Genetic and molecular investigations were carried out with Eurasian Drosophila melanogaster populations on the P-M system of hybrid dysgenesis. In 27 strains sampled from France to Middle Asia, a clear gradient exists between Western Europe, in which most modern strains are of the Q type, and eastern areas, in which M-cytotype strains predominate. Molecular analysis on individual flies was performed with two complementary probes of the cloned 2.9-kilobase P element. The results provide evidence for a gradually decreasing frequency of P elements from west to east, but the presence of P-homologous sequences has been ascertained in all of the wild M-cytotype populations analyzed. Moreover, some active P elements with GD sterility potential were revealed in the majority of M-cytotype populations when tested with a highly sensitive reference line. The gradual change in distribution of the polymorphic P family in Eurasia is discussed in relation to the structure of the elements together with the theories of P-M evolution and is interpreted as the present invasion of Eurasian populations by these elements.About 10% of the genome ofDrosophila melanogaster exists as dispersed moderately repetitive sequences belonging to different families (1). The P family is composed of mobile dispersed genetic elements implicated in the P-M system of hybrid dysgenesis. This phenomenon, which is manifested in certain interstrain hybrids, results in a number of correlated aberrant genetic traits-e.g., high frequencies of gonadal sterility (GD sterility), mutation, and male recombination (2). Three types of individuals, P, Q, and M, have been described on the basis of their cross-effect properties. Hybrids between P males and M females show dysgenic traits that are reduced or absent in the reciprocal hybrids. Q individuals do not exhibit GD sterility in any cross-combinations but produce mutations and male recombinations in crosses with M females (3, 4).In the P-M system, hybrid dysgenesis results from interactions between chromosomally linked factors (P factors) and a particular type of cellular state referred to as the M cytotype (5). The P factors are active genetic elements of the P family, whose members are heterogeneous in size [0.5-2.9 kilobases (kb)], but which share substantial sequence homology (6-8). All P and Q strains thus far examined bear 30-50 copies of the P family (7). Q individuals are thought to carry a subset of the P-element family that apparently lacks sterility potential while retaining mutator activity and other Pelement functions (7,9,10). Conversely, all long-established laboratory M strains that have been examined completely lack homology with the P-element family (7). Some strains showing the M cytotype but with some homology to P sequences have also been found in laboratory collections (7).In this paper, such strains will be called M', the term M strain being reserved for strains of the M cytotype with no P homology at all.The M-cellular state component of the P-M interaction may be considered as a "s...

Guanine residues in nucleic acids can be modified by treatment with N-acetoxy-N-2-acetylaminofluorene and its 7-iodo derivative in an in vitro nonenzymatic reaction. The modified nucleic acids (ribo or deoxyribo, single or double stranded) are recognized by specific antibodies. They can be immunoprecipitated or used as probes in hybridization experiments and detected by immunochemical techniques.

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