Stewart D T, Kenchington E R, Singh R K, Zouros E
Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
Genetics. 1996 Jul;143(3):1349-57. doi: 10.1093/genetics/143.3.1349.
Mussels of the genus Mytilus segregate for a maternally transmitted F lineage and a paternally transmitted M lineage of mitochondrial DNA. Previous studies demonstrated that these lineages are older than the species of the M. edulis complex and that the M lineage evolves faster than the F lineage. Here we show that the latter observation also applies to a region of the molecule with no assigned function. Sequence data for the mitochondrial COIII gene and the "unassigned" region of the F and M lineages of M. edulis and M. trossulus are used to evaluate various hypotheses that may account for the faster rate of evolution of the M lineage. Tests based on the proportion of synonymous and nonsynonymous substitutions suggest that the M lineage experiences relatively relaxed selection. Further support for this hypothesis comes from an examination of COIII amino acid substitutions at sites defined as either conserved or variable based on the pattern of variation in other mollusks and Drosophila. Most substitutions in the M lineage occur in regions that are also variable among non-Mytilus taxa. We suggest that these differences in selection pressure are a consequence of doubly uniparental mitochondrial DNA transmission in Mytilus.
贻贝属的贻贝区分出母系遗传的线粒体DNA的F谱系和父系遗传的M谱系。先前的研究表明，这些谱系比紫贻贝复合体的物种更为古老，并且M谱系的进化速度比F谱系更快。在此我们表明，后一观察结果也适用于分子中一个未指定功能的区域。利用紫贻贝和翡翠贻贝的F和M谱系的线粒体COIII基因及“未指定”区域的序列数据，来评估可能解释M谱系进化速度更快的各种假说。基于同义替换和非同义替换比例的测试表明，M谱系经历的选择相对宽松。对这一假说的进一步支持来自于对基于其他软体动物和果蝇的变异模式定义为保守或可变位点的COIII氨基酸替换的检查。M谱系中的大多数替换发生在非贻贝类群中也可变的区域。我们认为，选择压力的这些差异是贻贝线粒体DNA双亲单亲遗传的结果。
