The transformation of uninucleate amoebae of into multinucleate plasmodia is under the control of a multiallelic mating-type locus, . Plasmodium formation usually occurs only when amoebal fusion brings together two different alleles within a single cell; such plasmodium formation is termed crossing. Mutations () in the locus permit haploid amoebae to self, that is to form plasmodia in clonal cultures without amoebal fusion. By constructing diploid amoebae heterozygous for alleles, it was shown that the mutant alleles and were each dominant to . Non-selfing revertants of mutants may be generated as a result of further mutations () within the locus. Sixty-one revertants of this type were studied, derived from three mutants. Some revertants were able to cross with a tester strain. These ‘class I’ revertants failed to cross with one another or with similar revertants of two mutants; they appear all to lack a single gene function ( ) that is necessary in plasmodium development. Other revertants failed to cross with the tester. These ‘class II’ revertants also failed to cross with one another, but showed several different patterns of crossing when mixed with other strains. The behaviour of some of the class II revertants suggests that they lack a further gene function ( ) necessary in plasmodium development. The mutations may affect a - acting regulator of .


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