紅果蠅新性染色體之演化

Abstract

紅果蠅 (Drosophila albomicans) 具有一對新的性染色體 (3-X 與 3-Y)，使得其性染色體佔了整個基因組的 60%，而與此對新性染色體 3 號臂同源的仍為其同胞種-輝顏果蠅 (D. nasuta) 的 3 號體染色體。一般而言，Y 染色體多異染色質化且僅含少數具有活性的基因，但由於紅果蠅具有此癒合的新性染色體，尚處於染色體分化初期，這些同源序列是研究性性染色體與體染色體差別的好材料，本論文依此特性，探討諸如 Y 染色體上對雄性有利的性別拮抗因子的堆積、Y 染色體的退化以及性染色體分化的時間。由限制性雜合消去法 (SSH) 所獲得的紅果蠅雄性特殊表現基因的分析結果，並未顯示在新性染色體臂上的比例較高，但如預期地性染色體上的基因承受較大的選汰壓，並雄性特殊表現基因的演化速率較一般基因為快。並由紅果蠅性染色體上的二十五個對偶基因，其 X 與 Y 序列差異推測新性染色體臂分化的時間約七萬年。雖未發現紅果蠅新 Y 染色體有明顯的退化，但發現雲南族群的 Amyrel 基因有一個多態存在的退化因子，它的表現序列具有一個 206 bp 的缺失而且該序列的轉譯過早終止。表示 Y 染色體在演化初期雖未有顯著的退化現象，但族群中已出現退化因子。

Drosophila albomicans has a pair of neo-sex chromosomes which composed 60% of the genome (i.e., fused 3-X and 3-Y) and the homologous 3rd chromosome is still an autosome of its sibling species D. nasuta. Generally speaking, the Y chromosome contains only a small number of genes and its major components are heterochromatin. Because of the fusion events between the autosome and sex chromosomes, D. albomicans became a promising material to study the early stage of sex chromosomes. Homologous sequences on the 3rd chromosome can be used to study sex chromosome differentiation after the cessation of recombination, such as the accumulation of sexual antagonistic alleles favoring males, Y chromosome degeneration and the neo-sex chromosome divergence time of D. albomicans. The SSH cDNA library of males did not show a significant higher proportion of male specifically expressed genes located on this neo-sex chromosome arms, but their evolution rate is higher as expected. Also as expected, genes on the sex chromosome were shown to be under higher selection pressure, and found the evolution rate is higher of the male-specific expression genes than other ones. The divergence time of the neo-sex chromosomes is about 70,000 years according to the 25 alleles of X and Y homologous sequences. There’s no significant degeneration found on the neo-Y chromosome, however, an allele of the gene Amyrel on the Y chromosome with a 206 bp deletion and a premature stop codon was found polymorphically existed in the Yunnan population. It shows there’s no obvious degeneration of Y chromosome at this early stage; nevertheless, a degenerate allele polymorphically existed.