Mills G C
Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston 77550.
J Theor Biol. 1991 Sep 21;152(2):177-90. doi: 10.1016/s0022-5193(05)80451-5.
In this paper, the author notes the recommended definition of the word "homology" (i.e., indicating an ancestral relationship) and the recommended stipulation that "evidence for homology should be explicitly laid out". The postulated homology for somatic and testes-specific isozymes of cytochrome c is then examined, using recent data obtained from the study of cytochrome c genes. Consideration is also given to some newer findings of molecular biology and possibilities are considered for various types of change in the genome of an organism. Possible roles of introns, pseudogenes and multigene families are considered. The relationship of testes-specific cytochrome c to somatic cytochrome c is carefully considered from data obtained in experimental studies of genes of these two isozymes. If one assumes that these isozymes arose as a consequence of a gene duplication, data from rat and mouse genes indicate that the testes-specific isozyme has incorporated more amino acid changes than the somatic isozyme since the time of their divergence. However, when the 15 amino acid differences (testes-specific vs. somatic isozyme) are considered, there is virtually no similarity in these 15 positions of the testes-specific isozyme with any of the hypothetical ancestral sequences of the somatic isozyme. Nucleotide differences in cytochrome c genes have been evaluated by comparing genes for the two rodent cytochrome c isozymes to cytochrome c genes of fruit flies, chickens and humans. Comparisons of nucleotide substitution rates in genes for the two cytochrome c isozymes in rodents confirm the conclusions from amino acid sequence comparisons; namely, that more rapid nucleotide changes have occurred in the testes-specific cytochrome c gene, than in the somatic cytochrome c gene. Possible explanations for these findings are considered.
在本文中，作者指出了“同源性”一词的推荐定义（即表明祖先关系）以及“同源性证据应明确列出”的推荐规定。然后，利用从细胞色素c基因研究中获得的最新数据，对体细胞特异性和睾丸特异性细胞色素c同工酶的假定同源性进行了研究。同时也考虑了分子生物学的一些较新发现，并探讨了生物体基因组中各种类型变化的可能性。还考虑了内含子、假基因和多基因家族的可能作用。根据这两种同工酶基因实验研究获得的数据，仔细考虑了睾丸特异性细胞色素c与体细胞细胞色素c的关系。如果假设这些同工酶是基因复制的结果，来自大鼠和小鼠基因的数据表明，自分化以来，睾丸特异性同工酶比体细胞同工酶纳入了更多的氨基酸变化。然而，当考虑这15个氨基酸差异（睾丸特异性同工酶与体细胞同工酶）时，睾丸特异性同工酶的这15个位置与体细胞同工酶的任何假设祖先序列几乎没有相似性。通过比较两种啮齿动物细胞色素c同工酶的基因与果蝇、鸡和人类的细胞色素c基因，评估了细胞色素c基因中的核苷酸差异。对啮齿动物两种细胞色素c同工酶基因中核苷酸替代率的比较证实了氨基酸序列比较的结论；即，睾丸特异性细胞色素c基因中发生的核苷酸变化比体细胞细胞色素c基因更快。文中考虑了这些发现的可能解释。
