Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada.
PLoS One. 2010 Jan 29;5(1):e8954. doi: 10.1371/journal.pone.0008954.
Helicosporidia are achlorophyllous, non-photosynthetic protists that are obligate parasites of invertebrates. Highly specialized, these pathogens feature an unusual cyst stage that dehisces inside the infected organism and releases a filamentous cell displaying surface projections, which will penetrate the host gut wall and eventually reproduce in the hemolymph. Long classified as incertae sedis or as relatives of other parasites such as Apicomplexa or Microsporidia, the Helicosporidia were surprisingly identified through molecular phylogeny as belonging to the Chlorophyta, a phylum of green algae. Most phylogenetic analyses involving Helicosporidia have placed them within the subgroup Trebouxiophyceae and further suggested a close affiliation between the Helicosporidia and the genus Prototheca. Prototheca species are also achlorophyllous and pathogenic, but they infect vertebrate hosts, inducing protothecosis in humans. The complete plastid genome of an Helicosporidium species was recently described and is a model of compaction and reduction. Here we describe the complete mitochondrial genome sequence of the same strain, Helicosporidium sp. ATCC 50920 isolated from the black fly Simulium jonesi.
METHODOLOGY/PRINCIPAL FINDINGS: The circular mapping 49343 bp mitochondrial genome of Helicosporidium closely resembles that of the vertebrate parasite Prototheca wickerhamii. The two genomes share an almost identical gene complement and display a level of synteny that is higher than any other sequenced chlorophyte mitochondrial DNAs. Interestingly, the Helicosporidium mtDNA feature a trans-spliced group I intron, and a second group I intron that contains two open reading frames that appear to be degenerate maturase/endonuclease genes, both rare characteristics for this type of intron.
CONCLUSIONS/SIGNIFICANCE: The architecture, genome content, and phylogeny of the Helicosporidium mitochondrial genome are all congruent with its close relationship to Prototheca within the Trebouxiophyceae. The Helicosporidium mitochondrial genome does, however, contain a number of novel features, particularly relating to its introns.
Helicosporidia 是一类无叶绿素、专性寄生的原生动物，是无脊椎动物的专性寄生虫。这些病原体高度特化，具有不寻常的囊泡阶段，在感染的生物体内部裂开，并释放出丝状细胞，显示出表面突起，这些突起将穿透宿主肠道壁，最终在血淋巴中繁殖。长期以来，Helicosporidia 被归类为不定类群或其他寄生虫（如 Apicomplexa 或 Microsporidia）的近亲，但通过分子系统发育分析，它们令人惊讶地被确定为绿藻门的一员。涉及 Helicosporidia 的大多数系统发育分析都将它们置于 Trebouxiophyceae 亚组内，并进一步表明 Helicosporidia 与 Prototheca 属之间存在密切的亲缘关系。Prototheca 属的物种也是无叶绿素的病原体，但它们感染脊椎动物宿主，在人类中引起原质体病。最近描述了一种 Helicosporidium 物种的完整质体基因组，是一个紧凑和简化的模型。在这里，我们描述了从黑蝇 Simulium jonesi 中分离出的同一种菌株 Helicosporidium sp. ATCC 50920 的完整线粒体基因组序列。
方法/主要发现：Helicosporidium 的圆形图谱 49343 bp 线粒体基因组与脊椎动物寄生虫 Prototheca wickerhamii 非常相似。这两个基因组具有几乎相同的基因组成，并显示出比任何其他已测序的绿藻线粒体 DNA 更高的同线性水平。有趣的是，Helicosporidium mtDNA 具有一个转剪接的 I 组内含子，以及一个包含两个开放阅读框的第二个 I 组内含子，这两个开放阅读框似乎是退化的成熟酶/内切酶基因，这两种特征对于这种类型的内含子来说都是罕见的。
结论/意义：Helicosporidium 线粒体基因组的结构、基因组内容和系统发育都与其在 Trebouxiophyceae 中的 Prototheca 密切相关。然而，Helicosporidium 线粒体基因组确实包含了一些新的特征，特别是与它的内含子有关。
