Fate of DNA encoding hygromycin resistance after meiosis in transformed strains of Gibberella fujikuroi (Fusarium moniliforme)

Abstract

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 hygr gene from Escherichia coli to study meiotic stability of foreign DNA in this plant pathogenic fungus. Crosses with single-copy transformants segregated hygr:hygs in a 1:1 manner consistent with that expected for a Mendelian locus in a haploid organism. Multicopy transformants, however, segregated hygr:hygs 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 hygr 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 hygr sequences. When these phenotypically sensitive progeny were crossed with a wild-type strain that carried no hygr sequences, some of the progeny were phenotypically hygr. 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.