Cytogenetic mechanism and genetic consequences of thelytoky in the wasp Trichogramma cacoeciae

Abstract

In Hymenoptera, complete parthenogenesis, that is thelytoky, is a common phenomenon where virgin females produce only daughters. Thelytoky is often induced by bacteria of the genus Wolbachia, but can also be genetically determined by the insect itself, as in the genus Trichogramma where both forms exist. In order to compare these two forms of thelytoky, chromosome behaviour analysis in young eggs and genetic analysis of microsatellite markers were carried out in the wasp Trichogramma cacoeciae, where thelytoky is genetically determined. Microscopic studies revealed that during female gamete formation meiotic cells undergo only a single equational division followed by the expulsion of a single polar body. This absence of meiotic recombination and reduction corresponds well with the high levels of heterozygosity observed in females collected from the field and a nonsegregation pattern in the offspring of heterozygous females. We therefore concluded that diploidy in T. cacoeciae is maintained through an apomictic cloning mechanism and that the incidence of thelytoky under genetic control of the wasp differs entirely from the mechanism induced by Wolbachia infection, where thelytoky is restored through gamete duplication.

A new cytogenetic mechanism for bacterial endosymbiont-induced parthenogenesis in Hymenoptera

“…However, the mechanism of the Rickettsia-induced parthenogenesis described here maintains heterozygosity, and consequently takes away one of the barriers involved in the transition from sexual to thelytokous reproduction for species that commonly outbreed, such as most diplodiploid species. It is interesting to note that apomictic parthenogenesis is the most common form of parthenogenesis within diplodiploid arthropods (Suomalainen et al 1987) and has been found in some Hymenoptera where parthenogenesis is not associated with microbial infection ( Vavre et al 2004). Our results suggest that in the future we may detect the presence of parthenogenesis-inducing symbionts in diplodiploid species.…”

“…This cytological mechanism is similar to that of a non-bacteria-induced thelytoky in the parasitoid wasp Trichogramma cacoeciae and is also known as apomictic cloning ( figure 1a(ii); Vavre et al 2004). …”

“…A schematic of different mechanisms in thelytokous diploidization is given in figure 1: a(ii) apomictic parthenogenesis (e.g. Vavre et al 2004), b(ii) automictic terminal fusion (e.g. Rö ssler & DeBach 1973), b(iii) automictic central fusion (e.g.…”

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“…Third, when parthenogenesis-inducing endosymbionts have been found in a species, they are often assumed to occur in related parthenogenetic species, which is not necessarily true. Mechanisms underlying parthenogenesis can greatly differ between highly related species, as shown in Trichogramma (Vavre et al 2004) and Encarsia parasitoids wasps (Gokhman 2009) as well as Aptinothrips grass thrips (van der Kooi and Schwander 2014a), where parthenogenesis is caused by endosymbionts in some species, but via another mechanism in others. In conclusion, in 42% of the investigated haplodiploid species parthenogenesis is due to endosymbionts, but the actual frequency presumably is much lower.…”

Thelytokous parthenogenesis in the damselfly Ischnura hastata (Odonata, Coenagrionidae): genetic mechanisms and lack of bacterial infection

“…Furthermore, a preliminary test of microsatellite loci amplification in a sample of parthenogenetic females from the Azores detected only one clone , which suggests that recombination does not take place and that asexual populations from the Azores probably arose from a single event. Absence of recombination has been found to occur in Bdelloid rotifers (Welch and Meselson, 2000), in the parthenogenetic wasp Trichogramma cacoeciae (Vavre et al, 2004) and in parthenogenetic oribatid mites (Schaefer et al, 2006). In addition, in thelytokous females of Apis mellifera capensis, recombination rates are lower than in sexually reproducing females of the same species.…”