Ragan M A, Goggin C L, Cawthorn R J, Cerenius L, Jamieson A V, Plourde S M, Rand T G, Söderhäll K, Gutell R R
Canadian Institute for Advanced Research, National Research Council of Canada, Halifax, NS Canada.
Proc Natl Acad Sci U S A. 1996 Oct 15;93(21):11907-12. doi: 10.1073/pnas.93.21.11907.
Sequences of nuclear-encoded small-subunit rRNA genes have been determined for representatives of the enigmatic genera Dermocystidium, Ichthyophonus, and Psorospermium, protistan parasites of fish and crustaceans. The small-subunit rRNA genes from these parasites and from the "rosette agent" (also a parasite of fish) together form a novel, statistically supported clade. Phylogenetic analyses demonstrate this clade to diverge near the animal-fungal dichotomy, although more precise resolution is problematic. In the most parsimonious and maximally likely phylogenetic frameworks inferred from the most stably aligned sequence regions, the clade constitutes the most basal branch of the metazoa; but within a limited range of model parameters, and in some analyses that incorporate less well-aligned sequence regions, an alternative topology in which it diverges immediately before the animal-fungal dichotomy was recovered. Mitochondrial cristae of Dermocystidium spp. are flat, whereas those of Ichthyophonus hoferi appear tubulovesiculate. These results extend our understanding of the types of organisms from which metazoa and fungi may have evolved.
已测定鱼类和甲壳类动物的原生动物寄生虫——神秘属皮囊肿菌属、鱼醉菌属和胶孢子虫属代表的核编码小亚基rRNA基因序列。来自这些寄生虫以及“玫瑰花结病原体”（也是一种鱼类寄生虫）的小亚基rRNA基因共同形成了一个新的、有统计学支持的进化枝。系统发育分析表明，尽管更精确的分辨率存在问题，但该进化枝在动物-真菌二分法附近分化。在从最稳定比对的序列区域推断出的最简约和最可能的系统发育框架中，该进化枝构成后生动物的最基部分支；但在有限的模型参数范围内，以及在一些纳入比对不太好的序列区域的分析中，发现了另一种拓扑结构，即它在动物-真菌二分法之前立即分化。皮囊肿菌属物种的线粒体嵴是扁平的，而霍氏鱼醉菌的线粒体嵴则呈管状泡状。这些结果扩展了我们对后生动物和真菌可能进化而来的生物类型的理解。
