Haplonttisessa

Haplonttisessa, or haplontic life cycle, describes organisms in which the dominant multicellular stage is haploid. The diploid phase is restricted to a zygote formed by fertilization; this zygote immediately undergoes meiosis to produce haploid cells, which develop into the next haploid generation. There is no prolonged multicellular diploid stage.

In practice, the haploid organism grows and reproduces primarily through mitosis of haploid cells. Sexual reproduction proceeds when compatible haploid gametes fuse to form a diploid zygote, which then undergoes meiosis to restore haploidy. Some species also allow haploid individuals to reproduce asexually by mitosis. The diploid phase, when it occurs, is typically brief and often not multicellular.

This life cycle is common in many fungi and several groups of algae. In fungi, for example, the visible organism is generally haploid, with a transient diploid stage in the zygote that quickly produces haploid spores. In algae, haplontic patterns are similarly characterized by haploid vegetative forms and haploid spores following meiosis.

Haplonttisessa is contrasted with diplontic life cycles, where the multicellular stage is predominantly diploid (as in animals), and with haplodiplontic life cycles, which feature alternating generations of haploid and diploid multicellular organisms (as in many plants and some algae). The haplontic pattern emphasizes haploid dispersal units and a brief or non-multicellular diploid phase.