Locally Specific Patterns of Chromatid and Chromosome Interference in Neurospora.

NE of the fundamental problems concerning the mechanism of cross0 ing over is the question of chromatid interference. In the absence of chromatid interference the identity of the chromatids involved in two or more chiasmata or points of crossing over is random. Positive chromatid interference would exist if a crossover between one pair of chromatids reduced their chances of being involved in a second crossover with a resulting excess of 4-strand exchanges. Negative chromatid interference would exist if there was an excess of exchanges over that expected on a random basis in which a crossover between one pair of chromatids was followed by a second crossover between the same pair (a-strand exchange). The present paper and previous ones (LINDEGREN and LINDEGREN 1937, I 939) describe chromatid interference (non-random crossing over) in Neurospora. The conclusions reached in these studies are not in agreement with those of BEADLE and EMERSON (1935) on Drosophila. HUSKINS and NEWCOMBE (1941), however, demonstrated chromatid interference in Trillium by direct cytological observation. NEWCOMBE (1941) after a statistical analysis of BEADLE and EMERSON’S (1935) data, questions the validity of their interpretation. BONNIER and NORDENSKIOLD (193 7) failed to corroborate BEADLE and EMERSON’S findings. Our data show that when two crossovers occur symmetrically spaced on each side of the centromere, the same pair of strands is usually involved in each of the two exchanges, but in double exchanges involving certain other pairs of regions, the first exchange may interfere with a second exchange occurring between the same two chromatids. A pattern of exchanges involving negative chromatid interference between certain pairs of regions is associated with a pattern involving positive chromatid interference between other pairs of regions. There is a specific pattern characteristic of each pair of regions, and an excess of one type of exchange may occur between one pair of regions with a deficiency of that same type between another pair of regions. Directly related to the question of chromatid interference is the question of chromosome interference. If chromosome interference is positive,