Hansenula Holstii, a New Yeast Important in the Early Evolution of the Heterothallic Species of its Genus

Abstract

Hansenula holstii is judged to be the most primitive, or ancient, heterothallic species of the genus because it is so nearly exclusively haploid in the vegetative state that the diploid form has never been isolated from nature. This species ferments one sugar, rarely two, and the fermentation is slow. Even thin pellicles are seldom produced, and esters are not synthesized. H. holstii has the maximum requirement of the genus for vitamins. Its colonies are generally mucoid, and the cells are small. All of these characteristics of haploids contrast with the corresponding characteristics of those more recently evolved diploid yeasts which live independently of trees. H. holstii and H. capsulata, the latter being the most primitive homothallic species of the genus, live mainly in association with coniferous trees and bark beetles. From the taxonomic area in which these two species occur, a heterothallic and a homothallic line of species evolved into associations with deciduous trees and finally into free living forms. The physiological powers of these two lines increased with succeeding species. Two other lines, starting at the phylogenetic level of H. holstii and H. capsulata, developed into complete dependence upon coniferous trees, with concurrent gradual loss of the ability to cause gaseous fermentation. In all four lines the primary evolutionary characteristic was increase in the ratio of diploid to haploid vegetative cells. The proportion of diploid cells increased rapidly in the lines developing toward the free living state, but slowly in the lines developing toward dependence upon conifers. It is thus apparent that H. holstii and H. capsulata occupy important positions in the evolution of the genus. Generally, the more primitive a heterothallic species of yeast is, the more difficult it is to find sexually reactive forms in nature, and H. holstii is no exception to this rule. However, by mating ascosporic isolates back to their parents through two successive experiments, inbred mating types were obtained which mated abundantly among themselves but not with natural isolates. Although diploid cells are formed in such infinitesimally small numbers by primitive haploid species that diploid colonies are never isolated from nature, they may be produced in the laboratory, and pure ascosporogenous diploid cultures have been produced for the two primitive heterothallic species of Hansenula discovered to date. Hansenula holstii produces in good yield an extracellular phosphomannan that has interesting and potentially useful properties.