紅果蠅染色體演化

Abstract

由親緣關係及染色體核型 (karyotype) 的變化，推測大果蠅種群 (Drosophila immigrans species group) 中曾經發生過三次染色體癒合事件和一次跨著絲點的逆位 (pericentric inversion)。紅果蠅是這種群內唯一包含所有染色體改變事件，也是全世界三千多種果蠅中唯一在近期發生體染色體 (autosome) 分別和 X 及 Y 染色體癒合的物種。粗估紅果蠅與其同胞種 (輝顏果蠅) 的分歧時間約五十萬年之內，兩者最明顯的差別就在第 3 號體染色體有無與性染色體的癒合。利用 DNA 序列的相似性可得知同源序列在黃果蠅 (D. melanogaster) 染色體臂的位置。建立 PCR-RFLP 分子指標，利用雜交與回交後代是否性聯判斷是位於紅果蠅的新性染色體或是體染色體，依此推出紅果蠅的新性染色體 3 號臂為黃果蠅的 2R和3L 兩個染色體臂(亦即 Muller’s elements C and D)，而紅果蠅的體染色體則為黃果蠅的 2L和3R (亦即 Muller’s elements B and E)。然後利用原位雜合法 (in situ hybridization) 直接確認這些染色體臂的同源關係。藉此討論大果蠅種群的染色體演化過程。

Comparing karyotypes differences on a phylogenetic tree shows three chromosome fusion events and one pericentric inversion in the Drosophila immigrans species group. A young species, D. albomicans, included all those changes in its karyotype is the only one with neo-X and neo-Y chromosomes formed by autosome and X, Y chromosome fusions. The chromosome homology between D. albomicans and D. melanogaster may provide valuable information to study the evolutionary changes on the 3rd chromosome arm of D. albomicans neo-sex chromosomes. Chromosome arm homology was revealed by DNA sequence similarity of genes between these two species. Since no D. albomicans database available, we established several PCR-RFLP genetic markers for checking sex-linkage by D. albomicans and D. nusata hybrid crosses and established the hypothesis of chromosome arm relationship. The hypothesis was supported by sexual differences of those loci within certain isofemale strains, and then confirmed by in situ hybridization. Based on chromosome homology the process of chromosome evolution of D. immigans species group was discussed.