Map and function ofgadmutations inPhysarum polycephalum

Abstract

SUMMARYAmoebae and plasmodia are alternate vegetative forms in the life cycle of the acellular slime mouldPhysarum polycephalum. Haploid amoebae carrying heterothallic alleles of thematA(ormt) locus ordinarily form plasmodia only by crossing, but occasionally give rise to mutants that form plasmodia by selfing as well as by crossing. Twelve independently isolated mutants of this type have been studied. Eight carry mutations (termedgador greater asexual differentiation mutations) within approximately 0·2 map units ofmatA. Another mutation (gad-12) is linked neither tomatAnor to any of 9 other markers tested. The remaining three mutations are linked tomatAand map as follows:matA–0·5 units –gad-4–4 units –gad-6– 8 units –gad-11. One mutation,gad-11, has been tested in strains carrying each of the fivematAalleles (matAl, 2, 3, 4, andh) available in a common genetic background; the mutation is expressed with all five alleles. The mutationnpfF1(formerlyaptA1), which was isolated as a suppressor of selfing in Colonia (matAh) amoebae, suppresses the action of each of the 12gadmutations. The similarly isolated mutationnpfA1is also epistatic to eight of the mutations, but permits selfing withgad-5, 6, 12and13. For double mutant strains containinggad-12andgad-1, 2, 4, 6or11, the selfing behaviour of each double mutant differs from that of either single mutant. Mixtures ofgad−npfF1withgad+npf+amoebae readily form plasmodia, a result suggesting thatgadmutations are dominant or semi-dominant. We conclude that the commitment of a cell to differentiate into a plasmodium is under the control of a complex group of genes linked tomatA.