紅果蠅新Y染色體對新X染色體之依存

Abstract

果蠅屬中的一對新興同胞種：紅果蠅與輝顏果蠅 (分歧時間少於50萬年)能交配並產生可孕後代，來自於紅果蠅雄蟲與輝顏果蠅雌蟲雜交之後代雄蟲體型明顯的小於親代，然而來自於輝顏果蠅雄蟲與紅果蠅雌蟲所產生之後代雄蟲則呈現預期的體型大小。藉由將體型明顯偏小的雜交雄蟲分別與兩個親代物種的雌蟲進行回交得到第二子代雄蟲，其一維持和該雜交雄蟲相同的性染色體組合，另一獲得和紅果蠅相同的性染色體組合。前者之體型依舊，然後者之體型恢復正常，顯示紅果蠅新 X 染色體與新 Y 染色體間的相互依存配合的共演化關係。已知果蠅雄蟲缺乏重組，因此新 Y 染色體的退化成為 Y 染色體是無可避免的命運。在紅果蠅新 Y 染色體上之Amyrel 基因座具有一個帶有框架移轉缺失 (frameshift deletion) 的 Amyreld 對偶因子 (allele)，在種內多態 (polymorphic) 存在。在基因表現上，帶有 Amyreld 之品系其劑量補償現象 (亦即新 X 染色體出現了增加表現而新 Y 染色體則減少表現) 也是多態存在。在果蠅屬中多數物種，包括D. miranda 一個同樣具有新性染色體的物種，顯示區塊式 (block-by-block) 的劑量補償機制。在紅果蠅，在整個三號染色體上隨機的測試數個基因的表現模式包含一個鄰近於 Amyrel 的基因的表現模式並無顯示出支持紅果蠅的劑量補償是以區塊做為單位的方式進行。我們認為特定基因式的劑量補償機制可能是在 Y 染色體退化的極早期先出現，而區塊式的補償方式則是在 Y 染色體退化日漸嚴重後取而代之的機制。紅果蠅的極短分歧時間提供絕佳的機會以了解性染色體在極早期的演化過程。

Drosophila albomicans and D. nasuta are recently (< 0.5 MYA) diverged sibling species. The hybrid progeny are fertile but the hybrid males from a cross between D. nasuta females and D. albomicans males showed a significantly smaller body size comparing to parental species, but either F1 males from the reciprocal cross or F1 females from both crosses were normal. By backcrossing the F1 males to the two parental species, the F2 male body size of one cross remained the same while that of the other, which regained the neo-X for its neo-Y, increased. This supported the hypothesis of the cooption between the neo-X and neo-Y chromosomes of D. albomicans. For lack of recombination in Drosophila males, degeneration is an unavoidable fate for Y chromosomes. A polymorphic Amyrel locus in D. albomicans has a neo-Y allele with a frame-shift deletion (Amyreld), this could be regarded as a degenerated Y-linked allele. The expression pattern of it in some strains with Amyreld showed a dosage compensation phenomenon but it was polymorphic in populations. Drosophila species including D. miranda, another species with neo-sex chromosomes, are commonly observed to follow the block-by-block dosage compensation model. After examining loci sampled from the whole chromosome arm including one close to Amyrel, it’s found that D. albomicans may not adopt the same mechanism as the older species D. miranda (ca. 1 MY) does. We proposed that the gene-by-gene mechanism may emerge at the very early stage of Y chromosome degeneration. While the degeneration was more serious, the block-by-block mechanism might be advantageous and replace the original multiple gene-by-gene mechanisms. The very short divergence time of D. albomicans provides a great chance to explore sex chromosome evolution at a very early stage.