Mitochondrial genome conformation among CW-group chlorophycean algae

Most green algal taxa have circular-mapping mitochondrial genomes, whereas some have linear genome- or subgenomic-sized mitochondrial DNAs (mtDNA). It is not clear, however, if the circular-mapping genomes represent genome-sized circular molecules, if such circular molecules and the linear forms are the predominant in vivo mtDNA structures, or if the linear forms arose only once or multiple times among extant green algal lineages. We therefore examined the DNA components detected with homologous mtDNA probes after pulsed-field gel electrophoresis of total cellular DNA from the chlorophycean basal bodies displaced clockwise(CW)-group taxa Chlamydomonas reinhardtii and Chlamydomonas moewusii. For C. reinhardtii , the 15.8-kb linear mtDNA was the only DNA component detected, and there was no evidence of circular or large linear precursors of this DNA. In the case of C. moewusii , which is known to have a circular-mapping 22.9-kb mitochondrial genome, three DNA components were detected; these appeared to be circular (relaxed and supercoiled) and genome-sized linear DNA molecules, the latter of which likely resulted from random double-strand breaks in the circular forms during DNA isolation. In further studies, DNA from additional CW-group taxa was examined using conventional gel electrophoresis and DNA-filter blot analysis with C. reinhardtii and C. moewusii mtDNA probes. We conclude that all taxa from the Volvox clade (sensu Nakayama et al. 1996) of the CW-group have genome- or subgenomic-sized linear mtDNAs as their predominant mtDNA form and that these arose from a genome-sized circular form in an ancestor that existed near the base of this clade.