The killer phenomenon of yeasts was investigated in naturally occurring yeast communities. Yeast species from communities associated with the decaying stems and fruits of cactus and the slime fluxes of trees were studied for production of killer toxins and sensitivity to killer toxins produced by other yeasts. Yeasts found in decaying fruits showed the highest incidence of killing activity (30/112), while yeasts isolated from cactus necroses and tree fluxes showed lower activity (70/699 and 11/140, respectively). Cross-reaction studies indicated that few killer-sensitive interactions occur within the same habitat at a particular time and locality, but that killer-sensitive reactions occur more frequently among yeasts from different localities and habitats. The conditions that should be optimal for killer activity were found in fruits and young rots of Opuntia cladodes where the pH is low. The fruit habitat appears to favor the establishment of killer species. Killer toxin may affect the natural distribution of the killer yeast Pichia kluyveri and the sensitive yeast Cryptococcus cereanus. Their distributions indicate that the toxin produced by P. kluyveri limits the occurrence of Cr. cereanus in fruit and Opuntia pads. In general most communities have only one killer species. Sensitive strains are more widespread than killer strains and few species appear to be immune to all toxins. Genetic study of the killer yeast P. kluyveri indicates that the mode of inheritance of killer toxin production is nuclear and not cytoplasmic as is found in Saccharomyces cerevisiae and Kluyveromyces lactis.

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Representative strains (n = 61) of the yeast Pichia kluyveri from across the United States were studied for their ability to kill 71 other strains (representing 25 species) of yeast. This survey showed killing activity in 69%o of the P. kluyveri strains tested. More extensive analysis of killer activity of 197 P. kluyveri strains against strains of five tester species showed comparable activity (67% of strains tested). This activity was shown to be equally variable within localities, within regions, and across the continent. The genetic basis of the variability was ascertained by tetrad analysis and is most likely due to alleles segregating at three epistatic loci. Evidence for the idea that killer toxins have a role in excluding other yeasts from particular habitats is discussed.

ABERDEEN~We describe Pichia antillensis, a new species of yeast which is closely related to Pichia opuntiae. Pichia antillensis, 20 strains of which were isolated, is heterothallic and occurs in nature in both the haploid state and the diploid state. It produces asci with four hat-shaped spores, which are rapidly released upon maturity. The guanine-plus-cytosine content of its nuclear deoxyribonucleic acid is 33.4 to 33.7 mol%. The deoxyribonucleic acids of Pichia antillensis and Pichia opuntiae var. apuntiae show approximately 50% homology, but the two species are not interfertile. Moreover, the two species are separated geographically and by host plant. Pichia antillensis occurs principally in necrotic tissue of the columnar cactus Cephalocereus royenii in the West Indies, and Pichia opuntiae var. opuntiae occurs in Opuntia rots in Australia. The type strain of P. antillensis is strain UCD-FST 82-651A (= ATCC 56267 = CBS 7111).Recently, we have been studying a collection of yeasts that were obtained from decaying stems of cacti growing on islands in the Caribbean Sea. In this collection we discovered a heterothallic yeast which has a low-level physiological profile, a deoxyribonucleic acid (DNA) base composition of 33.4 to 33.7 mol% guanine plus cytosine (G+C), and, as far as now known, a restricted geographic and host plant distribution. The sexual, physiological, and macromolecular properties of this yeast are similar to those of three previously described cactus-specific yeasts, Pichia heedii (9), Pichia amethionina (13), and Pichia opuntiae (14), which are known from other geographic regions. Comparisons of the new isolates with strains of these three previously described species for sexual compatibility, physiological abilities, ecological specificity, and DNA-DNA sequence homology suggested that the new isolates comprise a new species in the genus Pichia. We named the new isolates Pichia antillensis because of their unique habitat in necrotic tissues of the columnar cactus Cephalocereus royenii on several islands of the Lesser Antilles. MATERIALS AND METHODSSamples of necrotic cactus tissue yielding the new yeast were collected from islands in the eastern West Indies in May 1982 (Table 1) during cruise CF-8205 of the research vessel Cape Florida in this region. The techniques used for yeast isolation and purification have been described previously (12). The yeast strains analyzed for nuclear DNA base composition and the strains used in the DNA-DNA homology sequence studies are listed in Table 2.The usual phenotypic characterization of the isolates was carried out by methods currently used in yeast taxonomy (16), but additional carbon compounds used in assimilation tests were also employed. These included D-glucosamine hydrochloride, N-acetyl-D-glucosamine, methahol, 2-propanol, acetone, and ethyl acetate; the last three compounds could be tested only by incorporating them at concentrations of 1.0% in yeast nitrogen base agar because in liquid yeast nitrogen base media they proved to be inhibitory, even i...