Hansenula Angusta, an Excellent Species for Demonstration of the Coexistence of Haploid and Diploid Cells in a Homothallic Yeast

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Abstract
SUMMARY Hansenula angusta is a species of yeast associated with deciduous trees and disseminated by bark beetles and fruit flies. It exists in nature predominantly in the haploid form but a small percentage of diploid cells are generally present. Ascospores impart a red color to the colonies when they are 4 to 8 days old, the diploid colonies having a denser color than the haploid colonies because diploid cells of homothallic yeasts generally sporulate sooner and more abundantly than the haploid cells. This species is recommended for classroom demonstration of the following general characteristics of homothallic yeasts which exist in both the haploid and diploid forms: Haploid cells are smaller than diploid. Haploid cells produce conjugation tubes whereas diploids do not, and the asci formed from haploids are smaller and are conjugated, while those produced by diploids are larger and are unconjugated. The haploid cells diploidize at a slow rate while growing vegetatively, but the diploids evidently must sporulate in order to return to the haploid state. Either form may be derived and maintained in practically pure state by selection and lyophilization.