Microorganisms associated with thelytoky in Aphytis lingnansensis compere (Hymenoptera : Aphelinidae)

Patterns and mechanisms in instances of endosymbiont-induced parthenogenesis

Female-producing parthenogenesis can be induced by endosymbionts that increase their transmission by manipulating host reproduction. Our literature survey indicates that such endosymbiont-induced parthenogenesis is known or suspected in 124 host species from seven different arthropod taxa, with Wolbachia as the most frequent endosymbiont (in 56-75% of host species). Most host species (81%, 100 out of 124) are characterized by haplo-diploid sex determination, but a strong ascertainment bias likely underestimates the frequency of endosymbiont-induced parthenogenesis in hosts with other sex determination systems. In at least one taxon, hymenopterans, endosymbionts are a significant driver of transitions from sexual to parthenogenetic reproduction, with one-third of lineages being parthenogenetic as a consequence of endosymbiont infection. Endosymbiont-induced parthenogenesis appears to facilitate the maintenance of reproductive polymorphism: at least 50% of species comprise both sexual (uninfected) and parthenogenetic (infected) strains. These strains feature distribution differences similar to the ones documented for lineages with genetically determined parthenogenesis, with endosymbiont-induced parthenogens occurring at higher latitudes than their sexual relatives. Finally, although gamete duplication is often considered as the main mechanism for endosymbiont-induced parthenogenesis, it underlies parthenogenesis in only half of the host species studied thus far. We point out caveats in the methods used to test for endosymbiont-induced parthenogenesis and suggest specific approaches that allow for firm conclusions about the involvement of endosymbionts in the origin of parthenogenesis.

Insect Sex Determination Manipulated by Their Endosymbionts: Incidences, Mechanisms and Implications

The sex-determining systems of arthropods are surprisingly diverse. Some species have male or female heterogametic sex chromosomes while other species do not have sex chromosomes. Most species are diploids but some species, including wasps, ants, thrips and mites, are haplodiploids (n in males; 2n in females). Many of the sexual aberrations, such as sexual mosaics, sex-specific lethality and conversion of sexuality, can be explained by developmental defects including double fertilization of a binucleate egg, loss of a sex chromosome or perturbation of sex-determining gene expression, which occur accidentally or are induced by certain environmental conditions. However, recent studies have revealed that such sexual aberrations can be caused by various groups of vertically-transmitted endosymbiotic microbes such as bacteria of the genera Wolbachia, Rickettsia, Arsenophonus, Spiroplasma and Cardinium, as well as microsporidian protists. In this review, we first summarize the accumulated data on endosymbiont-induced sexual aberrations, and then discuss how such endosymbionts affect the developmental system of their hosts and what kinds of ecological and evolutionary effects these endosymbionts have on their host populations.

Effects of parthenogenesis and geographic isolation on female sexual traits in a parasitoid wasp

Population divergence in sexual traits is affected by different selection pressures, depending on the mode of reproduction. In allopatric sexual populations, aspects of sexual behavior may diverge due to sexual selection. In parthenogenetic populations, loss-of-function mutations in genes involved in sexual functionality may be selectively neutral or favored by selection. We assess to what extent these processes have contributed to divergence in female sexual traits in the parasitoid wasp Leptopilina clavipes in which some populations are infected with parthenogenesis-inducing Wolbachia bacteria. We find evidence consistent with both hypotheses. Both arrhenotokous males and males derived from thelytokous strains preferred to court females from their own population. This suggests that these populations had already evolved population-specific mating preferences when the latter became parthenogenetic. Thelytokous females did not store sperm efficiently and fertilized very few of their eggs. The nonfertility of thelytokous females was due to mutations in the wasp genome, which must be an effect of mutation accumulation under thelytoky. Divergence in female sexual traits of these two allopatric populations has thus been molded by different forces: independent male/female coevolution while both populations were still sexual, followed by female-only evolution after one population switched to parthenogenesis.

Sexual functionality of Leptopilina clavipes (Hymenoptera: Figitidae) after reversing Wolbachia-induced parthenogenesis

Females infected with parthenogenesis-inducing Wolbachia bacteria can be cured from their infection by antibiotic treatment, resulting in male production. In most cases, however, these males are either sexually not fully functional, or infected females have lost the ability to reproduce sexually. We studied the decay of sexual function in males and females of the parasitoid Leptopilina clavipes. In western Europe, infected and uninfected populations occur allopatrically, allowing for an investigation of both male and female sexual function. This was made by comparing females and males induced from different parthenogenetic populations with those from naturally occurring uninfected populations. Our results indicate that although males show a decay of sexual function, they are still able to fertilize uninfected females. Infected females, however, do not fertilize their eggs after mating with males from uninfected populations. The absence of genomic incompatibilities suggests that these effects are due to the difference in mode of reproduction.

Wolbachia density and host fitness components in Muscidifurax uniraptor (Hymenoptera: pteromalidae)

Intracellular bacteria of the genus Wolbachia are found in a variety of arthropod hosts, where they cause various reproductive disorders. Attempts to study the fitness advantages and disadvantages of carrying these symbionts have yielded contradicting results. Using various doses of the antibiotic rifampicin, we were able to manipulate the density of Wolbachia in the uniparental parasitoid Muscidifurax uniraptor (Hymenoptera: Pteromalidae). The effect of different titers of the symbiont on the fecundity, reproductive rate, longevity, survival rate, and sex ratio of the host was measured. The data gathered show that following antibiotic treatments, the percentage of males rises at low doses of rifampicin and then drops again. The total sex ratio of offspring produced by treated mothers was positively correlated with the numbers of Wolbachia found in eggs laid by these females. No significant effects were detected with regard to the other studied fitness components. It is concluded that in M. uniraptor, Wolbachia are not posing any burden on the life history trait studied.

Wolbachia pipientis: microbial manipulator of arthropod reproduction

The alpha-proteobacterium Wolbachia pipientis is a very common cytoplasmic symbiont of insects, crustaceans, mites, and filarial nematodes. To enhance its transmission, W. pipientis has evolved a large scale of host manipulations: parthenogenesis induction, feminization, and male killing. W. pipientis's most common effect is a crossing incompatibility between infected males and uninfected females. Little is known about the genetics and biochemistry of these symbionts because of their fastidious requirements. The affinity of W. pipientis for the microtubules associated with the early divisions in eggs may explain some of their effects. Such inherited microorganisms are thought to have been major factors in the evolution of sex determination, eusociality, and speciation. W. pipientis isolates are also of interest as vectors for the modification of wild insect populations, in the improvement of parasitoid wasps in biological pest control, and as a new method for interfering with diseases caused by filarial nematodes.

Phylogenetic analysis of parthenogenesis-inducing Wolbachia in the genus Aphytis (Hymenoptera: Aphelinidae)

Parthenogenesis-inducing intracellular bacteria of the genus Wolbachia are found in a variety of parasitoid wasp genera. The presence of Wolbachia in the uniparental Aphytis species A. lingnanensis Compere, A. diaspidis (Howard), A. chilensis Howard, and A. chrysomphali (Mercet) was tested using primers specific for the ftsZ gene. The symbiont was detected in all of these species. Wolbachia ftsZ genes that were sequenced from the four hosts show a high degree of similarity. Both the PCR with specific primers for group 'A' and phylogenetic analysis place these Wolbachia in group 'A'. The fact that the tested Aphytis species belong to different phylogenetic groups and harbour what seem to be almost identical Wolbachia, lends credence to the horizontal transmission hypothesis.

Parthenogenesis-inducing microorganisms in Aphytis (Hymenoptera: Aphelinidae)

Production of males in uniparental lines of two species in the parasitic wasp genus Aphytis was induced by rifampicin, and male sexual functioning was determined. Wolbachia-specific 16S rDNA primers were used in a PCR in order to: (1) assess correlation between thelytokous reproduction and the presence of Wolbachia; (2) detect the loss of Wolbachia DNA in uniparental A. lingnanensis following antibiotic treatments, with or without the presence of a host; and (3) clone and sequence part of the Wolbachia 16S rDNA from the uniparental Aphytis species for phylogenetic studies. Males produced viable sperm that was transferred to the female spermatheca following mating. However, sperm failure to effect egg fertilization resulted in all-male progeny. Wolbachia were found in the two uniparental (A. lingnanensis and A. diaspidis) but not in the two biparental (A. lingnanensis and A. melinus) Aphytis lines tested. They can be detected in wasps up to 7 days following antibiotic treatments, regardless of the presence of host. The 16S rDNA for the symbionts in the two Aphytis species is virtually identical, and is most closely related to the Wolbachia found in Muscidifurax uniraptor (Pteromalidae).