GERM-LINE TRANSPOSITION OF THE COPIA RETROTRANSPOSON IN DROSOPHILA-MELANOGASTER IS RESTRICTED TO MALES BY TISSUE-SPECIFIC CONTROL OF COPIA RNA LEVELS

Germ line transposition rates of the retrotransposon copia were direct ly measured in males and females of an inbred Drosophila melanogaster line, 2b3, which is highly polymorphic for copia insertion sites. The elevated germ line transposition rate of copia in this line (3-8 x 10( -3) per generation per element) is confined to males, with transpositi on in females being undetectable under the conditions of the experimen t but at most 50-fold lower than the rate for males. To determine the molecular basis of this effect, copia RNA levels were measured in whol e bodies and germ lines of male and female flies of both the unstable 2b3 line and a stable line, Oregon RC-iso, which shows normal rates of copia transposition. Both male and female 2b3 flies contain much more copia RNA than flies of the stable line. However, 2b3 male germinal t issues contain much higher levels of copia RNA than the equivalent fem ale tissues. The highest copia expression is detected in maturing prim ary spermatocytes. Our data show that high rates of germ line copia tr ansposition are restricted to males by tissue-specific control of RNA levels and suggest that transposition of copia only occurs in fly tiss ues containing more than a relatively high threshold level of copia RN A.