Non-mendelian female sterility in Drosophila melanogaster: hereditary transmission of I factor.

Systematic crosses between various strains of Drosophila melanogaster lead in some cases to partly sterile F1 females. Two main classes of strains, inducer and reactive, may be recognized on the basis of the fertility of F1 female progeny. Females which may show incomplete sterilty (SF female) arise only when reactive females are crossed with inducer males, other crosses, including the reciprocal, producing only fertile F1 females. SF sterility appears as the result of an interaction between two factors, R brought into the initial cross by the reactive mother and maternally inherited, and I brought by the inducer father. The present paper reports on the hereditary transmission of I factor. It is shown that when transmitted through heterozygous males, bearing chromosomes of both inducer and reactive origin, I factor may be strictly linked to any one of the three major chromosomes of inducer strains. Such chromosomes carrying I factor were called inducer chromosomes. When transmitted through heterozygous females, this Mendelian behavior fails to hold, and non-inducer chromosomes coming from reactive strains may become inducer independently of the production of recombined gametes. This phenomenon was called chromosomal contamination. This contamination occurs even between nonhomologous chromosomes.