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Abstract
The evolutionary affinities among several morphologically and physiologically similar species of the ascogenous yeast genus Hansenula were studied by a technique which involved genome comparisons in combination with nuclear deoxyribonucleic acid base composition determinations. The results revealed a new member of the genus Hansenula that was recovered five times in 1968 from slime exudates of Populus trichocarpa (cottonwood trees) in British Columbia and Alaska. This new species is named Hansenula populi because of its specific habitat in exudates of cottonwood trees. H. populi is homothallic and occurs naturally in the haploid state. Conjugation between vegetative cells precedes sporulation, and up to four hat-shaped spores are produced in zygotes. The spores are liberated from the asci soon after formation. H. populi resembles Hansenula nonfermentans and Hansenula dryadoides but differs from these species in habitat, guanine-plus-cytosine content of the nuclear deoxyribonucleic acid, maximum growth temperature, and ability to assimilate several carbon compounds. The type strain of H. populi is UCD-FS&T 68-628C (= CBS 8094 = ATCC 48773).
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