Gibberella pulicaris transformants: state of transforming DNA during asexual and sexual growth

A genetically fertile, trichothecene-producing plant pathogen, Gibberella pulicaris (Fusarium sambucinum), was transformed with three different vectors: cosHyg1, pUCH1, and pDH25. All three vectors carry hph (encoding hygromycin B phosphotransferase) as the selectable marker. Transformation frequency was 0.03 transformants per mumg of DNA for pDH25 and 0.5 for pUCH1 or cosHyg1. The vector DNA sequences integrated at different sites into the fungal genome. Transformants were classified into three types based upon distinctive integration patterns: type A contained a single, intact copy of the vector at one site per genome; type B contained multiple tandem copies or a combination of single and multiple tandem copies at one or more sites per genome; type C contained a partial vector copy at one site per genome. While the transformants with cosHyg1 and pUCH1 were type A or B, type C was unique to pDH25 transformants. Type A and C transformants were both meiotically and mitotically stable. However, type B multiple inserts were unstable in mitosis and meiosis since: (1) multiple tandem copies were deleted; (2) rearrangements occurred during premeiosis; and (3) inserts in one of the type B transformants became methylated during premeiosis. Differential expression of transforming sequences between spore germination and mycelial growth was also observed among type B transformants. The ability to transform G. pulicaris with the resulting varied features of integration patterns and the behavior of transforming DNA during mitosis and meiosis provides a means to isolate, manipulate, and study cloned genes in this mycotoxin-producing plant pathogen.