DIRECT DETERMINATION OF RETROTRANSPOSON TRANSPOSITION RATES IN DROSOPHILA-MELANOGASTER

Rates of transposition and excision of the Drosophila melanogaster ret rotransposon elements mdg3, 297, Doc, roo and copia were estimated dir ectly, by in situ hybridization analysis of their cytological insertio n sites in 31 replicates of a highly inbred line that had accumulated spontaneous mutations for approximately 160 generations. Estimated tra nsposition rates of Doc, roo and copia were, respectively, 4.2 x 10(-5 ), 3.1 x 10(-4) and 1.3 x 10(-3); no transpositions of 297 nor mdg3 we re observed. Rates of transposition of copia varied significantly amon g sublines. Excisions were only observed for roo elements, at a rate o f 9.0 x 10(-6) per element per generation. Copy number averaged over t hese element families increased 5.9 %; therefore, in these lines the m agnitude of the forces opposing transposable element multiplication we re weaker than transposition rates. Estimated total genomic mutation r ates from transposition are of the same order as the nucleotide mutati on rate in this species.