Chloroplast genomes and phylogenetic analysis of two species of <i>Oedocladium</i> (Oedogoniales, Chlorophyta)

The Oedogoniales comprises the genera <i>Oedogonium, Oedocladium</i> and <i>Bulbochaete</i> which include more than 600 described species. However, only two chloroplast genome sequences have been reported to date, and little is known about the evolutionary relationships between these genera or their infrageneric classification. This group is economically important in astaxanthin production and the energy sector. We examined four <i>Oedocladium</i> strains from different locations in China. One was identified as <i>O. prescottii</i>, and the morphology and molecular data for the other three strains suggested that they were <i>O. carolinianum</i>. Our chloroplast (cp) genome sequences of <i>O. prescottii</i> (FACHB-2452) and <i>O. carolinianum</i> (FACHB-2453) were compared with previously reported Oedogoniales cp genomes. The cp genome of <i>O. carolinianum</i> FACHB-2453 was of similar size, structure and composition to that of <i>O. carolinianum</i> UTEX LB 1686, apart from two missing group I introns in the genes <i>psb</i>A and <i>psb</i>D. <i>O. prescottii</i> showed more differences with regard to genome size, structure, number of genes and introns, and repeats than the other Oedogoniales cp genomes. The inverted repeats (IRs) region of the <i>O. prescottii</i> cp genome had a 10-kb contraction. Synteny analysis of the four cp genomes showed a high degree of syntenic conservation in general; however, some structural variation at genus level was observed in <i>Oedocladium</i>. In phylogenetic analyses of <i>Oedocladium</i> using sequences of 18S rDNA, ITS and 59 cp protein-coding genes, species were placed on two separate branches within the <i>Oedogonium</i> clade, indicating that <i>Oedocladium</i> is a paraphyletic group.

鞘藻目（Oedogoniales）涵盖鞘藻属（Oedogonium）、枝鞘藻属（Oedocladium）与毛鞘藻属（Bulbochaete）三个类群，目前已正式描述的物种超过600个。但截至目前，仅公开了2条该目的叶绿体基因组（chloroplast genome）序列，学界对该目各属间的演化关系及其属下分类系统仍缺乏深入认知。该类群在虾青素（astaxanthin）生产与能源产业中具有重要经济价值。本研究采集了来自中国不同地域的4株枝鞘藻属（Oedocladium）菌株，其中1株经鉴定为普氏枝鞘藻（O. prescottii），其余3株的形态学与分子数据显示其隶属于卡罗琳枝鞘藻（O. carolinianum）。我们对普氏枝鞘藻（FACHB-2452）与卡罗琳枝鞘藻（FACHB-2453）的叶绿体（cp）基因组进行测序，并与已报道的鞘藻目叶绿体基因组开展比对分析。结果显示，卡罗琳枝鞘藻FACHB-2453的叶绿体基因组在大小、结构与组成上与已报道的UTEX LB 1686株卡罗琳枝鞘藻高度相似，仅在psbA与psbD基因中缺失了2个I类内含子。普氏枝鞘藻的叶绿体基因组在基因组大小、结构、基因与内含子数量以及重复序列方面，与其他鞘藻目物种的叶绿体基因组存在更为显著的差异。其叶绿体基因组的反向重复（IR）区域发生了10 kb的收缩。对4条叶绿体基因组的共线性分析显示，整体上保守性较高，但在枝鞘藻属内观察到了属水平的结构变异。基于18S rDNA、ITS序列以及59个叶绿体蛋白编码基因的枝鞘藻属系统发育分析显示，该属物种在鞘藻属演化支中分为两个独立分支，表明枝鞘藻属为并系类群。