Transposable elements in Drosophila that are responsible for one type of hybrid dysgenesis (q.v.). P elements have been cloned in E. coli plasmids. When DNA molecules carrying P elements are microinjected into Drosophila embryos, some P elements integrate into the germ-line chromosomes and are transmitted to the progeny of the injected flies. Active autonomous P elements are 2.9 kb in length and are flanked by inverted repeats that are 31 bp long. They have four exons (designated ORF 0, 1, 2, and 3). All four exons encode an 87 kDa transposase, and the first three exons specify a 66 kDa repressor. P elements were discovered in D. melanogaster, but they are absent from other sister species. However, P elements are common in D. willistoni and species related to it. D. melanogaster is believed to have received its P elements in the 1950s from D. willistoni. Horizontal transfer may have been accomplished by ectoparasitic mites. See Chronology, 1982, Bingham et al., Spradling and Rubin; 1991, Houck et al.; 1994, Clark et al.; horizontal mobile elements (HMEs), promiscuous DNA, transposable elements, transposon tagging.
Subjects: Genetics and Genomics.