Although the plant body of thallophytes are haploid (n) or gametic, yeasts show variation. Some species are haploid and others may be diploid or sporophytic (2n). Hence, 3 types of life cycle pattern are seen among the yeasts.
(i) Haplobiontic life cycle – (Saccharomyces octosporus)
In this case, the yeasts occur mostly as vegetative cells or ascospores, which are haploid (n) in nature. The diploid stage is very short and is represented by zygote (2n) or ascus. Thus the haploid phase is long and diploid phase is short. The ascospores release by the rupture of ascus wall, grow in size, divide and re divide by fission to form haploid vegetative cells. Some of the normal yeast cells behave as gametes and conjugate to form zygote which acts as ascus.
(ii) Diplobionitic life cycle –
In this case the vegetative yeast cells are diploid. Some of them act as asci. An ascus produces four ascospores (n) by meiosis, two of which are of positive (+ve) strain and the other two are of negative (-ve) strain. This is because of heterothallic nature of yeast species. Inside the ascus, the ascospores fuse in pairs to form diploid zygote. Each diploid zygote cell germinates by forming germ tube, from which diploid yeast cells are cut off.
In this case the diploid somatic phase is elaborate and haploid phase is very short being represented by ascospores (n).
(iii) Haplo-diplobiontic Life cycle (Saccharomyces cerevisiae):
In this type haplophase (n) and diplophase (2n) are equally represented and alternate in regular succession. In Saccharomyces cerevisiae, two haploid vegetative cells of opposite strains (+ve and -ve) unite with each other resulting in the formation of a diploid zygote (2n) which again by budding produces many diploid somatic cells Each diploid cell in turn behaves like an ascus and produces four haploid ascospores (of which two are +ve and two are -ve) through meiosis, which are finally liberated. They grow in size and multiply by budding and thus produce many haploid vegetative cells (n) of two different strains, (+ve and -ve).