[New evidence for the induction of mobile genetic element transpositions by severe heat shock]

The induction of retrotransposon 412 transpositions by stress was studied in detail. Males of an isogenic line carrying the radius incompletus (ri) mutation of the Mendelian gene were exposed to heavy heat shock (HHS). The procedure consisted of treatment at 37 degrees C for 1 h and at 4 degrees C for 1 h, with reciprocal changes of developmental temperature 3 times, sequentially; the males were then crossed with untreated females. The same males were crossed both on the fifth and ninth day after the HHS treatment. On the basis of in situ hybridization in 85 F1 larvae, 193 transpositions were identified. After treatment, the transposition rate increased by two orders of magnitude (compared to control) and amounted to 0.11 events per site of the original isogenic line per spermium per generation. Two hot sites (segments) of preferential transposition localization, 43B and 97CD, were detected after the first cross; these sites comprised more than 3/4 of all transpositions. Sperm from the first cross were exposed to HHS during the time period of 120 to 244 h after the appearance of the corresponding germline cells, probably at the stage of spermatid maturation. The overinduction of transpositions was shown to occur in these sites and at this stage. In the remaining sites, after the first cross, and in all sites, after the second cross, the rate of induced transpositions was (1.3-3.1) x 10(-2) events per site per spermium per generation, which is higher than in the control by an order of magnitude. This basic induction level was observed at all stages of spermatogenesis. The induction of transposition by heavy heat shock may be considered established.