FATE OF DNA ENCODING HYGROMYCIN RESISTANCE AFTER MEIOSIS IN TRANSFORMED STRAINS OF GIBBERELLA-FUJIKUROI (FUSARIUM-MONILIFORME)

Stability of foreign DNA transformed into a novel host is an important parameter in decisions to permit the release of genetically engineered microorganisms into the environment. Meiotic instability of transformed DNA has been reported in fungi such as Ascobolus, Aspergillus, and Neurospora. We used strains of Gibberella fujikuroi (Fusarium moniliforme) transformed with the hyg(r) gene from Escherichia coli to study meiotic stability of foreign DNA in this plant pathogenic fungus. Crosses with single-copy transformants segregated hyg(r):hyg(s) in a 1:1 manner consistent with that expected for a Mendelian locus in a haploid organism. Multicopy transformants, however, segregated hyg(r):hyg(s) in a 1:2 manner that was not consistent with Mendelian expectations for a chromosomal marker, even though two unrelated auxotrophic nuclear genes were segregating normally. Segregation ratios in crosses in which hyg(r) was introduced via the male parent did not differ significantly from crosses in which the transformed strain served as the female parent. Some of the sensitive progeny from the crosses with the multicopy transformants carried hyg(r) sequences. When these phenotypically sensitive progeny were crossed with a wild-type strain that carried no hyg(r) sequences, some of the progeny were phenotypically hyg(r). Some progeny from some crosses were more resistant to hygromycin than were their sibs or the transformant strains that served as their parents. Transformants passaged through a maize plant only rarely segregated progeny with the high levels of resistance. The mechanism underlying these genetic instabilities is not clear but may involve unequal crossing over or methylation or both. Further work with cloned genes with homology to sequences already present in the Fusarium genome is warranted.