Natural selection maintains the transcribed LTR retrotransposons in Nosema bombycis  

作　　者：Heng Xiang Guoqing Pan Ruizhi Zhang Jinshan Xu Tian Li Wenle Li Zeyang Zhou Zhonghuai Xiang 

机构地区：[1]Institute of Sericulture and Systems Biology, Southwest University, Beibei, Chongqing 400715, China [2]College of Life Science, Chongqing Normal University, Chongqing 400047, China [3]Biotechnology Research Center, Southwest University, Beibei, Chongqing 400715, China

出　　处：《Journal of Genetics and Genomics》2010年第5期305-314,共10页遗传学报（英文版）

基　　金：supported by the National Basic Research Program of China(No.2005CB121000);the project of Chongqing Science & Technology Commission(CSTC,No.2006AA5019 and 2009BB1241);the Programme of Introducing Talents of Discipline to Universities(No.B07045);State Development Fund at Risk of Callus Silk(No.M012005-000Y-00070)

摘　　要：Eight intact LTR retrotransposons （Nbr1-Nbr8） have been previously characterized from the genome of Nosema bombycis, a eu- karyotic parasite with a compact and reduced genome. Here we describe six novel transcribed Nbr elements （Nbr9-Nbr14） identified through either cDNA library or RT-PCR. Like previously determined ones, all of them belong to the Ty3/Gypsy superfamily. Retrotransposon diversity and incomplete domains with insertions （Nbr12）, deletions （Nbrll） and in-frame stop codons in coding regions （Nbr9） were detected, suggesting that both defective and loss events of LTR retrotransposon have happened in N. bornbycis genome. Analysis of selection showed that strong purifying selection acts on all elements except Nbr11. This implies that selective pressure keeps both these Nbrs and their functions in genome. Interestingly, Nbrll is under positive selection and some positively selected codons were identified, indicating that new functionality might have evolved in the Nbrll retrotransposon. Unlike other transposable elements, Nbrll has integrated into a conserved syntenic block and probably resulted in the inversion of both flanking regions. This demonstrates that transposable element is an important factor for the reshuffling and evolution of their host genomes, and may be maintained under natural selection.Eight intact LTR retrotransposons （Nbr1-Nbr8） have been previously characterized from the genome of Nosema bombycis, a eu- karyotic parasite with a compact and reduced genome. Here we describe six novel transcribed Nbr elements （Nbr9-Nbr14） identified through either cDNA library or RT-PCR. Like previously determined ones, all of them belong to the Ty3/Gypsy superfamily. Retrotransposon diversity and incomplete domains with insertions （Nbr12）, deletions （Nbrll） and in-frame stop codons in coding regions （Nbr9） were detected, suggesting that both defective and loss events of LTR retrotransposon have happened in N. bornbycis genome. Analysis of selection showed that strong purifying selection acts on all elements except Nbr11. This implies that selective pressure keeps both these Nbrs and their functions in genome. Interestingly, Nbrll is under positive selection and some positively selected codons were identified, indicating that new functionality might have evolved in the Nbrll retrotransposon. Unlike other transposable elements, Nbrll has integrated into a conserved syntenic block and probably resulted in the inversion of both flanking regions. This demonstrates that transposable element is an important factor for the reshuffling and evolution of their host genomes, and may be maintained under natural selection.

关 键 词：MICROSPORIDIA LTR retrotransposon transcribed purifying selection positive selection 

分 类 号：Q943[生物学—植物学] S884.21[农业科学—特种经济动物饲养]