| Summary: | 碩士 === 國立臺灣大學 === 昆蟲學研究所 === 96 === 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.
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