Shimmin L C, Miller J, Tran H N, Li W H
Human Genetics Center, SPH, University of Texas-Houston, USA.
Mol Biol Evol. 1998 Apr;15(4):449-55. doi: 10.1093/oxfordjournals.molbev.a025941.
The X-linked color pigment (opsin) locus is known to be highly polymorphic in the squirrel monkey and other New World monkeys. To see whether this is also the case for the autosomal (blue) opsin locus, we obtained 32 squirrel monkey and 30 human blue opsin gene sequences. No amino acid polymorphism was found in either the squirrel monkey sample or the human sample, contrary to the situation at the X-linked opsin locus. This sharp contrast in the level of polymorphism might be due to differences in gene expression between the autosomal and the X-linked loci. At the X-linked locus, heterozygote advantage can occur because, owing to X-inactivation, the two alleles in a heterozygote are expressed in different cone cells, producing two types of cone cell, whereas at the autosomal locus, heterozygote advantage cannot occur because the two alleles in a heterozygote are expressed in the same cone cells, producing only one type of cone cell (i.e., phenotypically a homozygote). From the sequence data, the levels of nucleotide diversity (pi, i.e., the number of nucleotide differences per site) are estimated: for the human sample, pi = 0.00% per nondegenerate site, 0.00% per twofold degenerate site, and 0.04% per fourfold degenerate site in the coding regions and 0.01% per site in intron 4; for the squirrel monkey sample, pi = 0.00% per nondegenerate site, 0.00% per twofold degenerate site, and 0.15% per fourfold degenerate site in the coding regions and 0.17% per site in intron 4. The blue opsin genes from the common and pygmy chimpanzees, the gorilla, the capuchin, and the howler monkey were also sequenced. Features critical to the function of the opsin are well conserved in all known mammalian sequences. However, the interhelical loops are, on average, actually more conservative than the transmembrane helical regions. In addition, these sequence data and those from some other genes indicate that the common and pygmy chimpanzees are not closely related, their divergence data being from one third to one half the date of the human-chimpanzee divergence.
已知X连锁颜色色素（视蛋白）基因座在松鼠猴和其他新大陆猴中具有高度多态性。为了探究常染色体（蓝色）视蛋白基因座是否也是如此，我们获得了32个松鼠猴和30个人类蓝色视蛋白基因序列。与X连锁视蛋白基因座的情况相反，在松鼠猴样本和人类样本中均未发现氨基酸多态性。多态性水平的这种鲜明对比可能是由于常染色体基因座和X连锁基因座之间基因表达的差异所致。在X连锁基因座，杂合子优势可能会出现，因为由于X染色体失活，杂合子中的两个等位基因在不同的视锥细胞中表达，产生两种类型的视锥细胞，而在常染色体基因座，杂合子优势不会出现，因为杂合子中的两个等位基因在相同的视锥细胞中表达，仅产生一种类型的视锥细胞（即表型上为纯合子）。根据序列数据，估计了核苷酸多样性水平（π，即每个位点的核苷酸差异数）：对于人类样本，编码区中每个非简并位点的π = 0.00%，每个两倍简并位点的π = 0.00%，每个四倍简并位点的π = 0.04%，内含子4中每个位点的π = 0.01%；对于松鼠猴样本，编码区中每个非简并位点的π = 0.00%，每个两倍简并位点的π = 0.00%，每个四倍简并位点的π = 0.15%，内含子4中每个位点的π = 0.17%。还对普通黑猩猩、倭黑猩猩、大猩猩、卷尾猴和吼猴的蓝色视蛋白基因进行了测序。视蛋白功能的关键特征在所有已知的哺乳动物序列中都得到了很好的保守。然而，平均而言，螺旋间环实际上比跨膜螺旋区域更保守。此外，这些序列数据以及来自其他一些基因的数据表明，普通黑猩猩和倭黑猩猩的亲缘关系并不密切，它们的分歧数据为人与黑猩猩分歧日期的三分之一到二分之一。
