Comparative genomics of mitochondria in chlorarachniophyte algae: endosymbiotic gene transfer and organellar genome dynamics.

Chlorarachniophyte algae possess four DNA-containing compartments per cell, the nucleus, mitochondrion, plastid and nucleomorph, the latter being a relic nucleus derived from a secondary endosymbiont. While the evolutionary dynamics of plastid and nucleomorph genomes have been investigated, a comparative investigation of mitochondrial genomes (mtDNAs) has not been carried out. We have sequenced the complete mtDNA of Lotharella oceanica and compared it to that of another chlorarachniophyte, Bigelowiella natans. The linear mtDNA of L. oceanica is 36.7 kbp in size and contains 35 protein genes, three rRNAs and 24 tRNAs. The codons GUG and UUG appear to be capable of acting as initiation codons in the chlorarachniophyte mtDNAs, in addition to AUG. Rpl16, rps4 and atp8 genes are missing in L.oceanica mtDNA, despite being present in B. natans mtDNA. We searched for, and found, mitochondrial rpl16 and rps4 genes with spliceosomal introns in the L. oceanica nuclear genome, indicating that mitochondrion-to-host-nucleus gene transfer occurred after the divergence of these two genera. Despite being of similar size and coding capacity, the level of synteny between L. oceanica and B. natans mtDNA is low, suggesting frequent rearrangements. Overall, our results suggest that chlorarachniophyte mtDNAs are more evolutionarily dynamic than their plastid counterparts.

绿藻门的绿藻纲藻类每个细胞拥有四个含DNA的区室，即细胞核、线粒体、质体和核形，后者是源自次生内共生体的残留细胞核。虽然质体和核形基因组的进化动态已被研究，但尚未对线粒体基因组（mtDNA）进行比较研究。我们对海洋洛氏藻（Lotharella oceanica）的完整线粒体DNA进行了测序，并将其与另一种绿藻纲藻类——纳氏绿藻（Bigelowiella natans）的线粒体DNA进行了比较。海洋洛氏藻的线性线粒体DNA大小为36.7千碱基对，包含35个蛋白质基因、3个核糖体RNA和24个转运RNA。除了AUG外，GUG和UUG密码子似乎也能够作为绿藻纲藻类线粒体DNA的起始密码子。海洋洛氏藻线粒体DNA中缺少Rpl16、rps4和atp8基因，尽管这些基因存在于纳氏绿藻线粒体DNA中。我们在海洋洛氏藻的核基因组中寻找并发现了带有剪接体内含子的线粒体Rpl16和rps4基因，这表明在这两个属分化之后发生了线粒体到宿主细胞核的基因转移。尽管海洋洛氏藻和纳氏绿藻的线粒体DNA大小和编码能力相似，但它们之间的共线性水平较低，这表明频繁的重排。总体而言，我们的结果表明绿藻纲藻类的线粒体DNA在进化上比其质体DNA更具动态性。