Summary: Auxotrophs derived by UV or -methyl--nitro--nitrosoguanidine mutagenesis from three North American, three European and four Australian tomato wilt isolates of were used to investigate heterokaryon compatibility. Ninety “crosses” involving race-1 and race-2 isolates were attempted using mixed conidial suspensions and a “hanging drop” technique. Crosses between North American and Australian isolates failed to produce heterokaryons over a 20 d period but within each of these two groups of isolates (with one exception) the frequency of heterokaryon formation was 100%. In crosses involving the three European isolates the frequency of heterokaryon formation varied from 0 to 100%. Two of the European isolates produced heterokaryons both with North American and with Australian isolates. Prototrophic diploid conidia were recovered from 13 of the 30 crosses that produced heterokaryons. The rate of diploid production was greatest in crosses showing 100% compatibility but rare diploids were recovered from two crosses which had produced no observable heterokaryotic growth on minimal medium. The formation of heterozygous diploids between race-1 and race-2 isolates of indicates the potential for the genetic analysis of pathogenicity through the parasexual cycle.


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