Haplodiploidy

Definition

A sex-determination system in which males develop from unfertilized eggs and are haploid (possessing a single set of chromosomes), while females develop from fertilized eggs and are diploid (possessing two sets of chromosomes). This system produces asymmetric genetic relatedness among siblings: sisters share 75% of their genes on average (higher than the 50% typical of diploid systems), while brothers share only 25% with sisters and 0% with each other. This genetic asymmetry underlies kin-selection explanations for eusociality and female-biased sex allocation in many haplodiploid lineages.

Etymology

From Greek haploos (single) + diploos (double) + -oid (resembling), referring to the alternating haploid and diploid phases in the life cycle.

Example

Honey bees (Apis mellifera) exhibit haplodiploidy: queens lay unfertilized eggs that become haploid drones (males), and fertilized eggs that become diploid workers or queens (females). A worker bee is more closely related to her sisters (r = 0.75) than to her own offspring (r = 0.5), which contributes to the evolutionary logic of sterile worker castes.

Synonyms

• Arrhenotoky

Related Terms

• Arrhenotoky
• thelytoky
• deuterotoky
• eusociality
• Kin selection
• haplodiploid hypothesis
• complementary sex determination
• Hymenoptera

Usage Notes

Haplodiploidy is sometimes treated as synonymous with arrhenotoky, though arrhenotoky technically refers to the developmental process (production of males from unfertilized eggs) while haplodiploidy describes the chromosomal state. Not all haplodiploid species are eusocial, and not all eusocial insects are haplodiploid (termites are diploid). The relatedness asymmetry assumes single-locus complementary sex determination and outbred mating; inbreeding or sl-CSD can alter these calculations. Some mites and thrips also exhibit haplodiploidy, but it is most studied in Hymenoptera.