Widespread horizontal transfer of the cerato-ulmin gene between Ophiostoma novo-ulmi and Geosmithia spp.

Preliminary work had shown the presence of a gene encoding class II hydrophobin cerato-ulmin (cu) from 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 proposed. We used PCR amplification to search for the cu gene in 70 Geosmithia spp. strains from different european locations (Czech Republic, Hungary, Spain, Italy) and host plants: 46 strains were derived from insect vectors infesting elm trees or from decaying elm trees and 24 strains were from insect vectors on plants other than elms. The gene was present in 52.1% of the strains from elm trees, while none of those isolated from non-elms possessed it. The presence of cu mRNA was assessed by real time PCR in the reference strain G. pallida IVV7 grown in liquid shaken culture for 4, 8, 12, 16 and 20 days. cu mRNA was present in G. pallida, even if in very low amount, reaching its maximum after 8 days of growth. The same time-point was used to test for cu expression in 7 isolates of different species, where cu mRNA was found in amounts comparable to IVV7. Finally, the induction of cu gene expression was tested in IVV7 grown on elm sawdust and in dual culture with O. novo-ulmi. In both conditions the expression level was increased, however the amount of cu mRNA remained extremely low thus raising the question of its functional significance.