Mitochondrial and Plastid Genomes of the Colonial Green Alga Gonium pectorale Give Insights into the Origins of Organelle DNA Architecture within the Volvocales

Volvocalean green algae have among the most diverse mitochondrial and plastid DNAs (mtDNAs and ptDNAs) from the eukaryotic domain. However, nearly all of the organelle genome data from this group are restricted to unicellular species, like Chlamydomonas reinhardtii, and presently only one multicellular species, the ∼4,000-celled Volvox carteri, has had its organelle DNAs sequenced. The V. carteri organelle genomes are repeat rich, and the ptDNA is the largest plastome ever sequenced. Here, we present the complete mtDNA and ptDNA of the colonial volvocalean Gonium pectorale, which is comprised of ∼16 cells and occupies a phylogenetic position closer to that of V. carteri than C. reinhardtii within the volvocine line. The mtDNA and ptDNA of G. pectorale are circular-mapping AT-rich molecules with respective lengths and coding densities of 16 and 222.6 kilobases and 73 and 44%. They share some features with the organelle DNAs of V. carteri, including palindromic repeats within the plastid compartment, but show more similarities with those of C. reinhardtii, such as a compact mtDNA architecture and relatively low organelle DNA intron contents. Overall, the G. pectorale organelle genomes raise several interesting questions about the origin of linear mitochondrial chromosomes within the Volvocales and the relationship between multicellularity and organelle genome expansion.团藻目绿藻（Volvocalean green algae）拥有真核生物域中最多样化的线粒体DNA（mtDNA）与质体DNA（ptDNA）之一。然而，该类群的细胞器基因组数据几乎均局限于单细胞物种，如莱茵衣藻（Chlamydomonas reinhardtii）；目前仅有一种多细胞物种——拥有约4000个细胞的卡特里团藻（Volvox carteri）——的细胞器DNA完成了测序。卡特里团藻的细胞器基因组富含重复序列，其质体DNA（ptDNA）是迄今已测序的最大质体基因组（plastome）。 本研究报道了群体型团藻目物种盘藻（Gonium pectorale）的完整mtDNA与ptDNA序列。该物种仅由约16个细胞构成，在团藻演化支（volvocine line）中，其系统发育位置相较于莱茵衣藻更接近卡特里团藻。盘藻的mtDNA与ptDNA均为呈环状作图特征的AT富集型分子，长度分别为16 kb与222.6 kb，编码密度分别为73%与44%。 该物种的细胞器DNA与卡特里团藻存在部分共有特征，例如质体区室中的回文重复序列（palindromic repeats）；但与莱茵衣藻的细胞器DNA更为相似，例如紧凑的线粒体基因组结构以及相对较低的细胞器DNA内含子含量。 总体而言，盘藻的细胞器基因组提出了若干值得探讨的科学问题，包括团藻目（Volvocales）内线粒体染色体线性化的起源，以及多细胞化与细胞器基因组扩张之间的关联。