Males of a Drosophila melanogaster isogenic line with a mutation of the major gene for radius incompletus (ri) were treated by standard light heat shock (3TC for 90 min) and by heavy heat shock (transfer of males from 37C for 2 hr to 4rC for 1 hr and back; this procedure was repeated three times). In the F1 generation of treated males mated with nontreated females of the same isogenic line, mass transpositions of copia-like mobile genetic element Dm-412 were found. The altered positions ofthe element seem nonrandom; five "hot spots" of transposition were found. Probabilities of transpositions were estimated after light heat shock and heavy heat shock and in the control sample. These probabilities were, respectively, 3.4 x 10-2, 8.7 x 10-2, and <4.1 x 10-4 transpositions per genome per occupied position per generation. Therefore, as a result of heat shock treatment, the probabilities of transpositions were two orders of magnitude greater than those of the control sample in the next generation after induction. Comparison of the results with those after stepwise temperature treatment shows that the induction depends on the intensity of the stress action (temperature treatment) rather than on the type of the stress action. (7) and HS regulation sites (8, 9). After HS treatment of flies, transcription and transposition of different MGEs seemed to be activated (9-11).From this evidence, speculations and hypotheses have been advanced about the genetic system of HS regulation in relation to heritable temperature effects in Drosophila (4, 5, 10-12). There have also been theories about the evolutionary importance of the genome system of MGE genomic patterns after environmental stress (temperature, etc.) (4,5,(10)(11)(12).However, the data are incomplete. Vasilyeva and coworkers (1-3, 13) in most of their work used heterogeneous Drosophila lines, and in situ hybridization was done many tens of generations after temperature treatment. Indeed, in some repeated experiments this interval was decreased to several generations with the same results. But the possibility of selective influence was not excluded completely. Junakovic et al. (6) found the effects in the next generation, but their restriction fragments were not compared with patterns of MGE localization in polytene chromosomes.In this paper (see also ref. 14), we present data that demonstrate multiple transpositions of copia-like MGE Dm-412 in the next generation after HS treatment. This work was done with an isogenic Drosophila line to decrease pattern variability in the control sample and after treatment. MATERIALS AND METHODSIsogenic Lines. The isogenic lines with a mutation of the major gene for ri were created by using marked balanced chromosomes (15): M5 (X chromosome), Cy/Pm (chromosome 2), D/Sb (chromosome 3). Of 20 isogenic lines, one (N 49) was used for the HS treatment, being most stable in its pattern of MGE Dm412 localization and expression of the character ri.Control Sample. Young males and virgin females were collected from mass cultures of isog...