Microheliella maris possesses the most gene-rich mitochondiral genome in Diaphoretikes

The mitochondrial genomes are very diverse, but their evolutionary history
is unclear due to the lack of efforts to sequence that of protists
(unicellular eukaryotes), which cover a major part of the eukaryotic tree.
Cryptista comprises cryptophytes, goniomonads, kathablepharids, and
Palpitomonas bilix, and their mitochondrial genomes (mt-genomes) are
characterized by various gene contents, particularly the presence/absence
of an ancestral (bacterial) system for the cytochrome c maturation system.
To shed light on mt-genome evolution in Cryptista, we report the complete
mt-genome of Microheliella maris, which was recently revealed to branch at
the root of Cryptista. The M. maris mt-genome was reconstructed as a
circular mapping chromosome of 61.2 kbp with a pair of inverted repeats
(12.9 kbp) and appeared to be the most gene-rich among the mt-genomes of
the members of Diaphoretickes (a mega-scale eukaryotic assembly including
Archaeplastida, Cryptista, Haptista, and SAR) studied so far, carrying 53
protein-coding genes. With this newly sequenced mt-genome, we inferred and
discussed the evolution of the mtgenome in Cryptista and Diaphoretickes.

线粒体基因组多样性极高，但由于对覆盖真核生物系统发育树主要分支的原生生物（单细胞真核生物）的线粒体基因组测序工作不足，其演化历史仍不明朗。隐藻总门（Cryptista）涵盖隐藻、角单胞虫类、凯发虫类及Palpitomonas bilix，其线粒体基因组（mitochondrial genomes, mt-genomes）的基因组成存在显著差异，尤其体现在细胞色素c成熟系统的祖先（细菌型）通路的有无特征上。为阐明隐藻总门的线粒体基因组演化历程，本研究报道了Microheliella maris的完整线粒体基因组，该物种近期被证实位于隐藻总门的系统发育根部。该线粒体基因组被重构为一条长度61.2 kbp的环状染色体，包含一对12.9 kbp的反向重复序列；其基因含量在目前已测序的泛真核类群（Diaphoretickes，涵盖原始色素体生物（Archaeplastida）、隐藻总门、定鞭藻门（Haptista）及SAR超类群的大型真核生物集合）成员的线粒体基因组中最高，共编码53个蛋白质编码基因。借助此次新测序的线粒体基因组，本研究对隐藻总门与泛真核类群的线粒体基因组演化进行了推演与讨论。