Haploidia

Haploidia, or haploidy, is the state of having a single complete set of chromosomes (n) in a cell or organism. In contrast, diploidy (2n) carries two homologous chromosome sets, and polyploidy involves more than two. Haploid genomes may occur in certain life stages or organisms, and in many groups they are transient or conditional during development.

Natural occurrence: In fungi, many algae, and bryophytes, the haploid phase forms the main body or a distinct multicellular stage. In animals, haploidy is rare, but it is a feature in some insects; in Hymenoptera, for example, males develop from unfertilized eggs and are haploid, whereas females are typically diploid. Plants and some algae also show haploid-dominant or haploid-dominated life cycle phases called gametophytes.

Genetic consequences: Because haploid genomes lack a second allele, all genetic variants are directly exposed to selection. Recessive deleterious mutations have immediate phenotypic effects, while beneficial mutations can spread without masking. Haploidy can affect developmental processes and fitness, depending on the species and ecological context.

Life cycles and reproduction: Haploid spores or gametes are produced by meiosis or mitotic divisions in some contexts. In alternation-of-generations systems, the haploid stage is the gametophyte, producing gametes by mitosis, while fertilization restores the diploid sporophyte.

Artificial haploidy: In plant breeding and genetics, haploids are induced to create doubled haploid lines that are completely homozygous. Production methods include haploid induction via specific crosses, anther or pollen culture, and genome elimination. Treatment with mitotic inhibitors such as colchicine can double chromosomes to produce stable DH lines quickly.