The Correlation Effect for a Histidine Locus of Neurospora Crassa.

OR different organisms rare wild type segregants have been observed in crosses F involving independently isolated mutants which all show the same phenotypic effect. In all cases in which markers on one or both sides of these mutational sites were present the segregants which are wild type with respect to the isolocaP markers are associated with outside marker combinations which in the crossing over theory would be assigned to 1, 2, and even 3 crossovers between and outside the isolocal markers. The correlation between these “recombinations” is so high that the occurrence of wild type segregants cannot be explained by statistically independent crossovers. The first report of this kind was given by DEMEREC (1928) who described the phenomenon as reverse mutation associated with crossing over in heterozygotes of Drosophila virilis. That neither statistically independent crossovers nor clusters of (reciprocal) crossovers can explain the results has most clearly been shown for Neurospora crassa by M. B. MITCHELL (1955 a,b), who found that of four asci containing a wild type segregant for the isolocal markers none contained the reciprocal double mutant. Further results have been published for Neurospora by GILES (1955), ST. LAWRENCE (1956), and DE SERRES (1956), for Aspergillus nidulans by PRITCHARD (1955) and for phages by STREISINGER and FRANKLIN (1956). Irregular asci have also been found in yeast by LINDEGREN (1955) and ROMAN (1956). In this situation several points of view can be or have been suggested. Agreement among all workers in the field seems now to exist for the statement that there occur regions of “intimate” (or “effective”) pairing statistically distributed along the chromosome. In these regions events take place which result in the irregular segregation for the isolocal markers and the correlated appearance of the described 1, 2 and 3 fold “recombinants”. Concerning the events within sucli a region of intimate pairing two different attitudes to the problem can be stated.