To characterize the relationship between P element activities and their structures, we cloned P elements from genomic libraries of three isogenic P and Q strains derived from natural populations in Japan. These P elements were mapped with Bamlll, Avail and Pstl and were classified by their size. The majority of P elements cloned were classified as either complete or relatively small P elements rather than medium size. The numbers of full length (2.9 kb) P elements per haploid genome of NP280 (P), AK194 (weak P) and WY113 (Q) were at least four, five and one, respectively. However, the 2.9 kb P element of WY113 was thought to be defective since this strain has no transposase activity. In our previous work, we demonstrated that the ORF 3-deleted P element is essential for P cytotype determination in WY113. A similar P element also exists in NP280, and this may have an important role for P cytotype determination in this strain. Two and one copies of the KP element, a deletion derivative of the P element, were found in NP280 and AK194, respectively. One of four complete P elements in NP280 was fully sequenced, and the base sequence was completely identical to that of pπ25.1 originally derived from the U.S.A. This result is consistent with the notion that these P elements have a relatively recent origin in Drosophila melanogaster.
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