Regulation of Development by the matA Complex Locus in Physarum polycephalum

Abstract

The transformation of uninucleate amoebae of Physarum polycephalum into multinucleate plasmodia is under the control of a multiallelic mating-type locus, matA. Plasmodium formation usually occurs only when amoebal fusion brings together two different matA alleles within a single cell; such plasmodium formation is termed crossing. Mutations (gadA) in the matA locus permit haploid amoebae to self, that is to form plasmodia in clonal cultures without amoebal fusion. By constructing diploid amoebae heterozygous for gadA alleles, it was shown that the mutant alleles gadA5 and gadA111 were each dominant to gadA +. Non-selfing revertants of gadA mutants may be generated as a result of further mutations (npf) within the matA locus. Sixty-one revertants of this type were studied, derived from three matA3 gadA mutants. Some revertants were able to cross with a matA3 gadA + tester strain. These ‘class I’ revertants failed to cross with one another or with similar revertants of two matA2 gadA mutants; they appear all to lack a single gene function (npfC +) that is necessary in plasmodium development. Other revertants failed to cross with the matA3 gadA + 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 (npfB +) necessary in plasmodium development. The gadA mutations may affect a cis-acting regulator of npfB +.