A considerable proportion of visible mutations is reported to be caused by the insertion of mobile genetic elements in Drosophila and other organisms. We estimated transposition rates of some Drosophia mobile elements by using the lines AW and JH in which spontaneous mutations have been accumulated independently for about 400 generations. Occupied sites of the mobile elements were detected by in situ hybridization on the salivary gland chromosomes sampled from 40 AW and 30 JH lines. The rates of insertion and excision of the copia and two copia-like elements, 412 and 17.6, are very low: Insertions occurred at up to 10-3 per second chromosome per generation (17.6) and excision occurred at about 10i-per site per generation (copia and 412). Insertions of the I and hobo elements occurred much more frequently. These estimates are not only important for assessing the actual rate of various types of mutations but also for developing an evolutionary theory of mobile elements themselves.A large amount ofgenetic variation commonly exists in natural populations of Drosophila and other organisms. Clearly, genetic variation is the source of adaptive microevolution and understanding the maintenance mechanism of such variation has been a core problem of evolutionary theory (1). Reliable estimate of spontaneous mutation rate is essential for quantitative analysis of genetic variation maintained in natural populations. Mutation rate is, however, known to be influenced by both environmental stresses and genomic components such as insertion sequences. It has been suggested that many of the spontaneous visible mutations are insertional ones (2). Recent molecular analysis of the white locus in Drosophila melanogaster suggests that the majority of spontaneous mutations may be due to insertion of moderately repetitive DNA or mobile elements (3). These elements are also shown to have induced lethal mutations (4), viability mutations (5, 6), and other mutations ofquantitative traits such as bristle number (7) (Fig. 1) The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
