Transposable and nontransposable elements similar to the I factor involved in inducer-reactive (IR) hybrid dysgenesis in Drosophila melanogaster coexist in various Drosophila species.

The I factor is a transposable element controlling inducer-reactive (IR) hybrid dysgenesis in Drosophila melanogaster, which occurs when males from the class of inducer strains are crossed with females from the class of reactive strains. Inducer strains contain several copies of the complete 5.4-kilobase (kb) I factor at various sites on the chromosomal arms; reactive strains contain no complete I factor. Incomplete and defective I elements occur at constant locations in pericentromeric heterochromatin of both types of strains. The 5.4-kb I factors transpose, whereas incomplete I elements do not transpose. The constant location of defective I elements in all strains indicates that they were in the genome before the spread of D. melanogaster throughout the world. Sequences homologous to I occur in other Drosophila species, and their distribution correlates with the phylogenetic relationships between species. We have studied the organization of I homologues in Drosophila simulans and Drosophila teissieri. These species seem to contain both transposable I elements, even though their structure may differ from that of the 5.4-kb I factors of the inducer strains of D. melanogaster, and nontransposable I elements, which are always at the same place in the genome when different stocks of the same species are compared. These results suggest that both mobile and nonmobile I elements are very old components of the Drosophilidae genome.