QUANTUM AND CONTINUOUS EVOLUTION OF DNA BASE COMPOSITION IN THE YEAST GENUS PICHIA

This paper investigates the noncontinuous nature and evolution of the base composition of nuclear DNA (expressed as mol% guanine + cytosine) in species of the yeast genus Pichia (sensu Kurtzman, 1984b). The pattern of change in the G + C contents in species of this genus, which range from about 27 to 52 mol%, was evaluated. When specifically those species of Pichia were analyzed that have evolved in necroses of cactus species and associated Drosophila, a periodic change in the G + C contents of approximately 3.0-3.2 mol% was detected by a "bootstrapping" method, Fourier analysis, and a nonlinear trigonometric model. Pichia species occurring in exudates of broad-leaved deciduous trees or associated Drosophila and substrates such as soil and water ("other") showed a periodicity of 2.5-2.6 mol%, whereas species associated with conifers and associated bark beetles showed no significant periodicity. Periodicity in the most recent association (cactus and resident Drosophila) as compared to the lack of periodicity in the oldest association (conifer-beetle) may indicate mixed evolutionary processes. Low mol% G + C values appear more frequently in the relatively recent cactus and Drosophila-associated yeast species. In addition, low mol% G + C species do not display the ancestral bud-meiosis mode of sexual reproduction which occurs frequently in medium to high mol% G + C yeasts. It was found that the mol% G + C content of the Drosophila- and cactus-associated Pichia species is positively correlated with the number of compounds fermented or respired by these yeast species. Possible reasons for the periodic changes in mol% G + C content accompanying speciation include aneuploidy, allopolyploidy, the presence of nuclear plasmids, and regular differences in moderately repetitive portions of DNA. Since significant DNA complementarity is virtually limited to species within a relatively narrow G + C group, this suggests that there are at least two processes which alter the G + C content between species, one saltational and one continuous.