Transition of R- to I-type in inbred lines of Drosophila melanogaster.

In several branches derived from an inbred line of Drosophila melanogaster, transitions from R- to I-type in the I-R system have been observed. After a recent split into two branches of an R-type (+)-K subline, one branch showed, on retesting, an alteration to I-type. Genome from the R-type (+)-K branch was introduced into two R-type sublines with strong and fairly weak R-reactivity, respectively. The original R-type (+)-K and the two new derivates were each split into five sublines. The new I-type line was also split into five sublines. These fifteen R- and five I-type sublines were repeatedly tested for about 50 generations. One of the R-sublines with strong R-reactivity changes to I-type. None of the five I-type sublines showed any change in the reaction pattern. Another line, which had been derived by recombination from an I-type line, showed R-type reaction in repeated tests. After 23 generations of mass-mating, this line gave indications of inductivity in offspring from a single male. Subsequent splitting into five sublines confirmed that one of them rapidly switched to I-type. The consequences are discussed in relation to the models proposed to explain the existence of R- and I-lines.