Biedler James K, Tu Zhijian
Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
BMC Evol Biol. 2007 Jul 9;7:112. doi: 10.1186/1471-2148-7-112.
In contrast to DNA-mediated transposable elements (TEs), retrotransposons, particularly non-long terminal repeat retrotransposons (non-LTRs), are generally considered to have a much lower propensity towards horizontal transfer. Detailed studies on site-specific non-LTR families have demonstrated strict vertical transmission. More studies are needed with non-site-specific non-LTR families to determine whether strict vertical transmission is a phenomenon related to site specificity or a more general characteristic of all non-LTRs. Juan is a Jockey clade non-LTR retrotransposon first discovered in mosquitoes that is widely distributed in the mosquito family Culicidae. Being a non-site specific non-LTR, Juan offers an opportunity to further investigate the hypothesis that non-LTRs are genomic elements that are primarily vertically transmitted.
Systematic analysis of the ~1.3 Gbp Aedes aegypti (Ae. aegypti) genome sequence suggests that Juan-A is the only Juan-type non-LTR in Aedes aegypti. Juan-A is highly reiterated and comprises approximately 3% of the genome. Using minimum cutoffs of 90% length and 70% nucleotide (nt) identity, 663 copies were found by BLAST using the published Juan-A sequence as the query. All 663 copies are at least 95% identical to Juan-A, while 378 of these copies are 99% identical to Juan-A, indicating that the Juan-A family has been transposing recently in evolutionary history. Using the 0.34 Kb 5' UTR as the query, over 2000 copies were identified that may contain internal promoters, leading to questions on the genomic impact of Juan-A. Juan sequences were obtained by PCR, library screening, and database searches for 18 mosquito species of six genera including Aedes, Ochlerotatus, Psorophora, Culex, Deinocerites, and Wyeomyia. Comparison of host and Juan phylogenies shows overall congruence with few exceptions.
Juan-A is a major genomic component in Ae. aegypti and it has been retrotransposing recently in evolutionary history. There are also indications that Juan has been recently active in a wide range of mosquito species. Furthermore, our research demonstrates that a Jockey clade non-LTR without target site-specificity has been sustained by vertical transmission in the mosquito family. These results strengthen the argument that non-LTRs tend to be genomic elements capable of persistence by vertical descent over a long evolutionary time.
与DNA介导的转座元件（TEs）不同，逆转座子，尤其是非长末端重复逆转座子（non-LTRs），通常被认为水平转移的倾向要低得多。对位点特异性非LTR家族的详细研究表明其具有严格的垂直传递。需要对非位点特异性非LTR家族进行更多研究，以确定严格的垂直传递是与位点特异性相关的现象，还是所有非LTR的更普遍特征。Juan是一种Jockey进化枝非LTR逆转座子，最早在蚊子中发现，广泛分布于蚊科。作为一种非位点特异性非LTR，Juan为进一步研究非LTR是主要通过垂直传递的基因组元件这一假说提供了机会。
对约1.3 Gbp的埃及伊蚊基因组序列进行系统分析表明，Juan-A是埃及伊蚊中唯一的Juan型非LTR。Juan-A高度重复，约占基因组的3%。使用90%长度和70%核苷酸（nt）同一性的最小截止值，以已发表的Juan-A序列作为查询序列，通过BLAST发现了663个拷贝。所有663个拷贝与Juan-A至少95%相同，其中378个拷贝与Juan-A 99%相同，表明Juan-A家族在进化史上最近一直在转座。以0.34 Kb的5'UTR作为查询序列，鉴定出2000多个可能包含内部启动子的拷贝，这引发了关于Juan-A对基因组影响的问题。通过PCR、文库筛选和数据库搜索，获得了包括伊蚊属、奥库蚊属、骚蚊属、库蚊属、巨蚊属和韦氏蚊属在内的六个属的18种蚊子的Juan序列。宿主和Juan系统发育的比较显示总体一致，只有少数例外。
Juan-A是埃及伊蚊基因组的主要组成部分，并且在进化史上最近一直在逆转座。也有迹象表明Juan最近在广泛的蚊子物种中活跃。此外，我们的研究表明，一种没有靶位点特异性的Jockey进化枝非LTR在蚊科中通过垂直传递得以维持。这些结果强化了非LTR倾向于成为能够在漫长进化时间内通过垂直遗传而持续存在的基因组元件这一观点。
