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

Abstract

Vertically transmitted endosymbiotic bacteria, such as Wolbachia, Cardinium and Rickettsia, modify host reproduction in several ways to facilitate their own spread. One such modification results in parthenogenesis induction, where males, which are unable to transmit the bacteria, are not produced. In Hymenoptera, the mechanism of diploidization due to Wolbachia infection, known as gamete duplication, is a post-meiotic modification. During gamete duplication, the meiotic mechanism is normal, but in the first mitosis the anaphase is aborted. The two haploid sets of chromosomes do not separate and thus result in a single nucleus containing two identical sets of haploid chromosomes. Here, we outline an alternative cytogenetic mechanism for bacterial endosymbiont-induced parthenogenesis in Hymenoptera. During female gamete formation in Rickettsia-infected Neochrysocharis formosa (Westwood) parasitoids, 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 a non-segregation pattern in the offspring of heterozygous females. We conclude that diploidy in N. formosa is maintained through a functionally apomictic cloning mechanism that differs entirely from the mechanism induced by Wolbachia.

Feminization and the collapse of haplodiploidy in an asexual parasitoid wasp harboring the bacterial symbiont Cardinium

“…Cardinium is required to feminize diploid (male) embryos and must interact with elements of the host sex determination system. This contrasts with the recent discovery of a PI-Rickettsia able to induce female development by only diploidization of unfertilized eggs in the parasitoid Neochrysocharis formosa; in this system antibiotic treatment of infected females causes the production of haploid males (Adachi- Hagimori et al, 2008).…”

“…In Wolbachia-infected parthenogenetic wasps the bacteria are responsible for diploidy restoration in unfertilized eggs (Stouthamer and Kazmer, 1994;Gottlieb et al, 2002;Pannebaker et al, 2004). Recently, a Rickettsia has also been found to cause diploidy restoration in a thelytokous chalcidoid species (Adachi- Hagimori et al, 2008). In E. hispida, the observation that antibiotic treatment causes the production of diploid sons suggests either a very early role, or no role for Cardinium in diploidy restoration of unfertilized eggs.…”

Occurrence of arrhenotoky and thelytoky in a parasitic wasp Venturia canescens (Hymenoptera: Ichneumonidae): Effect of endosymbionts or existence of two distinct reproductive modes?

“…Single locus-CSD appears to constrain the infection by endosymbiont-induced thelytoky because, for instance in the case of infection with Wolbachia, thelytokous induction involves gamete duplication in order to restore the diploidy and results in completely homozygous diploid offspring (Stouthamer & Kazmer, 1994). However, the recent discovery of Rickettsia inducing thelytoky in Neochrysocharis formosa (Hymenoptera: Eulophidae) through a cytogenetic mechanism that does not modify heterozygosity (Adachi-Hagimori et al, 2008) justifies the investigation of reproductive parasites inducing parthenogenesis in species with s1-CSD, like V. canescens.…”

Check out these males

“…In light of this assumption, cytological observations revealed that the mechanism by which diploidy is restored in Wolbachiainfected species is gamete duplication (the collapse of the first embryonic mitotic division) (for example, Stouthamer and Kazmer, 1994) and in Rickettsiainfected Neochrysocharis formosa it is apomixis (reductive meiosis does not occur; Adatchi- Hagimori et al, 2008). It was assumed that the males produced as a result of antibiotic treatments were haploid and so their ploidy status was not actually determined in any of the pertinent studies reported so far.…”