On the evolutionary importance of genetic recombination from molecular aspects

從分子角度看基因重組在演化上的重要性

張筱涵(Hsiao-Han Chang)

指導教授 ： 丁照棣 ； 汪宏達

國立清華大學/生命科學院/生物科技研究所/碩士(2008年)

https://doi.org/10.6843/NTHU.2008.00214

摘要

Sexual recombination is widespread and its advantage is one of the most fundamental questions in evolutionary biology. Theoretical models explain the benefit of recombination by demonstrating that lack of recombination would reduce selection efficacy. Here, the evolutionary importance of recombination was investigated from two empirical approaches. First, the neo-sex chromosome system in Drosophila albomicans was used as a model to assess evolutionary patterns of homologous genes with and without recombination. The results show that the non-synonymous nucleotide changes on the neo-X and the non-recombining neo-Y chromosome are not significantly different and polymorphism is absent on the neo-Y chromosome of Chitou population. It indicates that selection efficacy on the neo-Y chromosome is still maintained but the effective population size of the neo-Y chromosome is severely reduced due to selection on the non-recombining chromosome or high variation of reproductive ability. Second, selection efficacy in regions with different levels of recombination rates was studied to examine the influence of variable recombination rates. Efficacy of negative selection against the accumulation of transposable elements in regions with different recombination rates was compared in D. melanogaster and D. simulans. The results reveal that different levels of recombination rates could influence the capacity of selection from the correlation between the densities of transposable elements and recombination rates. In conclusion, the advantage of recombination is supported by higher efficacy of negative selection in high recombining regions; however, the reduced selection efficacy has not been found on the young neo-Y chromosomes of D. albomicans, suggesting the influence of recombination on selection efficacy by could not be detected within a short time.

關鍵字

重組 ； 果蠅 ； 選汰效率 ； 新性染色體

並列摘要

有性生殖的遺傳重組非常普遍，它的優點是演化生物學上最重要的問題之一。理論模型藉由說明失去重組會減低選汰效率來解釋重組的好處。這裡，兩種實驗的方法被用來研究重組在演化上的重要性。第一，紅果蠅的新性染色體系統被用來當作模型去探討同源基因在有跟沒有重組下的演化模式。溪頭族群的結果顯示，在neo-X與不能重組的neo-Y上非同義的核苷酸改變並無顯著差異，且neo-Y上沒有多型性。這指出，neo-Y上選汰效率仍被維持著，但neo-Y的有效族群大小可能受到不能重組的染色體上的選汰或是雄性間生殖能力差異很大的影響而嚴重的減低了。第二，藉由研究不同重組速率區域的選汰效率，探討不同重組速率所造成的影響。在黃果蠅與擬黃果蠅中，對不同重組速率區域針對轉位子累積的負向選汰效率做了比較。結果顯示，從轉位子密度與重組速率的相關性看來，不同程度的重組速率會影響選汰的能力。總之，高重組區域有較高的負向選汰效率支持了重組的優點；然而在紅果蠅年輕的neo-Y染色體中，並沒有看到選汰效率的減低，這代表重組對選汰的影響可能無法在短時間內被偵測到。

並列關鍵字

recombination ； Drosophila ； selection efficacy ； neo-sex chromosomes

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