Evidence for the somatic transposition of mobile DNAs in life forms other than mammals is mixed. Tc1, the major DNA transposon of Caenorhabditis elegans, is able to transpose in somatic cells , whereas in Drosophila species, P elements and the I factor, an L1-like non-LTR retrotransposon, appear limited to germ-cell mobility [3, 4]. Now, Eickbush and Eickbush, in a study published in Mobile DNA , find that transposition of another Drosophila autonomous retrotransposon, R2, can occur in somatic cells during early embryonic development. R2 is a non-LTR retrotransposon with many features in common with mammalian L1 elements, most notably its ability to reverse transcribe and integrate in a single step directly into chromosomal DNA . However, R2 differs from L1 in that it inserts at a single site in the 28S rRNA genes whereas L1 can enter the genome at a very large number of essentially random, short consensus sequences. In addition, the endonucleases encoded by the two elements differ in their position within the element and their enzymatic type (restriction enzyme type IIs for R2 and apurinic/apyrimidinic endonuclease for L1).
Using single-step PCR, Eickbush and Eickbush found evidence for 15 somatic early embryonic insertions in 7 of 29 flies studied. This number of somatic insertions is clearly a minimum, and one wonders how many more insertions would have been detected with two-step, nested PCR. The detected insertions were present in multiple tissues of both the adult and larval stages, and had all the characteristics of authentic R2 insertions. They occurred at, or very close to, the 28S rRNA gene insertion site of R2. Many had a few non-templated nucleotides at the 5' end, and all were 5' truncated. Because the same somatic insertions were found in different tissues, the timing of many events could be estimated as early in development, before the differentiation of tissues, including the germ line.
A previous study of 213 R2 insertions in the offspring of a single female fly found 32 new insertions . Twenty-seven of these were clear germline events, occurring in a single fly each. The remaining five appeared to be derived from somatic events because the identical insertion occurred in more than one fly, meaning that one parent was a germline mosaic and had the same insertion present in many but not all germ cells. It was also previously known that R2 retrotransposition occurs much less frequently in males than in females, and now Eickbush and Eickbush suggest that perhaps all of male R2 retrotransposition may be due to germline mosaicism. Thus, it appears that the incidence of somatic R2 retrotransposition is not very dissimilar from that of germline R2 insertion.