DYNAMIC EQUILIBRIUM BETWEEN INSERTION AND EXCISION OF P-ELEMENTS IN HIGHLY INBRED LINES FROM AN M' STRAIN OF DROSOPHILA-MELANOGASTER

Six highly inbred lines of Drosophila melanogaster extracted from an M ' strain (in the P/M system of hybrid dysgenesis) were studied for the evolution of the number and chromosomal location of complete and defe ctive P elements through generations 52-200. These lines possessed ful l-sized P elements but differed in their cytotype (M or P). Three line s with P cytotype and full-sized P elements at site IA had a constant P copy number over generations with low rates of insertion and excisio n. Three lines with M cytotype and at least one full-sized P element a ccumulated P copies over the generations and reached a plateau near ge neration 196, at which rates of transposition and excision were equal to 1.2 x 10(-3) to 3 x 10(-3) events per element per generation. At th at time these three lines still presented an M cytotype, produced tran sposase, and were able to regulate P copy number. The similarity at eq uilibrium between insertion and excision rates was exactly what was ex pected from theoretical models for a self-regulated element. The large number of generations necessary to attain the equilibrium in copy num ber indicates, however, that caution may be de rigueur when testing th eoretical models of copy-number containment based on transposition and excision-rate comparison.