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Arrhenotoky (from Ancient Greek ἄρρην árrhēn "male" and τόκος tókos "birth"), also known as arrhenotokous parthenogenesis, is a form of parthenogenesis in which unfertilized ovum develop into males. In most cases, parthenogenesis produces exclusively female offspring, hence the distinction.
In the most commonly used sense of the term, arrhenotoky is synonymous with haploid arrhenotoky or haplodiploidy: the production of haploid males from unfertilized eggs in having a haplodiploid sex-determination system. Males are produced parthenogenetically, while diploid females are usually produced biparentally from fertilized eggs. In a similar phenomenon, parthenogenetic diploid eggs develop into males by converting one set of their chromosomes to heterochromatin, thereby inactivating those chromosomes. This is referred to as diploid arrhenotoky or parahaploidy.
Arrhenotoky occurs in members of the insect order Hymenoptera (, , and ) and the Thysanoptera (thrips). The system also occurs sporadically in some , Hemiptera, Coleoptera (), Scorpiones (Tityus metuendus) and .
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