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Isolates of Phytophthora infestans were collected from all potato growing regions of Poland during the blight seasons of 1987-1989. All 1987 isolates were of Al mating type and were sensitive to metalaxyl. In 1988 and 1989, 46.5% and 55.3% of the isolates were sensitive to metalaxyl, respectively. The percentage of highly resistant (R) isolates increased from 25.6% in 1988 to 39.5 % in 1989; however the percentage of intermediately resistant (J) isolates decreased during that period from 27.9% to 5.3 %. A significant association was observed between the Al compatibility type and metalaxyl resistance. The A2 mating type first appeared in 1988, and its frequency increased from 4.7 % of the population in 1988 to 47.6% in 1989. Coincident with this change in mating type frequency, changes in ploidy levels of isolates were observed. Whereas 3% of the 1988 isolates were diploid, 90% of the 1989 A2 isolates and 28.6% of the 1989 A1 isolates were diploid. The approximate 1:1 ratio of the two mating types encountered in 1989, and the predominance of diploidy, indicates that the Polish population of P. infestans has the potential to become sexual. [ABSTRACT FROM AUTHOR]

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Clones of myxamoebae which fail to undergo a direct morphogenetic change into plasmodia have been isolated from cultures of the Philippine-1 isolates of Didymium iridis. These clones have been maintained continuously for a period of more than 4 years without producing plasmodia. It has been demonstrated by Feulgen-DNA cytophotometry that the mean nuclear DNA content of such clones is equivalent to the haploid replicated (2C) value. Conversely, myxamoebae which produce plasmodia directly have a mean nuclear DNA content which is equivalent to the diploid replicated (4C) value. The haploid Philippine-1 clones have been demonstrated to be cross compatible with isolates bearing the mating alleles A1, and A3 through A11, and incompatible with isolates bearing the mating allele A2. Furthermore, plasmodia which result from the crosses Ph-1n (A2) Hon 1-2 (A1), (A1), Ph-1n (A2) x Pan 2-4 (A7) and Ph-1n (A2) x Pan 2-7 (A8) have mean nuclear DNA values which are equivalent to the replicated diploid value, and produce spores which bear the mating alleles in the predicted ratios. Such data demonstrate that the Ph-1n clones are sexually compatible with haploid myxamoebae of heterothallic isolates. The significance of the isolation of these haploid clones with regard to a recent hypothesis related to the evolution of developmental systems in the Myxomycetes is discussed.