| Summary: | 碩士 === 國立臺灣大學 === 昆蟲學研究所 === 91 === Drosophila albomicans and D. nasuta are sibling species with the same appearance, but can only be distinguished by the number of chromosomes: 2n = 6 in D. albomicans and 2n = 8 in D. nasuta. Yu et al. (1999) proposed a “Stepwise chromosome evolution” hypothesis to explain how the karyotype of Drosophila albomicans becoming fused 3-X and 3-Y from 3,X and 3,Y. The process includes the maintenance of the mutant 3-X, which has a higher fitness value, awaits for the 3-Y fusion, and then the two fused chromosomes reach fixation because of the structural compatibility during meiosis. But experimental confirmation of the process in a population containing four kinds of sex chromosomes is still needed. Two hybrid populations each with 3 replicates were established to verify this expected fixation of the fused chromosomes. The frequency changes of the 2nd and 4th autosomes have no discrepancy with previous studies, but the trends of the sex chromosomes were quite different in these two populations. The frequencies of the fused chromosomes in one replicate of population II full filled the expectation, those of another replicate remained the same, and the other decreased. The original stock used to establish population I contains a meiotic driver on its 3-X chromosome. To avoid the complication, which might be caused by this factor, in our experiment, we constructed the population through a special design to keep the structure but get rid of the meiotic driver. To our surprise, 3 replicates of population I showed concordant drop of the frequencies of fused chromosomes. May it be the meiotic driver a main cause to raise the frequency of 3-X? To test the hypothesis that meiotic drive can increase the 3-X frequency, populations with three sex chromosome types (3-X; 3,X; 3,Y) were established. The result showed that meiotic driver has little influence to the frequency changes, the 3-X chromosomes from those original stocks with or without the driver consistently showed higher fitness as predicted. The experiment of population I avoided not only meiotic driver but also an advantageous element on 3-X. This study also showed that the laboratory isofemale stocks have plenty genetic variation, and it makes a 100% proof of the expectation impossible. The result of population II can only support the possibility of the fixation of fused chromosomes due to structural compatibility.
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