Many inbred and isofemale lines derived from wild populations of Drosophila melanogaster were tested for gonadal dysgenic sterility, male recombination and snw secondary mutation. Among them, we have found strains whose dysgenic offspring show negligible sterility, and undetectable male recombination and snw mutation. They can be considered to be neutral strains in the strict sense. Such neutral strains appear to carry only defective P elements in their genomes. Taking the observations of Karess & Rubin (1984) into account, it is suggested that some defective P elements retain the function necessary for P cytotype. Cytotype determination mechanisms are discussed.
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