Instability in the ctMR2 strain of Drosophila melanogaster: role of P element functions and structure of revertants.

Simultaneous multiple transpositions and long-term genetic instability have been described in the ctMR2 strain of Drosophila melanogaster and its derivatives. This strain originated from a cross that was dysgenic in the P-M system. While spontaneous instability declined over 2 years, instability has been reactivated by backcross to the progenitor P element bearing strain MRh12/Cy. We show here using germline transformation that active P factor alone cannot mimic the effect of this cross, suggesting that MRh12/Cy contains some other activator. In addition, we have observed that ct+ exceptional progeny arise in the F1 as well as the F2 generations. Molecular analysis of X chromosomes from some ct+ progeny indicates that phenotypic reversion of the ct mutation can arise through two unrelated mechanisms.