Less-than-diploid I. orientalis cells are produced. The cells have at least one unpaired chromosome and may be haploid, i.e., are missing one member of each pair of chromosomes that are present in the wild-type strains. The less-than-diploid cells are useful fermentation strains, performing similarly to diploid strains that are otherwise similarly engineered. The less-than-diploid strains can be mated to produce diploids, which themselves are useful fermentation strains. The less-than-diploid strains are also useful as host strains for producing further genetically modified strains that can be less-than-diploid or mated to produce diploids.