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

Abstract

Wickerham stated (1951) that the ratio of diploid to haploid cells is the dominant characteristic determining the evolutionary level attained by a species of Hansenula. This relationship holds whether the species is homothallic or heterothallic, and whether the yeast belongs to the phylogenetic lines which developed toward the free-living state or toward more dependence upon trees and insects. Other characteristics, such as increasing size of cells and transition from mucoid, to butyrous, to mat colonies, also reveal evolutionary succession in the various lines. Physiological characteristics, particularly the number of vitamins required from the environment, the number of sugars fermented, and the rate of fermentation, also indicate evolutionary position. The physiological reactions, however, became stronger in lines developing toward the freeliving state, and weaker in the lines developing toward greater dependence upon insects and trees. The genus Hansenula, as we now know it, apparently originated as a conifer-inhabiting species. The yeasts in the lines pointing toward independence from trees evolved successively as species dependent upon coniferous trees, then as species dependent upon deciduous trees, and finally as species living independently in soil and water. The completely dependent lines of Hansenula became limited in habitat to the coniferous