A new heterothallic species of Pichia has been recovered 25 times from widely separated cactus substrates. The organism has been named Pichia opuntiae because the sexually most compatible strains were isolated from Opuntia inermis in Australia. Two varieties are designated based on differences in physiology, habitat, and geographic distribution. P. opuntiae var. opuntiae has a maximum temperature for growth of 30 to 33°C and assimilates citric acid strongly, but assimilation of cellobiose is latent, weak, or negative. P. opuntiae var. thermotolerans grows well at 37°C, but not at 39°C; it assimilates cellobiose strongly but does not assimilate citric acid. Ecologically, P. opuntiae var. opuntiae is associated with Opuntia inermis (tribe Opuntiaeae, subtribe Opuntiinae) in Australia; P. opuntiae var. thermotolerans is associated with species of the cactus tribe Pachycereeae, subtribe Pachycereinae, from various locations in the North American Sonoran Desert. A discussion of the physiological and host-plant shifts for these two varieties and three similar cactophilic yeasts is presented. The base composition of the nuclear deoxyribonucleic acid of P. opuntiae var. opuntiae (average of four strains) is 33.64 ± 0.25 mol% guanine plus cytosine and that of P. opuntiae var. thermotolerans (average of 3 strains) is 33.13 ±0.23 mol% guanine plus cytosine. The type strain of P. opuntiae and of the type variety, P. opuntiae var. opuntiae, is UCD-FS&T 77-40 (= ATCC 36836 = CBS 7010). The type strain of P. opuntiae var. thermotolerans is UCD-FS&T 76-211 (= ATCC 36834 = CBS 7012).
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