haplodiploidy

Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, while females develop from fertilized eggs and are diploid. This pattern is characteristic of many Hymenoptera, including bees, wasps, and ants, and is also observed in some thrips and other arthropods. In the typical arrangement, a queen or laying female can control whether an egg is fertilized; fertilized eggs yield diploid females, while unfertilized eggs yield haploid males. Males possess only the maternal set of chromosomes because they arise from unfertilized eggs and receive no paternal genetic contribution.

The system is sometimes referred to as arrhenotoky, in reference to male development from unfertilized eggs. Some species show deviations or alternative schemes, such as thelytoky, a form of parthenogenesis where females develop from unfertilized eggs or fertilized eggs, producing diploid females without paternal input. In other cases, species can exhibit mixed modes or facultative parthenogenesis, indicating that the reproductive strategy is not uniform across all haplodiploid taxa.

Evolutionary and ecological implications: Haplodiploidy affects relatedness within families, often resulting in higher relatedness between sisters than between mothers and offspring. This relatedness pattern is thought to influence the evolution of cooperative breeding and, in some groups, eusocial behavior. However, the social and ecological outcomes of haplodiploidy are shaped by multiple factors beyond the genetic mechanism of sex determination.