green algae and (2) reports thefirst example of fragmented and scrambled mitochondrial LSU rRNA codingregions in a green algal taxon outside the Chlamydomonas group. The presentstudy (1) suggests that the scrambling of the mitochondrial rRNA codingregions may have occurred early in the evolution of fragmented andscrambled mitochondrial rRNA genes within the chlorophycean green algalgroup, most likely in parallel with the fragmentation events, (2) proposesrecombination as a possible mechanism involved in the evolution of thesemitochondrial rRNA genes, and (3) presents a hypothetical pathway forconverting continuous mitochondrial rRNA genes into the highly fragmentedand scrambled rRNA coding regions of Chlamydomonas through a series ofrecombinatorial events between short repeated sequences.