Pélisson A, Teysset L, Chalvet F, Kim A, Prud'homme N, Terzian C, Bucheton A
Centre de Génétique Moléculaire du CNRS (UPR 9061), l'Université P. & M. Curie, Paris VI, Gif-sur-Yvette, France.
Genetica. 1997;100(1-3):29-37.
The gypsy element of Drosophila melanogaster is the first retrovirus identified so far in invertebrates. According to phylogenetic data, gypsy belongs to the same group as the Ty3 class of LTR-retrotransposons, which suggests that retroviruses evolved from this kind of retroelements before the radiation of vertebrates. There are other invertebrate retroelements that are also likely to be endogenous retroviruses because they share with gypsy some structural and functional retroviral-like characteristics. Gypsy is controlled by a Drosophila gene called flamenco, the restrictive alleles of which maintain the retrovirus in a repressed state. In permissive strains, functional gypsy elements transpose at high frequency and produce infective particles. Defective gypsy proviruses located in pericentromeric heterochromatin of all strains seem to be very old components of the genome of Drosophila melanogaster, which indicates that gypsy invaded this species, or an ancestor, a long time ago. At that time, Drosophila melanogaster presumably contained permissive alleles of the flamenco gene. One can imagine that the species survived to the increase of genetic load caused by the retroviral invasion because restrictive alleles of flamenco were selected. The characterization of a retrovirus in Drosophila, one of the most advanced model organisms for molecular genetics, provides us with an exceptional clue to study how a species can resist a retroviral invasion.
果蝇中的吉普赛元件是迄今为止在无脊椎动物中鉴定出的首个逆转录病毒。根据系统发育数据，吉普赛与LTR逆转录转座子的Ty3类属于同一组，这表明逆转录病毒在脊椎动物辐射之前就从这类逆转元件进化而来。还有其他一些无脊椎动物逆转元件也可能是内源性逆转录病毒，因为它们与吉普赛元件具有一些结构和功能上类似逆转录病毒的特征。吉普赛元件受果蝇的一个名为弗拉门戈的基因控制，该基因的限制性等位基因使逆转录病毒处于抑制状态。在允许的品系中，功能性吉普赛元件高频转座并产生感染性颗粒。位于所有品系着丝粒周围异染色质中的有缺陷的吉普赛原病毒似乎是黑腹果蝇基因组中非常古老的组成部分，这表明吉普赛元件在很久以前就侵入了这个物种或其祖先。当时，黑腹果蝇可能含有弗拉门戈基因的允许等位基因。可以想象，由于弗拉门戈基因的限制性等位基因被选择，该物种在逆转录病毒入侵导致的遗传负荷增加中得以存活。对果蝇（分子遗传学中最先进的模式生物之一）中的一种逆转录病毒的表征，为我们研究一个物种如何抵抗逆转录病毒入侵提供了一个特殊线索。
