國立臺灣大學昆蟲學系暨研究所張慧羽2006-07-252018-06-292006-07-252018-06-292000http://ntur.lib.ntu.edu.tw//handle/246246/12555紅果蠅 (Drosophila albomicans) 的新- X (neo-X) 染色體與其新-Y (neo-Y) 染色體 是由原始的性染色體與第三對體染色體癒 合而成。種化過程 (speciation) 中的兩次癒 合事件所牽涉到的選汰作用 (selection forces) 是本研究探討的主題。利用紅果蠅 與其同胞種輝顏果蠅 (D. nasuta) 仍能成功 雜交的特性，我們所提出的 "階段性染色 體進化" 假說有機會得以驗證。 "階段性 染色體進化" 是推測在進化的過程中第一 次的性染色體癒合可以在族群中留存很長 的一段時間直到第二次性染色體癒合的發 生。其後選汰力就會促成兩個癒合型性染 色體在族群中發生了固定化 (fixation)。 若此假說成立，則紅果蠅進化研究上 的一些爭議當可迎刃而解。例如：本研究 室以往曾兩度建立紅果蠅和輝顏果蠅的雜 交族群，均維持長期 (i.e., 超過二十代) 的 多態性，但印度 Dr. Ranganath 研究室所 建立的雜交族群卻很快地 (i.e., 不超過十 代) 達到染色體核型的固定。若階段性染 色體進化假說是正確的，則這個矛盾現象 很顯然是由於族群起始狀態的不同所致。 為了驗證這個假說，我們曾構築四種具有 不同染色體組成的實驗族群，兩種代表第 一階段：原始型 X 與兩種 Y、原始型 Y 與兩種 X；兩種代表第二階段：癒合型 X 與兩種 Y、癒合型 Y 與兩種 X。雖然研究 結果間接地支持這個假說，仍存在一些不 確定性，即第一階段驗證癒合的性染色體 在族群中以多態性的方式留存，而第二階 段卻是從癒合的性染色體已達固定的情況 下去探討，而從多態性到固定化仍有一段 距離。因此在這次的實驗中我們將構築同 時具有原始型 X 、原始型 Y 、癒合型 X 與癒合型 Y 四種染色體型式的實驗族群。 我們預測癒合型 X 與癒合型 Y 最後將固 定在該族群中，也就是在四型染色體並存 的狀態下，選汰力會促成兩種癒合型性染 色體的固定化。 兩個雜交族群的結果顯示：E1 雜交族 群長期飼養後仍維持3-X 與3-Y 染色體多 態型；而 E2 雜交族群在二十代的累代培 養過程中3-X 與3-Y 的染色體被快速地排 除。這個不符合預期的結果卻意外地顯示 我們在實驗過程中刻意迴避的減數分裂驅 動 (meiotic drive) 效應可能在紅果蠅染色 體進化過程中扮演重要的角色。我們接下 來的研究重點就是減數分裂驅動因子和染 色體進化的關係。The neo-X and neo-Y chromosomes of Drosophila albomicans were formed by two fusion events of the ancestral sex and 3rd chromosomes during evolution. The main purpose of this study is to show chromosomal selection forces involved during evolution. Fortunately, D. albomicans can produce fertile hybrids with its sibling species, D. nasuta. Therefore, it is possible to test our hypothesis of “stepwise 2 chromosome evolution”. In our “stepwise chromosome evolution” hypothesis, the new sex chromosome formed by the first fusion event can be maintained under a polymorphic condition in the population for a long time waiting for the occurrence of the second fusion event. The two fused chromosomes will be fixed in the population by selection forces. This hypothesis can solve some of the long lasting issues in the study of D. albomicans evolution. For instance: the hybrid populations established in our laboratory maintained polymorphic for at least twenty generations, whereas the hybrid populations established in Dr. Ranganath's laboratory in India soon became fixed. If our “stepwise chromosome evolution” hypothesis is correct, this phenomenon is apparently due to different initial conditions. In order to test this hypothesis, we had established four different types of populations to represent the two stages of chromosome evolution. They were fixed ancestral X with two kinds of Ys, fixed ancestral Y with two kinds of Xs for stage I, and fixed neo-X with two kinds of Ys, fixed neo-Y with two kinds of Xs for stage II. The result of that study supported our hypothesis indirectly, but there still exists some uncertainty between the two stages, i.e., in stage II, the populations were initiated from a condition that the first fused chromosome had been fixed. However, the data only supported that polymorphism can be maintained at stage I. Therefore, we planed to establish a population with all four types of chromosomes simultaneously, and see if the two fused chromosomes will go fixation as we predicted. The result of two replicated experiments revealed that karyotype polymorphism exists in E1 hybrid population after long-term cultivation, but the 3-X and 3-Y chromosomes are excluded rapidly after 20 generations. This unexpected result accidentally showed that the meiotic drive, which we tried to avoid in our experiments, might play an important role in the chromosome evolution of D. albomicans. The investigation of the relationship between meiotic driver and chromosome evolution of this species is in progress.application/pdf39354 bytesapplication/pdfzh-TW國立臺灣大學昆蟲學系暨研究所染色體進化紅果蠅輝顏果蠅勞勃松氏(Robertsonian) 變化chromosomal evolutionDrosophila albomicansD. nasutaRobertsonian changereporthttp://ntur.lib.ntu.edu.tw/bitstream/246246/12555/1/892313B002031.pdf