Type conversions among the three types, P, Q and M, of the P-M hybrid dysgenesis in
Drosophila melanogaster were examined. Among 54 isofemale lines established from two natural populations, about 30% of the lines have shown type conversions during laboratory culture for two years. These were M→Q, Q→M, P→Q and P→M in order of frequency. In hybrid lines (H
MP) started from M×P crosses, flies receiving a P type chromosome rapidly changed their cytotype away from the M toward the P or the Q type. However, in flies consisted of only M strain-derived chromosomes, cytotype conversions were comparatively slow. Similarly, P factor activities of hybrid lines increased with generation number in flies having a P type chromosome in the genome but not in flies without it. M strain-derived chromosomes were isolated to make the H
MP[e] series after several generations of exposure to the P type chromosome. In these series, P type lines appeared when the P type chromosome was removed in early generations of the H
MP lines. P factors did not increase, however, with the same procedures in the later generations of the H
MP lines. Circumstances related to type conversions in the P-M system were discussed: (1) P and Q factors and the M states must coexist in a genome and even in one chromosome. (2) P and Q factors or cytotype determinants transpose on a certain cytotype background. (3) How do the interactions between P or Q factors and cytotypes lead to expression of dysgenic traits?
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