The phenomenon of transposition induction by heavy heat shock (HHS) was studied. Males of a
Drosophila isogenic line with a mutation in the major gene radius incompletus (ri) were treated by
HHS (37 °C for 1 h followed by 4 °C for 1 h, with the cycle repeated three times) and crossed to
untreated females of the same line. The males were crossed 5 d after heat shock, and also 9 d after
HHS. Many transpositions were seen in the F1 larvae by in situ hybridization. The rate of induced
transposition was at least 2 orders of magnitude greater than that of the control sample, and was
estimated to be 0·11 events per transposable element copy per sperm. Two ‘hot’ subdivisions for
transpositions, induced probably during the post-meiotic stage of spermiogenesis, were found: 43B
and 97DE. Three-quarters of all transpositions were localized in these positions. In other sites the
rates of induced transpositions were (1·3–3·2)×10−2
events per occupied segment per sperm, 1 order of magnitude greater than those of the control.