紅果蠅族群間 neo-Y 染色體的分歧演化

Abstract

紅果蠅 (Drosophila albomicans) 具有一對新的性染色體 (neo-X 和 neo-Y)，此對染色體 (neo-sex) 新加入的染色體臂 (arm) 與其同胞種- 輝顏果蠅 (D. nasuta) 的第 3 號染色體同源。經實驗證明果蠅雄蟲不會發生染色體互換，因此當癒合的 neo-Y 染色體在族群中固定 (亦即頻率達 100%) 之後，這 neo-Y 臂就失去遺傳重組的機會。接著發現帶有紅果蠅 neo-Y 染色體的雜交雄蟲在行減數分裂產生配子時，有很高比例的染色體無分離 (non-disjunction) 現象，其所產生性染色體數不正常 (aneuploid) 的 3,X,X/neo-Y 雌蟲可孕，提供了很好的機會得以驗證 neo-Y 臂上具有隱性不良因子。這篇研究提出一個假說：不同的族群會各自在 neo-Y臂上累積隱性不良的因子，並透過不同品系經由特殊設計的雜交流程及 neo-Y 臂重組品系 (recombinant strains) 的建立，採用分子指標進行互補測試 (complementation test) 驗證該假說。#163.5-IA 品系的 neo-Y 臂上少有 3 個隱性不良因子， #281.4 品系至少有 2 個。不僅在隱性不良因子的檢測上，它們的位置不同；在另一檢測，所有選用的指標均可互補，兩筆資料均支持 “不同品系間所含隱性不良因子不同” 的假說。

Drosophila albomicans (2n = 6) has a pair of metacentric neo-sex chromosomes with a large portion of the neo-sex arm originally derived from an autosome homologous to the 3rd chromosome of its sibling species, D. nasuta. It’s been experimentally confirmed that males of these species including their hybrids do not perform genetic recombination during meiosis. Therefore, since the fixation of neo-Y chromosome, the neo-Y arm has been devoid of meiotic recombination. Hybrid males with neo-Y showed high non-disjunction rate and the resulting fertile aneuploid 3,X,X/neo-Y females offered a good opportunity to verify whether neo-Y arm contains recessive deleterious alleles. The finding that no X,neo-Y/neo-Y males were observed if the neo-Y arms were from the same strain while those males survived if the neo-Y arms were from different strains supported our hypothesis: “Different populations accumulated recessive deleterious alleles independently”. Because neo-Y arm can recombine with the 3rd chromosome from D. nasuta in XXY hybrid females, recombinant strains containing specific neo-Y arm fragments were established. On the neo-Y arm of strain #163.5-IA at least three recessive deleterious alleles at different regions were confirmed, and on that of strain #281.4 there at least two at different regions. Besides, all of the markers showing recessive deleterious alleles can be complemented. These results supported our hypothesis on differentiated neo-Y arms among D. albomicans populations.