How to recognize and diagnose chromosome rearrangements

In most eukaryotes, chromosome rearrangements are recognized cytologically by visibly altered chromosome structure, or genetically by altered linkage relations. The first translocations in Neurospora were detected and verified using these methods (McClintock 1945, Houlahan et al. 1949). However, characteristics of Neurospora provide an easier method that depends on ascospore pigmentation, and this has been used to identify most of the Neurospora rearrangements that have been detected since 1950. Viable ascospores are black, but ascospores that contain a recombination-generated deficiency are inviable and fail to darken. (These are referred to as 'hyaline' or 'white'.) Presence of a heterozygous rearrangement is signaled by the production of unpigmented ascospores in characteristic ratios. Preliminary diagnosis of the type of rearrangement can be made by observing patterns of black and white ascospores in individual asci. Diagrams showing the meiotic events that give rise to the various ascus types can be found in Perkins (1974, 1986, 1997) and Perkins and Barry (1977).