異位表現 Odysseus 造成有絲分裂的缺陷

Abstract

在遺傳學的觀點上，種化基因指的是會造成物種間生殖隔離的基因。Odysseus (OdsH) 是第一個被發現，會造成果蠅的兩個姊妹種，擬黃果蠅和模里西斯果蠅的雜交雄性子代不孕的基因。根據DNA序列分析結果顯示，OdsH是由unc-4發生基因重複事件所產生，兩者都含有一個同源結構區，在黃果蠅種亞群中，擬黃果蠅和模里西斯果蠅的同源結構區有快速演化的現象。根據之前的研究顯示， 在黃果蠅中，OdsH主要表現在雄性成蟲的精巢，其功能被推測為藉由加速精子的生成率，以提高雄性的生殖率，但是到目前為止分子機制還不清楚。隨著技術的進步，果蠅的基因功能機制，可以利用UAS/Gal4系統，以異位表現的方式進一步分析。根據三個物種的OdsH在黃果蠅雌雄生殖系統表現的結果顯示，只有模里西斯果蠅的OdsH會造成雄性和雌性成蟲生殖器官發育異常，黃果蠅和擬黃果蠅的OdsH則不會。而當三個物種的OdsH在果蠅眼睛表現，都會造成嚴重的發育缺陷，造成小眼。根據胚胎以及幼蟲眼睛的觀察，當黃果蠅的OdsH表現時，會使得細胞週期受到干擾，並且是在細胞分裂期產生缺陷。因此，推測OdsH功能作用的分子機制為，參與細胞週期中的細胞分裂。結合最近在細胞株的觀察，發現OdsH可以結合到染色體中節的區域，OdsH在擬黃果蠅和模里西斯果蠅間造成的雄性子代不孕，可能符合centromere-drive model。

Genes that contribute to the reproductive isolation between species are known as “speciation genes’. Odysseus (OdsH) is a hybrid male sterility gene between Drosophila mauritiana and D. simulans. According to sequence analysis, OdsH duplicated from unc-4 recently in the Drosophila genome preserving the intron exon structure and the paired-like homeodomain. Among D. melaongaster subgroup, the homeodomain of OdsH in D. mauritiana, and D. simulans evolved rapidly. Previous studies showed that OdsH is highly expressed in the testes, and the function of OdsHmel is to enhance male fertility by accelerating the rate of sperm maturation. Three OdsH alleles from D. mauritiana, D. simulans, and D. melanogaster were ectopically expressed in D. melanogaster with the Gal4/UAS system to uncover the molecular mechanism for the function of OdsH. In germline expression, OdsHmau caused developmental defect in the reproductive organs. Germline expression of OdsHmel in females caused the overlapping of nuclei in the embryos and embryonic lethality. In the eyes, all three alleles caused critical defect in the development resulting in small-eye phenotype. Results from embryos and eye discs indicate that cell cycle was perturbed at the mitotic phase with the expression of OdsHmel. Therefore, the molecular mechanism of OdsH appears to play a role in the M phase of cell cycle regulation. Combined with the recent data in the cell culture indicating that OdsH can bind to pericentric satellite DNAs, the mechanism for OdsH to cause hybrid male sterility between D. mauritiana and D. simulans supports the centromere- drive model.