Perry J, Ashworth A
Section of Gene Function and Regulation Chester Beatty Laboratories Institute of Cancer Research, 237 Fulham Road, London, SW3 6JB, UK.
Curr Biol. 1999 Sep 9;9(17):987-9. doi: 10.1016/s0960-9822(99)80430-8.
Genes evolve at different rates depending on the strength of selective pressure to maintain their function. Chromosomal position can also have an influence [1] [2]. The pseudoautosomal region (PAR) of mammalian sex chromosomes is a small region of sequence identity that is the site of an obligatory pairing and recombination event between the X and Y chromosomes during male meiosis [3] [4] [5] [6]. During female meiosis, X chromosomes can pair and recombine along their entire length. Recombination in the PAR is therefore approximately 10 times greater in male meiosis compared with female meiosis [4] [5] [6]. The gene Fxy (also known as MID1 [7]) spans the pseudoautosomal boundary (PAB) in the laboratory mouse (Mus musculus domesticus, C57BL/6) such that the 5' three exons of the gene are located on the X chromosome but the seven exons encoding the carboxy-terminal two-thirds of the protein are located within the PAR and are therefore present on both the X and Y chromosomes [8]. In humans [7] [9], the rat, and the wild mouse species Mus spretus, the gene is entirely X-unique. Here, we report that the rate of sequence divergence of the 3' end of the Fxy gene is much higher (estimated at 170-fold higher for synonymous sites) when pseudoautosomal (present on both the X and Y chromosomes) than when X-unique. Thus, chromosomal position can directly affect the rate of evolution of a gene. This finding also provides support for the suggestion that regions of the genome with a high recombination frequency, such as the PAR, may have an intrinsically elevated rate of sequence divergence.
基因根据维持其功能的选择压力强度以不同速率进化。染色体位置也会产生影响[1][2]。哺乳动物性染色体的假常染色体区域（PAR）是一个小的序列同源区域，是雄性减数分裂期间X和Y染色体之间 obligatory 配对和重组事件的发生位点[3][4][5][6]。在雌性减数分裂期间，X染色体可以沿着其全长配对和重组。因此，与雌性减数分裂相比，PAR中的重组在雄性减数分裂中大约高10倍[4][5][6]。基因Fxy（也称为MID1[7]）跨越实验室小鼠（小家鼠，C57BL/6）中的假常染色体边界（PAB），使得该基因的5'端三个外显子位于X染色体上，但编码蛋白质羧基末端三分之二的七个外显子位于PAR内，因此存在于X和Y染色体上[8]。在人类[7][9]、大鼠和野生小鼠物种斯氏小家鼠中，该基因完全是X染色体特有的。在这里，我们报告说，当Fxy基因的3'端处于假常染色体状态（存在于X和Y染色体上）时，其序列分歧率比处于X染色体特有状态时高得多（同义位点估计高170倍）。因此，染色体位置可以直接影响基因的进化速率。这一发现也为如下观点提供了支持，即基因组中具有高重组频率的区域，如PAR，可能具有内在升高的序列分歧率。
