Department of Ecology, Evolution and Organismal Biology, Iowa State University, Ames, Iowa 50011, USA.
Integr Comp Biol. 2007 Nov;47(5):734-43. doi: 10.1093/icb/icm045. Epub 2007 Jun 13.
Animal mitochondrial DNA (mtDNA) is usually depicted as a small and very economically organized molecule with almost invariable gene content, stable gene order, a high rate of sequence evolution, and several unorthodox genetic features. Sampling across different animal phyla reveals that such a description applies primarily to mtDNA of bilaterian animals (such as arthropods or chordates). By contrast, mitochondrial genomes of nonbilaterian animals (phyla Cnidaria, Placozoa, and Porifera) display more variation in size and gene content and, in most cases, lack the genetic novelties associated with bilaterian mtDNA. Outside the Metazoa, mtDNA of the choanoflagellate Monosiga brevicollis, the closest unicellular out-group, is a much larger molecule that contains a large proportion of noncoding DNA, 1.5 times more genes, as well as several introns. Thus, changes in animal mtDNA organization appear to correlate with two main transitions in animal evolution: the origin of multicellularity and the origin of the Bilateria. Studies of mtDNA in nonbilaterian animals provide valuable insights into these transitions in the organization of mtDNA and also supply data for phylogenetic analyses of the relationships of early animals. Here I review recent progress in the understanding of nonbilaterian mtDNA and discuss the advantages and limitations of mitochondrial data sets for inferences about the phylogeny and evolution of animals.
动物线粒体 DNA（mtDNA）通常被描绘为一个小而非常经济组织的分子，具有几乎不变的基因含量、稳定的基因顺序、高序列进化率和几种非正统的遗传特征。在不同的动物门中进行采样表明，这种描述主要适用于两侧对称动物（如节肢动物或脊索动物）的 mtDNA。相比之下，非两侧对称动物（门刺胞动物、扁盘动物和多孔动物）的线粒体基因组在大小和基因含量上显示出更多的变化，并且在大多数情况下缺乏与两侧对称 mtDNA 相关的遗传新颖性。在 metazoa 之外，最接近的单细胞外群粘体虫 Monosiga brevicollis 的 mtDNA 是一个更大的分子，包含大量非编码 DNA、1.5 倍以上的基因以及几个内含子。因此，动物 mtDNA 组织的变化似乎与动物进化中的两个主要转变相关：多细胞生物的起源和两侧对称动物的起源。非两侧对称动物 mtDNA 的研究为 mtDNA 组织的这些转变提供了有价值的见解，并为早期动物的系统发育分析提供了数据。在这里，我回顾了非两侧对称动物 mtDNA 理解的最新进展，并讨论了线粒体数据集在推断动物的系统发育和进化方面的优势和局限性。
