THE GENETIC CONSEQUENCES OF ANAPHASE BRIDGE FORMATION IN DROSOPHILA

Abstract

ROSSING over between ring and normal X chromosomes of D. meluno-c gaster was first studied by L. V. MORGAN (1933); the later work of STURTEVANT and BEADLE (1936) provided an understanding of the behavior of anaphase bridges resulting from crossing over in inversion heterozygotes. The present work represents an attempt to interpret the data from ring/rod heterozygotes using the rules derived from inversion heterozygotes by the above workers, with emphasis on a discrepancy between the observed and expected frequencies of that class of gamete with no X chromosome, the nullo-X class. It will be shown that this discrepancy, a deficiency of the nullo-X class, occurs also in experiments involving certain types of X inversion heterozygotes, and the bearing of the phenomenon on the nature of chro-matid separation will be discussed. CONSEQUENCES OF CROSSING OVER IN THE RING/ROD HETEROZYGOTE The genetic consequences of crossing over in ring/rod heterozygotes are shown in figure 1. In the single exchange tetrad (El), a first anaphase tie insures the passage of a noncrossover chromatid, either a ring or rod, to the egg nucleus. The two-strand double exchange class (E2-2s) gives rise to rings and rods, of both crossover and noncrossover types; all four should be recovered equally frequently. One of the two types of three-strand double exchange (E2-3sa) produces a crossover rod chromatid, and a noncrossover ring, with a first anaphase bridge insuring the passage of either one or the other to the egg nucleus. The other type of three-strand two exchange tetrad (E2-3sb) forms a tricentric complex, with a free noncrossover rod. Finally, the four-strand double class (E2-4s) gives rise to a double chromatid bridge at first anaphase which, like the double bridges resulting from crossing over within inversion heterozygotes, would be expected to prevent the passage of an X chromatid to the egg. It is from this class that the nullo-X eggs are assumed to be derived. The ratio of crossover rod chromatids to nullo-X eggs originating in two exchange tetrads will provide the theoretical expectation with which the observed results may be compared. If all four types of two exchange tetrads