Kurek R, Reugels A M, Lammermann U, Bünemann H
Institut für Genetik, Heinrich-Heine-Universität Düsseldorf, Germany.
Genetica. 2000;109(1-2):113-23. doi: 10.1023/a:1026552604229.
Fertility genes on the heterochromatic Y chromosome of various Drosophila species are unique for several reasons. Most of them are megabase-sized. Their expression is restricted to premeiotic spermatocytes and often associated with unfolding of huge species-specific lampbrush loops. Molecular analysis of the orthologous dynein genes Dhc-Yh3, DhDhc7(Y) and DeDhc7(Y) on the Y chromosome of the three species D. melanogaster, D. hydei and D. eohydei, respectively, revealed that the megabase gene size as well as the species-specific morphology of the corresponding lampbrush loops kl-5, Threads and diffuse loops result from huge introns and their specific sequence composition, whereas the majority of all 20 introns in each of the three genes is in a size of 45-72 bp. The loop-specifying introns are extreme exceptions due to extended assemblies of degenerated transposable elements and/or large clusters of satellite DNAs. Here we use sequence information from the complete intron sets of three orthologous Y chromosomal dynein genes to deduce a scenario for an evolutionary pathway leading to the megabase-sized genes on the heterochromatic Y chromosome of Drosophila. The obvious bias between very small and species-specific mega introns is explained as the result of an autocatalytic mode of intron growth. An initial coincidental hit by a single transposable element extends the size of a 50 bp intron for about two orders of magnitude and determines it for preferential extension by similar insertion events. This phase of continuous moderate growth is followed by rapid size enlargements by repeating amplifications generating extended clusters of satellite DNA. Size control by recombination, on the other hand, is suppressed in Drosophila males by achiasmatic meiosis.
各种果蝇物种的异染色质Y染色体上的生育基因因其几个原因而独特。它们中的大多数是兆碱基大小。它们的表达仅限于减数分裂前的精母细胞，并且通常与巨大的物种特异性灯刷环的展开相关。分别对黑腹果蝇、海德氏果蝇和拟海德氏果蝇Y染色体上的直系动力蛋白基因Dhc - Yh3、DhDhc7(Y)和DeDhc7(Y)进行分子分析，结果表明，兆碱基基因大小以及相应灯刷环kl - 5、Threads和弥散环的物种特异性形态是由巨大的内含子及其特定的序列组成导致的，而这三个基因中所有20个内含子的大多数大小为45 - 72 bp。由于退化转座元件的扩展组装和/或卫星DNA的大簇，指定环的内含子是极端例外情况。在这里，我们利用来自三个直系Y染色体动力蛋白基因完整内含子集的序列信息，推断出一条导致果蝇异染色质Y染色体上兆碱基大小基因的进化途径。非常小的内含子和物种特异性的大内含子之间明显的偏差被解释为内含子生长的自催化模式的结果。单个转座元件的初始偶然命中将一个50 bp内含子的大小扩展了大约两个数量级，并决定其通过类似的插入事件进行优先扩展。这个持续适度生长的阶段之后是通过重复扩增产生卫星DNA的扩展簇而导致的快速大小增大。另一方面，在果蝇雄性中，由于无交叉减数分裂，重组对大小的控制受到抑制。
