Thelytoky in Hymenoptera with Venturia canescens and Leptopilina clavipes as Case Studies

Abstract

The insect order of Hymenoptera comprises around 200.000 described species of ants, bees, wasps and sawflies, many of which serve important ecological and economic functions. All Hymenoptera have a haplodiploid mode of reproduction. Males always develop from unfertilized eggs and are haploid. Females are always diploid and can develop from both fertilized and unfertilized eggs. Within haplodiploidy, arrhenotoky is the most common mode of reproduction: unfertilized eggs develop into males that are haploid and 100% related to their mother, whereas fertilized eggs yield diploid females with a haploid complement of both parents. Thelytoky is a less common mode of reproduction. Thelytokous females develop parthenogenetically from unfertilized eggs after restoration of diploidy and are 100% related to their mother. Two distinctive classes of thelytoky can be distinguished based upon the causal mechanism: thelytoky can be induced by nuclear genes or be based on cytoplasmic genes including microorganisms. Most thelytokous hymenopterans reproduce by some form of automixis: both terminal fusion and central fusion have been found, while most cases of microbe-induced thelytoky are a form of gamete duplication. These different mechanisms can have a number of important implications for the genetic make-up of individuals and the amount and structure of genetic variation in populations. We discuss these implications and their evolutionary consequences, with a special focus on the ichneumonid parasitoid wasp Venturia canescens, in which thelytoky has a genetic basis, and the figitid parasitoid wasp Leptopilina clavipes, which has Wolbachia-induced thelytoky.