果蠅之無重組染色體的演化

Abstract

三個經由紅果蠅 (Drosophila albomicans, 2n = 6) 雌蟲與輝顏果蠅 (D. nasuta, 2n = 8) 雄蟲雜交而得到的實驗室雜交品系 (H10、Hn1 和 Hn2) 被用來研究染色體停止重組之後發生的改變。由於這些雜交品系的雄蟲具有特殊核型 (2n = 7)，以致於一條源自於揮顏果蠅的第三號染色體，在品系中如同 Y 染色體，成為另一條無重組染色體。我們稱呼這條無重組的三號染色體為 “類似 Y 的 (Y-like)” 染色體；經由交配與回交的實驗，利用 PCR-RFLP 分子標記，去驗證是否有性別拮抗因子或隱性致死因子堆積在這條無重組的染色體上。較老的 (H10，約 280代) 類似 Y 的染色體和較新的 (Hn1 與 Hn2，約 20 代) 相比，的確顯示較高的隱性不良影響。相對的，有一條較新的類似 Y 的染色體顯示性別拮抗因子，而較老的卻沒有。我們提出的假說是：性別拮抗因子的堆積發生在染色體停止重組的早期，後來則由於全面性的隱性不良因子的堆積而不復顯現。此外，這個實驗系統的性染色體間發生兩種形式的共適應：1. 減數分裂驅動因子 (meiotic driver) 與其抑制者 (suppressor) 間的交互作用，及 2. 類似 Y 的染色體對它同源染色體的依賴性；都可能是導致性別拮抗因子在晚期觀察不到的原因。

Three hybrid strains, each started with a cross between one Drosophila albomicans female and one D. nasuta male, were used as a resource to study evolutionary changes on a non-recombining chromosome because they contained a fixed Y-like chromosome. Sexually antagonistic alleles and recessive deleterious alleles on this non-recombining Y-like chromosome can be revealed by proper cross schemes. Old (ca. 280 generations) Y-like chromosomes did show an increased accumulation of recessive deleterious alleles comparing to new (ca. 20 generations) ones. Only one new Y-like chromosome showed sexually antagonistic effect on both female viability and male fertility, but none was found in old ones. We proposed that the accumulation of sexually antagonistic alleles may occur on a Y-like chromosome at an early stage after the cease of recombination, but not at a later stage because the accumulation of recessive deleterious alleles is much more overwhelming. Furthermore, two forms of coadaptation between sex chromosomes, the interaction between a meiotic driver and its suppressor, and dependence of a Y-like chromosome on its homologue, were also probable reasons why sexual antagonism was not observed on old Y-like chromosomes.