Sunnucks P, Hales D F
School of Biological Sciences, Macquarie University, Sydney.
Mol Biol Evol. 1996 Mar;13(3):510-24. doi: 10.1093/oxfordjournals.molbev.a025612.
Polymerase chain reaction (PCR) products corresponding to 803 bp of the cytochrome oxidase subunits I and II region of mitochondrial DNA (mtDNA COI-II) were deduced to consist of multiple haplotypes in three Sitobion species. We investigated the molecular basis of these observations. PCR products were cloned, and six clones from one individual per species were sequenced. In each individual, one sequence was found commonly, but also two or three divergent sequences were seen. The divergent sequences were shown to be nonmitochondrial by sequencing from purified mtDNA and Southern blotting experiments. All seven nonmitochondrial clones sequenced to completion were unique. Nonmitochondrial sequences have a high proportion of unique sites, and very few characters are shared between nonmitochondrial clones to the exclusion of mtDNA. From these data, we infer that fragments of mtDNA have been transposed separately (probably into aphid chromosomes), at a frequency only known to be equalled in humans. The transposition phenomenon appears to occur infrequently or not at all in closely related genera and other aphids investigated. Patterns of nucleotide substitution in mtDNA inferred over a parsimony tree are very different from those in transposed sequences. Compared with mtDNA, nonmitochondrial sequences have less codon position bias, more even exchanges between A, G, C and T, and a higher proportion of nonsynonymous replacements. Although these data are consistent with the transposed sequences being under less constraint than mtDNA, changes in the nonmitochondrial sequences are not random: there remains significant position bias, and probable excesses of synonymous replacements and of conservative inferred amino acid replacements. We conclude that a proportion of the inferred change in the nonmitochondrial sequences occurred before transposition. We believe that Sitobion aphids (and other species exhibiting mtDNA transposition) may be important for studying the molecular evolution of mtDNA and pseudogenes. However, our data highlight the need to establish the true evolutionary relationships between sequences in comparative investigations.
线粒体DNA（mtDNA COI-II）细胞色素氧化酶亚基I和II区域803 bp的聚合酶链反应（PCR）产物被推断在三种麦长管蚜属物种中由多个单倍型组成。我们研究了这些观察结果的分子基础。PCR产物被克隆，每个物种从一个个体中选取六个克隆进行测序。在每个个体中，发现了一个常见序列，但也有两到三个不同的序列。通过从纯化的mtDNA测序和Southern印迹实验表明，这些不同的序列是非线粒体的。所有七个测序完成的非线粒体克隆都是独特的。非线粒体序列具有高比例的独特位点，并且在排除mtDNA的情况下，非线粒体克隆之间很少有共同特征。从这些数据中，我们推断mtDNA片段已分别发生转座（可能转入蚜虫染色体），其频率仅在人类中已知与之相当。在密切相关的属和其他研究的蚜虫中，转座现象似乎很少发生或根本不发生。在简约树推断的mtDNA中核苷酸替换模式与转座序列中的模式非常不同。与mtDNA相比，非线粒体序列的密码子位置偏倚较小，A、G、C和T之间的交换更均匀，非同义替换的比例更高。尽管这些数据与转座序列比mtDNA受到的限制更少一致，但非线粒体序列中的变化并非随机：仍然存在显著的位置偏倚，以及可能过量的同义替换和保守推断的氨基酸替换。我们得出结论，非线粒体序列中推断的一部分变化发生在转座之前。我们认为麦长管蚜属蚜虫（以及其他表现出mtDNA转座的物种）可能对研究mtDNA和假基因的分子进化很重要。然而，我们的数据强调在比较研究中需要确定序列之间真正的进化关系。
