Chromosomal distribution of P and I transposable elements in a natural population of Drosophila melanogaster

The chromosomal distribution of P and I transposable elements was studied, by in situ hybridisation, in 25 isofemale lines of Drosophila melanogaster collected at Nasr'Allah in Tunisia. An important interline variability for the number of copies of both elements was revealed. The mean number of copies per line was 31.3 for P and 21.0 for I. Certain chromosome arms had a higher frequency of copies than others: arm 3R had the highest frequency of I elements; the X chromosome had the highest frequency of P elements and the lowest frequency of I elements. For both P and I elements the number of copies on the different chromosome arms is independent. Furthermore, there is no significant correlation between the number of copies of P and the number of copies of I for a given line. A study of the localisation of hybridisation sites on the X chromosome revealed the existence of preferred regions for each family. The population studied was of type M' in the P-M system of hybrid dysgenesis. There is no direct relationship between the M potential of an isofemale line and its number of copies of P elements. These results are compared with those of other investigators and the consequences for cytotype determination are discussed.