Department of Biology, University of Missouri-St. Louis, USA.
Mol Biol Evol. 2010 Mar;27(3):537-42. doi: 10.1093/molbev/msp283. Epub 2009 Nov 23.
Haemosporidian parasites of birds and mammals reproduce asexually inside nucleated and nonnucleated host erythrocytes, respectively. Because of these different parasite environments and because bird parasites are paraphyletic, we evaluated whether patterns of parasite molecular evolution differ between host groups. We compared two mitochondrial (mt) genes and one apicoplast gene across mammal Plasmodium, bird Plasmodium, and bird Parahaemoproteus. Using a molecular phylogenetic approach, we show that the parasite mt cytochrome b (cyt b), mt cytochrome oxidase I (COI), and the apicoplast caseinolytic protease C (ClpC) exhibit similar levels of sequence divergence, yet each gene tree presents a strikingly different pattern of internal versus terminal branch lengths. In cyt b, the ratio of nonsynonymous (NS)-to-synonymous substitutions (d(N)/d(S)) is markedly elevated along the internal branch linking mammalian and avian parasites despite the sister relationship between mammal and bird Plasmodium. This is not the case for either COI or ClpC. When NS substitutions are excluded from the parasite cyt b alignment, the resulting phylogenetic tree resembles that of COI (both with and without NS substitutions). The high d(N)/d(S) ratio in the cyt b branch separating avian and mammalian parasites and a mammal-parasite codon bias suggest that adaptive evolution has distinguished mammal and bird parasites.
鸟类和哺乳动物的血孢子虫寄生虫分别在有核和无核的宿主红细胞内进行无性繁殖。由于这些寄生虫所处的环境不同，而且鸟类寄生虫是并系的，我们评估了寄生虫分子进化的模式是否因宿主群体而异。我们比较了哺乳动物疟原虫、鸟类疟原虫和鸟类疟原虫中的两种线粒体（mt）基因和一种质体基因。通过分子系统发育方法，我们表明寄生虫 mt 细胞色素 b（cyt b）、mt 细胞色素氧化酶 I（COI）和质体天冬氨酸蛋白酶 C（ClpC）表现出相似的序列差异水平，但每个基因树呈现出内部与末端分支长度截然不同的模式。在 cyt b 中，尽管哺乳动物和鸟类疟原虫是姐妹关系，但在连接哺乳动物和鸟类寄生虫的内部分支上，非同义（NS）与同义取代（d（N）/ d（S））的比值明显升高。COI 或 ClpC 则不是这样。当将寄生虫 cyt b 比对中的 NS 取代排除在外时，得到的系统发育树类似于 COI（均包含和不包含 NS 取代）。在分离鸟类和哺乳动物寄生虫的 cyt b 分支中，高 d（N）/ d（S）比值和哺乳动物寄生虫的密码子偏向表明，适应性进化已经区分了哺乳动物和鸟类寄生虫。
