Double Crossing over in Neurospora.

CROSSOVER involves exchange of genetic material between only two of the A four products of meiosis. We should like to know whether there are any special properties of these two strands of the tetrad which enable them to participate in crossing over. Such knowledge might help us understand the mechanics of the crossing over process and would perhaps shed some light on the physical nature of the chromosomes themselves. One approach to this problem is the study of strand relationships in double crossovers (two crossovers in the same tetrad). This type of study can only be made in a situation in which a t least two of the products of meiosis can be recovered and analysed genetically. Analysis of attached-X females in Drosophila permits a comparison of the frequencies of 2-strand and 4-strand double crossovers (BEADLE and EMERSON 1935; BONNIER and NORDENSKIOLD 1937; WELSHONS 1955). Tetrad analysis in Neurospora permits a direct scoring of 2-strand, 3-strand, and 4-strand double crossovers. With random association of non-sister strands, we would expect these three types of double crossover to occur with a frequency ratio of 1 : 2 : 1. Any significant deviation from this ratio is considered to be a demonstration of “chromatid interference”. LINDEGREN and LINDEGREN (1939, 1942) studied double crossing over in neighboring regions of the same chromosome arm of Neurospora, and they found no evidence of chromatid interference. However, the same authors (LINDEGREN and LINDEGREN 1937, 1942) reported that among double crossovers involving regions in opposite chromosome arms, the majority involved only two strands. Recent work has not confirmed this report of an excess of 2-strand double crossovers. HOWE (1956), in a study of the same linkage group as was used by LINDEGREN and LINDEGREN, found no evidence of chromatid interference across the centromere. The findings of the present report, involving double crossing over across the centromere of another linkage group of Neurospora crassa, are in complete agreement with those of HOWE.