Electrophoretic karyotypes of the elm tree pathogen Ophiostoma ulmi (sensu lato)

Characterization of three DNA transposons in the Dutch elm disease fungi and evidence of repeat-induced point (RIP) mutations

Transposable elements (TEs) are fundamental components of eukaryotic genomes and can contribute in various ways to genome plasticity and evolution. We describe here the first three DNA transposons in the Dutch elm disease (DED) pathogens Ophiostoma ulmi and O. novo-ulmi, named OPHIO1, OPHIO2 and OPHIO3. We demonstrate that OPHIO transposons, which show high homology to Fot1/pogo TEs within the Tc1/mariner superfamily, have different distribution patterns and specificity in the DED fungi and that interspecific hybrids could act as genetic bridges for transmission of TEs between closely related fungal species. OPHIO3 was found to have undergone repeat-induced point mutations (RIP). We have also developed a complementary method to Margolin's ratios based on the computation of cumulative transition scores (CTS) in order to visualize rapidly RIP signatures on individual DNA strands of OPHIO transposons and TEs found in other ascomycete fungi.

Molecular karyotype of the phytopathogenic fungus Sclerotinia sclerotiorum

Molecular techniques have been used to characterize different field isolates of Sclerotinia sclerotiorum, an ubiquitous phytopathogen. Chromosomal DNA resolved by pulsed-field gel electrophoresis (PFGE) revealed that S. sclerotiorum contains at least 16 chromosomes ranging from 1.5 Mb to 4.0 Mb. The size of the haploid genome was estimated to be 43.5 Mb. Six field isolates with different levels of virulence on sunflower germlings or green beans were differentiated by random amplification of polymorphic DNA (RAPD), and analysed by clamped homogeneous electric field electrophoresis. This analysis revealed few chromosome-length polymorphisms among these strains. Chromosomal DNA hybridization indicated that the endopolygalacturonase-encoding pg1 gene is localized on the smallest chromosome of all the strains, whereas the ribosomal DNA mapped to different-sized chromosomes. The less-aggressive strain was characterized by the presence of a supernumary small band, presumably consisting of dsRNA. In contrast to numerous other phytopathogenic fungi, this study reveals a strong karyotypic stability among the strains of S. sclerotiorum which may be preserved by the sexual mode of reproduction of this species

Inheritance of chromosome-length polymorphisms in Ophiostoma ulmi (sensu lato)

We have investigated the mitotic and meiotic transmission of chromosome-length polymorphisms in Ophiostoma ulmi s.l., the causal agent of Dutch elm disease. The North-American aggressive (NAN) strain CESS16K has an atypical electrophoretic karyotype, carrying two chromosome-sized DNAs (chDNAs) that have not been observed in other members of the NAN biotype. Independent CESS16K chDNA preparations, even after repeated inoculation and recovery from the elm host, and analysis of 16 progeny strains after a cross between the NAN strains FG245Br-O and CESS16K, demonstrated that these unique chDNAs are integral components of the CESS16K genome. Analysis of the progeny, by electrophoretic karyotyping and hybridizations with probes specific to individual chDNAs, presented evidence that genome rearrangements can occur as a consequence of meiosis. Even though novel electrophoretic karyotypes and a novel-sized chromosome were observed in the karyotypes of the progeny strains, the low level of reassortment between the chromosomes carrying length polymorphisms presented evidence that there are constraints to genome plasticity for this fungus.