HYALINE MUTANTS FROM VERTICILLIUM-DAHLIAE, AN EXAMPLE OF SELECTION AND CHARACTERIZATION OF STRAINS FOR HOST-PARASITE INTERACTION STUDIES

The objective of the present work was to select stable well-characteri zed strains of Verticillium dahliae that could be used as biological t ools in genetic and plant-microbe interaction studies. Hyaline mutants , known for their stability in pathogenicity were chosen for the study . Diversity in pathogenicity was found among hyaline subclones obtaine d from a defoliating wild-type clone, but not within those from nondef oliating ones. Most subclones from the defoliating clone had parental pathotypes, but one (V7-2) exhibited weak pathogenicity. This subclone (V7-2), together with a highly virulent one (V7-7) deriving from the same defoliating parent clone (7), were selected for further character ization, because of their differences in pathogenicity. When studied o n the basis of their growth requirements, the two subclones expressed marked differences. V7-7 grew better than V7-2 over a wider range of t emperature conditions. Both subclones grew similarly in media suppleme nted with NH4 as nitrogen source, but in those with NO3, V7-7 grew mor e vigorously than V7-2 and only the former could grow when NO2 was use d. In spite of these differences, the two subclones were found to belo ng to the same vegetative compatibility group, confirming their geneti c proximity. These results highlight the physiological and genetic com plexity inherent in V. dahliae. In view of their characteristics, the clones obtained in this study should prove to be valuable tools in fur thering the understanding of genetic and host-V. dahliae interactions.