An unexpectedly large and loosely packed mitochondrial genome in the charophycean green alga Chlorokybus atmophyticus

The Chlorokybus genome differs radically from its 42,424-bp Mesostigma counterpart in size, gene order, intron content and density of repeated elements. At 201,763-bp, it is the largest mtDNA yet reported for a green alga. The 70 conserved genes represent 41.4% of the genome sequence and include nad10 and trnL(gag), two genes reported for the first time in a streptophyte mtDNA. At the gene order level, the Chlorokybus genome shares with its Chara, Chaetosphaeridium and bryophyte homologues eight to ten gene clusters including about 20 genes. Notably, some of these clusters exhibit gene linkages not previously found outside the Streptophyta, suggesting that they originated early during streptophyte evolution. In addition to six group I and 14 group II introns, short repeated sequences accounting for 7.5% of the genome were identified. Mitochondrial trees were unable to resolve the correct position of Mesostigma, due to analytical problems arising from accelerated sequence evolution in this lineage.The Chlorokybus and Mesostigma mtDNAs exemplify the marked fluidity of the mitochondrial genome in charophycean green algae. The notion that the mitochondrial genome was constrained to remain compact during charophycean evolution is no longer tenable. Our data raise the possibility that the emergence of land plants was not associated with a substantial gain of intergenic sequences by the mitochondrial genome.Green algae belonging to the class Charophyceae gave rise to all land plant species [1,2]. In contrast to the large diversity of land plants, only a few thousand charophycean species are living today. Charophycean green algae and land plants form the lineage Streptophyta [3], whereas all of the other extant green algae belong to the sister lineage Chlorophyta [2]. Six monophyletic groups are currently recognized in the Charophyceae: the Mesostigmatales [4] represented by the scaly biflagellate Mesostigma viride that has long been regarded as a member of the Prasinophyce