Horizontal gene transfer of the cerato-ulmin gene is widespread between Ophiostoma novo-ulmi and Geosmithia spp.

Preliminary work had shown the presence of a gene encoding the class II hydrophobin cerato-ulmin (cu) in the phytopathogenic fungus Ophiostoma novo-ulmi in the unrelated species Geosmithia pallida. As the two species coexist inside elm trees occupying the same habitat but different ecological niches, a horizontal gene transfer event was hypothesized. PCR amplification was used to search for the cu gene in 57 isolates of Geosmithia spp. from different European countries (Czech Republic, Hungary, Spain, and Italy), showing that sequences homologues to the cu gene are present in all the isolates analyzed so far. Cu gene expression was also evaluated by real-time PCR in the reference isolate G. pallida IVV7 grown in liquid shaken culture for 4, 8, 12, 16 and 20 days, showing maximum level at eight hours. Seven isolates were therefore analyzed after eight days of growth, and all showed expression of the cu gene. Finally, when IVV7 was grown for four days on elm sawdust plates, cu expression was induced to a level 2.5-fold higher as compared with the control.