Demiploidy

One-sentence definition. Demiploidy (often called haplodiploidy) is a reproductive system in which females develop from fertilized diploid eggs while males develop from unfertilized haploid eggs, so males have only one copy of every chromosome.

One-sentence analogy. In a demipoloid system, think of males as walking around with only half a library — they have one copy of every book, while females have the full two-copy collection, and males came from the extra library copy that was never checked out.

Why it matters. Haplodiploidy is evolutionarily relevant because haploid males have no heterozygosity to hide recessive deleterious mutations, creating strong purifying selection. This is hypothesized to lower chromosome numbers over time: haplodiploid mite species have significantly lower chromosome numbers (~2n=5 fewer) than diplodiploid relatives, with low chromosome number evolving prior to haplodiploidy. Bull’s haploid-viability hypothesis predicts this association — lineages with already-low chromosome numbers are the ones that successfully transition.

Where you meet it in the wiki.

• Haplodiploidy evolution — detailed treatment of origins and consequences.
• Karyotype evolution overview — demiploidy as a karyotype state.
• Chromosome number evolution — correlation between ploidy and chromosome count.

Primary citation.

“Both models confirm that haplodiploid species have significantly lower chromosome numbers than diploid species (approximately 2n = 5 fewer chromosomes, PMCMC < 0.001 for both the taxonomic and phylogenetic models).” — Blackmon et al. 2015, Finding 1

Prerequisites: polyploidy, autosome Next, learn about: dysploidy, effective population size