Retrotransposition of R2 elements in somatic nuclei during the early development of Drosophila

PCR assays were used to identify somatic R2 insertions in isolated adult tissues and larval imaginal discs of Drosophila simulans. R2 somatic mosaics were detected encompassing cells from individual tissues as well as tissues from multiple body segments. The somatic insertions had 5' junction sequences characteristic of germline insertions suggesting they represented authentic retrotransposition events.Body segments are specified early in Drosophila development, thus the detection of the same somatic insertion in cells from multiple tissues suggested that the R2 retrotransposition events had occurred before the blastoderm stage of Drosophila development. R2 activity at this stage, when embryonic nuclei are rapidly dividing in a common cytoplasm, suggests that some retrotransposition events appearing as germline events may correspond to germline mosaicism.Mobile element insertions during the development of somatic tissues provide no benefit to the element, as these insertions are not transferred to subsequent generations. Thus in animals, where the separation of somatic and germline tissues is established early, the ability of a mobile element to generate new insertions in somatic tissues would most likely be selected against. Consistent with this prediction, early studies in Drosophila melanogaster showed that P element transpositions were dependent upon a germline-specific RNA splicing component [1], and I elements were only transcribed in ovaries [2]. However, counter to this model, mobile elements in other animals have been shown to generate new insertions in somatic tissues (for example, Tc1 elements in Caenorhabditis elegans [3], L1 elements in mammals [4,5]).Several explanations can be put forward for the somatic activity of mobile elements. First, somatic events are inconsequential to the host and thus there is little selective pressure for a mobile element to evolve specificity to the germline. Second, somatic events are harmful, however it is risky for a mo