ABSTRACT
Retroelements form a large and diverse family of mobile elements that can be found in all eultaryotes. They all share a common mechanism o f replication in that they propagate by reverse transcription o f A intermediates, and integrate their genetic information into ehe genome o f the host cell. The
A into D N A is the reverse Eanscr?ptase (RT) (Varmus and Swanstrom, 1985). RT were used to determine the evolutionary relationships among the retro-
elements (Xiong and Eickbusb. 1990; Springer et al., 1995). The phylogeny determjned in this way fits well with the general organization of these eiernents, i ding the liucleocidic strdctures o f the 5' and 3' extremities, the nurnber o f open reading frames ( S) , and the organization o f the polymerase domain. This classification pennits the distil-icdon o f t major classes o f e u k q o d c retcoe!emerits
(Table 10.1). This chapter will focus on three of those ciasses: long terminal repeat-retrotransposons (LTR-RT), endogenous retroviruses (EnRV); and exogenous retroviruses (ExRV). Two of these (ETRRT and EnRV) are present in the genomes of insects, are integrated in the genome of the germ cells, and are transmitted vertically like cellular genes. The "true" retroviruses (ExRV) propagate strictiy horizontally by cell-to-cell infection, and so far have been described only in vertebrates. The ExRV class is included in this chapter for two main reasons: (1) some stnlctural and functional properties have not yet been described for LT y may be infen-ed given similarities with Ex and (2) a large number of tool ed on ExRV. The chapter will compare structuraand func'tionai properiies of these three classes of retroelements and emphasize their simiParities where possible. The characteristics of the Gypy EnRV element from Drosophila will then be developed. Finally. the potential use of the EnRV as tools for insect transgenesis will be discuss.
