Parthenogenesis and Sex Determination in the Order Hymenoptera (Insecta): Patterns and Mechanisms

ABSTRACT The order Hymenoptera, which has ~160 000 described species, is the largest within the class Insecta, in which all members are capable of parthenogenesis. This review represents a synthesis of the previously existing and new reports and hypotheses on different cases and mechanisms of parthenogenesis in Hymenoptera, including genetic and cytological aspects of sex determination as well as the role of specific bacterial symbionts. Arrhenotokous parthenogenesis and haplodiploidy are likely to represent two key ancestral genetic features of this order. Multiple independent transitions to diploid, and, occasionally, to triploid thelytoky took place across different hymenopteran clades. Arrhenotoky, that is, haploid males and diploid females respectively developing from unfertilized and fertilized eggs, is implemented in Hymenoptera either through single‐locus (or sometimes multiple‐locus) complementary sex determination (CSD) or, at least in some taxa with high levels of inbreeding, via genomic imprinting. Similarly to other insects, sex determination in this order is performed by specific gene cascades, with transformer and doublesex usually being the main master and actuator genes, respectively. In a few special cases, males are produced through paternal genome elimination. Thelytoky in Hymenoptera can be sporadic, facultative or obligate, and, in terms of the presence/absence of the reductional meiotic division, either automictic or apomictic. Different types of automixis in this order include gamete duplication as well as central or terminal fusion. Arrhenotoky and thelytoky can coexist within the same species, whereby thelytokous individuals sometimes become intraspecific social parasites. Thelytoky in Hymenoptera, usually in the form of gamete duplication, is often induced by endosymbiotic microorganisms, and this type of parthenogenesis is almost exclusively restricted to parasitoids. These endosymbionts must therefore possess specific genes which ensure both egg diploidization and feminization of the developing individual, either in the form of one‐step or, more frequently, two‐step thelytoky. Although recent data and hypotheses significantly contribute to our understanding of hymenopteran reproduction, a number of questions deserve further investigation.