Pichia barkeri, a New Yeast Species Occurring in Necrotic Tissue of Opuntia stricta

Abstract

We describe Pichiu barkeri, a new cactophilic yeast species occurring in necrotic tissue of Opuntia stricta in New South Wales, Australia, and on several islands in the Caribbean Sea. The new species is homothallic and appears to occur in nature in the haploid state. After conjugation between a mother cell and a bud, four hat-shaped spores are produced that are rapidly released from the ascus. The range of guanine-plus-cytosine contents for the nuclear deoxyribonucleic acids of 22 strains is 35.7 to 36.6 mol% (average, 36.1 mol%; standard deviation, 0.2 mol%). The type strain of P. barkeri is strain UCD-FST 83-994.3 (ATCC 64111, CBS 7256) isolated at Discovery Bay, Jamaica.The rotting stems and cladodes of Opuntia species are specific habitats for a diversity of cactus-specific yeast species (1,2,7,11). During surveys of yeasts associated with rots of Opuntia species in Australia (1, 2), we isolated a number of strains that were initially identified by the system of Lodder (5) as Pichia kluyveri. Because P. kluyveri had been recovered only rarely in earlier surveys of columnar cacti (12), we initially concluded that these strains might have converged on the P . kluyveri phenotype or might be fermentative forms of Pichia cactophila. Subsequent characterization by molecular criteria (6) showed that the isolates in question differed from P. kluyveri in having a nuclear deoxyribonucleic acid (DNA) guanine-plus-cytosine (G + C) content approximately 6 mol% higher than that of P. kluyveri (-36 versus -30 mol%), virtually precluding conspecificity (6). Ensuing experiments, using the technique of DNA-DNA hybridization, provided additional evidence that the Australian isolates represented a different species, although retaining a distant relationship with P. kluyveri. DNA-DNA hybridization experiments also showed that the Australian isolates are distinct from P. cactophila, which has a similar base composition (-36 mol% G+C). During subsequent explorations of the yeast floras associated with cacti occurring on various islands in the Caribbean Sea, we isolated several additional strains of the new species, Pichia barkeri, also associated with necrotic tissue of Opuntia stricta.