Haplodiploid sex determination occurs in a wide range of animals, especially in Hymenoptera, where a fertilized egg develops into a diploid female and unfertilized into a haploid male. However, recent studies on diploid functional males in some wasps suggest that the simple addition of paternal gene by fertilization may not be enough to explain female offspring production in the sex determination system. Recently, activation of sex determination gene (tra) was found to have a pivotal role in determining the sex of Nasonia vitripennis. In N. vitripennis, tra is activated only on the paternal genome (i.e. sperm) not on the maternal counterpart (i.e. egg). Such parent specific activation of a gene is controlled by a epigenetic factor, DNA methylation. However, in Trichogramma kaykai, Wolbachia induces female offspring production without sperm. Therefore all female offspring are clonal to the maternal gene. This violates the role of activated sex determination gene (tra) from sperm in the wasp. We hypothesize that Wolbachia has an ability to activate the gene by demethylation. This hypothesis indicates that the target of sex ratio distorting endosymbionts may be an upstream gene. It will enhance our understanding of evolution of haplodiploid sex determination.

The α-proteobacterium Wolbachia pipientis infects a wide range of arthropods and filarial nematodes. Wolbachia is maternally inherited and is known to induce reproductive anomalies such as cytoplasmic incompatibility, feminization, male killing and parthenogenesis induction (PI). Trichogramma kaykai is a tiny wasp that parasitizes on lepidopteran eggs. When a female of the wasp is infected with PI-Wolbachia, the female produces female offspring via gamete duplication without the aid of sperm. As she ages, however, the fraction of male offspring increases. In this study, we investigated the effect of host genetic background on the expression of sex ratio between isofemale lines. Virgin females of six isofemale lines were allowed to lay eggs individually for 10 days. There was the positive relationship between female age and the offspring sex ratio. Furthermore, the sex ratio was significantly different among isofemale lines, implying that the host genetic background had an effect on the sex ratio. Based on the results, evolution of symbiosis in terms of sex ratio and future experimental design are discussed.

The Wolbachia bacterium is known to induce reproductive anomalies in various insect taxa such as cytoplasmic incompatibility, feminization, male killing and parthenogenesis. It is hypothesized that the degree of reproductive anomalies is dependent on the bacterial infection density. In this study, we attempted to test the hypothesis using the tiny egg wasp, Trichogramma kaykai that has served as the model system of parthenogenesis where an unfertilized egg develops into a female due to the bacterial infection. So far this is only found in haplodiploid organisms. The results show that 1) as mothers aged, they started producing male offspring, 2) the sex ratio was negatively correlated with the bacterial infection density, 3) female offspring were more than six times heavily infected with Wolbachia than male offspring in the species. In conclusion, female offspring production, parthenogenesis, is as a function of the Wolbachia bacterial density in this species.