A Third Multiallelic Mating-type Locus in Physarum polycephalum

SUMMARY: Sexual development (crossing) in Physarum polycephalum occurs when two haploid amoebae fuse to form a diploid plasmodium. The imz locus influences the maximum pH at which crossing can occur. A new allele of imz has been identified, bringing the total number of alleles to three. Contrary to earlier findings, it has been shown that all homoallelic combinations of imz alleles display a similar pH limit for crossing, which is lower than that for imz-heteroallelic combinations. It is concluded that imz is a mating compatibility locus; thus the mating-type system of P. polycephalum comprises three multiallelic loci: matA, matB and imz. It is proposed that imz be renamed matC.