P-element transposition in Drosophila melanogaster: influence of size and arrangement in pairs

In Drosophila melanogaster, several factors have been suggested to influenc e the rates of P-element transposition and excision. including position eff ects, size and structure of the elements and differences in transposase sou rce. We have investigated the effect of the size of the starting P-element on the rates of excision and transposition. Four transgenes localized at th e same insertion site on the X chromosome and which differ by the number of copies of an internal repeated sequence, were studied. Transgenes with siz es ranging from 11 kb to 22 kb excise at similar rates, and size does not c orrelate with the differences in transposition rate between them. We also s tudied the behavior of double P-elements, located at the same site and arra nged in various configurations: nested, contiguous or separated by a few ba se pairs, in the same or reverse orientation. These double P-elements displ ay different mobilities depending on the arrangement of the two transgenes. Transposition and excision rates were also studied for an insertion bearin g four transgenes in very close proximity. Our results suggest that several neighboring elements could excise together. We also propose a new model to explain the formation of all the double P-elements we describe.