Candida albicans and Yarrowia lipolytica as alternative models for analysing budding patterns and germ tube formation in dimorphic fungi.

The site for bud selection and germ tube emission in two yeasts, Candida albicans and Yarrowia lipolytica, was analysed. Both dimorphic organisms display different patterns of budding, which also differ from those described for Saccharomyces cerevisiae. C. albicans, which is diploid and (until now) lacks a known sexual cycle, buds in an axial budding pattern. During the yeast-hypha transition induced by pH, serum, N-acetylglucosamine (GlcNAc) or temperature, germ tube emergence occurs at approximately 50% in a polar manner, while the other 50% of cells show non-polar germ tube emission. Y. lipolytica, in which most of the natural isolates are haploid and which has a well characterized sexual cycle, buds with a polar budding pattern independently of the degree of ploidy. Germ tube emission during the yeast-hypha transition in both haploid and diploid cells generally occurs at the pole distal from the division site (bipolar). The addition of hydroxyurea (HU), an inhibitor of DNA synthesis, also produces different effects. In its presence, and therefore in the absence of DNA synthesis, the yeast-hypha transition is completely abolished in Y. lipolytica. By contrast, in C. albicans germ tube emission in the presence of HU is similar to that observed in control cultures for at least 90 min under induction conditions. These results demonstrate that, rather than a single developmental model, several models of development should be invoked to account for the processes involved in the morphological switch in yeasts (the yeast-hypha transition).