The frequency of genetic recombination in Coprinus lagopus may be modified by heat and cold shock. By removal of samples from a fruiting body before and after temperature treatment, it is possible to study the ultrastructure of chromosomes at the time recombination frequency (between den+ x +me-1) can be modified. The sensitive period for temperature effects and, therefore, probably the time of crossing over, commences with the formation of the synaptinemal complex (S.C.) and ends with its disappearance, i.e. during the entire existence of the S.C. It is concluded that recombination is an event subsequent to the formation of the S.C. and is independent of the process of its formation. It is suggested that the event takes place at the synaptic centre.
The Synaptinemal Complex and Four-Strand Crossing Over
