果蠅脂肪酸延長酶基因之分子演化與表現

Abstract

果蠅的脂肪酸延長酶（fatty acid elongase）基因家族擁有許多重要功能，例如維持脂質恆定、雄性生殖與費洛蒙生合成。在黃果蠅（Drosophila melanogaster）的20個延長酶基因當中，4個是雄性專一表現，1個是雌性專一；4個雄性專一的延長酶皆表現於生殖系統，另外有2個兩性都會表現的延長酶對精子形成不可或缺；而1個雄性專一與1個雌性專一的延長酶參與費洛蒙生合成，因此和性別與物種辨識有關。然而，延長酶基因家族的演化，與其對於演化的創新貢獻仍不清楚。為了回答以上問題，我首先系統性的尋找20種果蠅中的延長酶同源基因（homolog），然後在成蟲期測試10種果蠅的基因，是否於兩性中表現。我發現各種果蠅的延長酶數量介於14到21個，許多演化支系經歷多次基因複製與喪失。成蟲期時，大部分延長酶同源基因皆表現於兩性，少數則是性別專一或兩性皆不表現；大部分性別專一表現的延長酶基因在各個親緣關係接近的直系同源基因（ortholog）中，性別表現形態並不一致，意謂調控改變可能對兩性雙型性（sexual dimorphism）的演化非常重要。接著進一步分析各個延長酶直系同源基因的同義取代/異義取代率，以及測試某些密碼子是否受到正向選汰（positive selection）。大部分延長酶基因的同義取代/異義取代率都小於1，意謂淨化選汰（purifying selection）可能為延長酶演化的主要影響力；然而有些延長酶基因上的密碼子，也許受到正向選汰影響。總之，本研究認為果蠅延長酶的基因複製與調控改變，皆可能對於果蠅演化有創新貢獻。

Fatty acid elongase gene family in Drosophila plays several important roles such as lipid homeostasis, male reproduction, and pheromone biosynthesis. Among 20 fatty acid elongase members in D. melanogaster, four and one have been reported to be male-specific and female-specific, respectively. All of four male-specific elongases are expressed in the reproductive system. In addition, two bisexually- expressed elongases are essential for spermatogenesis. One male-specific and one female-specific elongases are involved in the biosynthesis of pheromones that associated with the sex and species recognition. However, the evolution and contributions of evolutionary novelty of elongase gene family in Drosophila are unclear. To address these questions, I first conducted a systematic search of elongase homologs in 20 Drosophila species, and then surveyed the gene expression of adult flies in 10 species. I found the number of elongase genes in each species range from 14 to 21, and multiple gene gains and losses events occurred in several lineages. Most elongase homologs were bisexually expressed, and few homologs were sex-specific or not expressed in adult flies. Most of the sex-specific elongase homologs displayed distinct sexual expression patterns between closely related species, suggesting that regulatory changes played an important role on the evolution of sexual dimorphism. The free-ratio analysis was further performed to evaluate the dN/dS ratio of each lineage in elongase orthologs, and NSsites analysis was used to reveal whether certain codons of each elongase ortholog were under positive selection. The dN/dS values of most lineages were less than one, suggesting that most elongase genes were under purifying selection. Nevertheless, some codons of several orthologs might be under positive selection. In summary, both gene duplication and regulatory changes may contribute to the evolutionary novelty of the elongase genes in Drosophila.