Mangé A, Prudhomme J C
Centre de Génétique Moléculaire et Cellulaire, Université Claude Bernard Lyon I, Centre National de la Recherche Scientifique, Villeurbanne, France.
Mol Biol Evol. 1999 Feb;16(2):165-72. doi: 10.1093/oxfordjournals.molbev.a026099.
The cytoplasmic actin genes BmA3 and BmA4 of Bombyx mori were found clustered in a single genomic clone in the same orientation. As a similar clustering of the two cytoplasmic actin genes Ha3a and Ha3b also occurs in another lepidopteran, Helicoverpa armigera, we analyzed the sequence of the pair of genes from each species. Due to the high conservation of cytoplasmic actins, the coding sequence of the four genes was easily aligned, allowing the detection of similarities in noncoding exon and intron sequences as well as in flanking sequences. All four genes exhibited a conserved intron inserted in codon 117, an original position not encountered in other species. It can thus be postulated that all of these genes derived from a common ancestral gene carrying this intron after a single event of insertion. The comparison of the four genes revealed that the genes of B. mori and H. armigera are related in two different ways: the coding sequence and the intron that interrupts it are more similar between paralogous genes within each species than between orthologous genes of the two species. In contrast, the other (noncoding) regions exhibited the greatest similarity between a gene of one species and a gene of the other species, defining two pairs of orthologous genes, BmA3 and HaA3a on one hand and BmA4 and HaA3b on the other. However, in each species, the very high similarities of the coding sequence and of the single intron that interrupts it strongly suggest that gene conversion events have homogenized this part of the sequence. As the divergence of the B. mori genes was higher than that of the H. armigera genes, we postulated that the gene conversion occurred earlier in the B. mori lineage. This leads us to hypothesize that gene conversion could also be responsible for the original transfer of the common intron to the second gene copy before the divergence of the B. mori and H. armigera lineages.
家蚕的细胞质肌动蛋白基因BmA3和BmA4被发现以相同方向聚集在一个单一的基因组克隆中。由于在另一种鳞翅目昆虫棉铃虫中也存在两个细胞质肌动蛋白基因Ha3a和Ha3b的类似聚集现象，我们分析了每个物种中这对基因的序列。由于细胞质肌动蛋白具有高度保守性，这四个基因的编码序列很容易比对，从而能够检测非编码外显子和内含子序列以及侧翼序列中的相似性。所有四个基因都在密码子117处插入了一个保守内含子，这是在其他物种中未发现的原始位置。因此可以推测，所有这些基因都源自一个携带该内含子的共同祖先基因，该内含子是在一次单一的插入事件后出现的。对这四个基因的比较表明，家蚕和棉铃虫的基因以两种不同方式相关：每个物种内的旁系同源基因之间的编码序列和打断它的内含子比两个物种的直系同源基因之间更相似。相反，其他（非编码）区域在一个物种的基因和另一个物种的基因之间表现出最大的相似性，定义了两对直系同源基因，一方面是BmA3和HaA3a，另一方面是BmA4和HaA3b。然而，在每个物种中，编码序列和打断它的单个内含子的高度相似性强烈表明基因转换事件使这部分序列同质化。由于家蚕基因的分歧高于棉铃虫基因的分歧，我们推测基因转换在家蚕谱系中发生得更早。这使我们假设基因转换也可能是在蚕蛾科和棉铃虫谱系分化之前，将共同内含子最初转移至第二个基因拷贝的原因。
