RESUMO
The yeast communities from slime fluxes of three deciduous trees (Prosopis juliflora, Populus fremontii and Quercus emoryi) and the necroses of two cacti (Opuntia phaeacantha and Carnegiea gigantea) were surveyed in the region of Tucson, Arizona. In addition, the yeasts carried by dipterans associated with the fluxes or necroses (Drosophila carbonaria, D. brooksae, D. nigrospiracula, D. mettleri, and Aulacigaster leucopeza) were sampled. The results indicate that each host sampled had a distinct community of yeasts associated with it. The dipterans, which can act as vectors of the yeasts, deposited yeasts from other sources in addition to those found on their associated hosts. It is argued that host plant physiology is relatively more important than the activity of the vector in determining yeast community composition. Furthermore, the average number of yeast species per flux or necrosis is not different from the average number of yeast species per fly. It is hypothesized that the vector may affect the number of species per individual flux or not, and that the number is lower than the rot or necrosis could potentially support.
RESUMO
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.
