MATHEMATICS: CHOW AND IGUSA

map and the strength of the heterocaryon as judged by enzyme activity, growth rate, or time required for growth to commence from mixed conidial inocula. Thus heterocaryons between mutants damaged in adjacent cistrons have low adenylosuccinase activities, and activities increase with increasing distance to the maximum value of 25 per cent of wild type for mutants separated by 2-5 cistrons. Possible interpretations of these results are discussed and the hypothesis is proposed that interallelic complementation involves the formation of defective gene products by different mutant nuclei in a heterocaryon. Cytoplasmic recombination of such gene products either at the primary (RNA) or secondary (polypeptide) level, perhaps by a process analogous to crossing over, could yield enzymatically active protein.