Stepwise chromosome evolution in Drosophila albomicans

A 'stepwise chromosome evolution model' is proposed to elaborate the two st ages of the sex chromosome evolution of Drosophila albomicans. To explain t he occurrence of both a 3-X and a 3-Y in this species it is proposed that t hese chromosomes evolved sequentially. The selection forces acting in the t wo stages are different. The genic composition in the first fused sex chrom osome of D. albomicans was supposed to be selectively superior to the primi tive unfused chromosomes, thus the fused chromosome could overcome the meio tic structural incompatibility and be maintained in the population during t he first stage. When the second fusion occurred, the newly fused sex chromo some was preferentially selected, as it could provide a meiotic pairing par tner for the first fusion. The fused sex chromosomes were eventually fixed in the population. Based on this model, we tried to determine which fused c hromosome, 3-X or 3-Y, established first. The karyotyping result of four ex perimental hybrid populations showed that the fused 3-X, as opposed to 3-Y, chromosome has higher fitness values. However, without the coexistence of 3-X, 3-Y is excluded rapidly. Therefore, 3-X is likely to be the first fuse d chromosome in the stepwise chromosome evolution model. The formation and fixation of the fused 3-Y chromosome occurred subsequently.