| Summary: | 博士 === 國立臺灣大學 === 昆蟲學研究所 === 90 === Drosophila albomicans belongs to the D. nasuta subgroup of the D. immigrans species group. We’re interested in a pair of sibling species, namely, D. albomicans (2n=6) and D. nasuta (2n=8), for that they’re morphologically indistinguishable, but with fixed structural difference of chromosomes. Whether the two species come into overlap in distribution is not clear. Since these two species produce fertile hybrid offsprings in the laboratory, whereas no hybrids have ever been found in nature near the border of their distributions, it is important to know whether isolating mechanism is strong enough to maintain species integrity should hybridization occur. The first part of this study is to evaluate the extent and mode of reproductive isolation between these two taxa. Our researches show that there is incipient reproductive isolation between these two species, including conspecific sperm precedence (postmating prezygotic isolation) and partial sterility of hybrid F2 male (postzygotic isolation).
To explain the fixation of both 3-X and 3-Y in D. albomicans, it is proposed that it evolved stepwise with 3-X is likely to be the first fused chromosome maintained in the population. The F2 progeny of a cross between Indian D. nasuta males and D. albomicans females showed a low male percentage. Abnormal sex ratio is resulted from non-Mendelian segregation of X-linked chromosomes instead of non-disjunction of sex chromosomes. Evidence indicated that a sex ratio distorter located on the 3-X chromosome, suppressor on the 3-Y chromosome of D. albomicans, and a receptor on the 3,Y chromosomes of D. nasuta. The sex ratio distorter presented in D. albomicans was silenced because of the suppressor in it’s own species, but functional because the sensitive 3,Y in interspecific hybrids. Our result suggested that the sex ratio distorter located in 3-X chromosome might increase the frequency of 3-X chromosome due to meiotic drive at the beginning of the chromosome evolution (Robertsonian mutation).
Hybrid strains were established with fixed 3-X and 3,Y chromosomes. Due to the inviability of aneuploidy, separate 3rd and Y chromosomes have to remain in males from generation to generation. They were tested to see whether or not the accumulation of sexual antagonistic mutations does occur. In addition, degeneration due to the accumulation of recessive lethal or deleterious mutations on the 3rd chromosomes is predicted for the lack of recombination and “ Muller’s ratchet” effect. One sexually antagonistic allele about viability was detected but the frequency was low. And there were no recessive lethals significantly accumulated on unfused 3rd chromosomes in hybrid strains on the hybrid neo-Y chromosome
Finally, parthenogenetic D. albomicans was used to explore how parthenogens can coexist with its sexual relatives. Females from a parthenogenetic stock have a high propensity to mate. The expression of parthenogenesis appears on the absence of males. The parthenogenetic capacity of hybrid F1 offspring indicates dominance of its genetic elements. And this capacity is probably controlled by only a few loci due to the parthenogenetic capacity still maintain in hybrid strains after genetic recombination for many generations of sexual reproduction. These characteristics are advantageous for the maintenance of genetic elements for parthenogenesis.
Based on all studies described above we provide a basis to explore the evolutionary genetics of the intriguing species, D. albomica
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