果蠅脂肪酸去飽和酶2的分子演化

Abstract

去飽和酶1，去飽和酶2和脂肪酸去飽和酶2在具性費洛蒙功用的果蠅表皮碳氫化合物的合成過程中扮演重要的角色。而脂肪酸去飽和酶2在黃果蠅和賽昔爾果蠅中參與合成雌性所特有的費洛蒙中的二烯化合物。但是我們仍然不了解它在其他種果蠅中所扮演的角色。我在十二種果蠅的基因體中找出脂肪酸去飽和酶2可能的同源基因並加以分析。而結果顯示脂肪酸去飽和酶2可能是經由反轉錄移位所產生的基因，產生的時間點可能在水果果蠅和果蠅亞屬分化之前，但在後者中失去。我也發現脂肪酸去飽和酶2在黃果蠅種亞群中和雌雄二型性表皮碳氫化合物有直接關連，此基因只會表現在具有二型性表皮碳氫化合物的果蠅；在單型性果蠅則沒有。為了要研究脂肪酸去飽和酶2在雌雄單型性表皮碳氫化合物的果蠅中是否有發生退化現象，我分析了擬黃果蠅，亞庫巴果蠅和德氏果蠅的族群，結果在二十個擬黃果蠅的品系中找到一個品系有二十五個鹼基對的缺失，在十六個德氏果蠅的品系內發現其中四個品系帶有無義突變，而在二十個亞庫巴果蠅品系中，有十四種插入和刪除的序列缺失，導致只有三個品系具有完整的編碼序列。脂肪酸去飽和酶2在三種果蠅中有不同程度的突變累積，代表它在各種果蠅中的退化可能是獨立發生的。脂肪酸去飽和酶2提供了一個研究基因重複的分子演化的模式。

Three fatty acid desaturase genes, namely desat1, desat2 and Fad2, in Drosophila involve in the production of cuticular hydrocarbons (CHs) that serve as sex pheromones. Among them, Fad2 is responsible for female-specific diene pheromone biosynthesis in D. melanogaster and D. sechellia. However, the role of Fad2 in other Drosophila species is still unknown. To address this question, the Fad2 homologs in 12 Drosophila species were analyzed. Based on the sequence information, Fad2 is originated by retrotransposition which took place before the split of Drosophila and Sophophora and subsequently lost in the Drosophila lineage. Furthermore, in D. melanogaster species subgroup, Fad2 is expressed only in the species with dimorphic CHs. These results suggest that Fad2 is responsible for sexually dimorphic CHs in the D. melanogaster species subgroup. To understand whether sequence degeneration also occurred in the non-expressed Fad2, populations of three CH monomorphic species, i.e., D. simulans, D. teissieri, and D. yakuba, were surveyed. A 25-bp deletion was observed in one of 20 D. simulans alleles. One non-sense mutation occurred in 4 of 16 D. teissieri sequences. In D. yakuba, 14 deletions and non-sense mutation resulted in that only 3 out of 20 samples keep full-length Fad2. The different levels of mutation accumulation indicate that the degeneration of Fad2 happened independently in each species. In summary, Fad2 provides a good model to study the molecular evolution after gene duplication.