Host and symbiont genetic contributions to fitness in a Trichogramma-Wolbachia symbiosis

Bacterial Community Survey of Wolbachia-Infected Parthenogenetic Parasitoid Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) Treated with Antibiotics and High Temperature

Wolbachia has been shown to induce thelytokous parthenogenesis in Trichogramma species, which have been widely used as biological control agents around the world. Little is known about the changes of bacterial community after restoring arrhenotokous or bisexual reproduction in the T. pretiosum. Here, we investigate the emergence of males of T. pretiosum through curing experiments (antibiotics and high temperature), crossing experiments, and high-throughput 16S ribosomal RNA sequencing (rRNA-seq). The results of curing experiments showed that both antibiotics and high temperatures could cause the thelytokous T. pretiosum to produce male offspring. Wolbachia was dominant in the thelytokous T. pretiosum bacterial community with 99.01% relative abundance. With the relative abundance of Wolbachia being depleted by antibiotics, the diversity and relative content of other endosymbiotic bacteria increased, and the reproductive mode reverted from thelytoky to arrhenotoky in T. pretiosum. Although antibiotics did not eliminate Wolbachia in T. pretiosum, sulfadiazine showed an advantage in restoring entirely arrhenotokous and successive bisexual reproduction. This study was the first to demonstrate the bacterial communities in parthenogenetic Trichogramma before and after antibiotics or high-temperature treatment. Our findings supported the hypothesis that Wolbachia titer-dependence drives a reproduction switch in T. pretiosum between thelytoky and arrhenotoky.

The effect of the endosymbiont Wolbachia on the behavior of insect hosts

As one of the most successful intracellular symbiotic bacteria, Wolbachia can infect many arthropods and nematodes. Wolbachia infection usually affects the reproduction of their hosts to promote their own proliferation and transmission. Currently, most of the studies focus on the mechanisms of Wolbachia interactions with host reproduction. However, in addition to distribution in the reproductive tissues, Wolbachia also infect various somatic tissues of their hosts, including the brain. This raises the potential that Wolbachia may influence some somatic processes, such as behaviors in their hosts. So far, information about the effects of Wolbachia infection on host behavior is still very limited. The present review presents the current literature on different aspects of the influence of Wolbachia on various behaviors, including sleep, learning and memory, mating, feeding and aggression in their insect hosts. We then highlight ongoing scientific efforts in the field that need addressing to advance this field, which can have significant implications for further developing Wolbachia as environmentally friendly biocontrol agents to control insect-borne diseases and agricultural pests.

Wolbachia dominate Spiroplasma in the co-infected spider mite Tetranychus truncatus

Wolbachia and Spiroplasma are both maternally inherited endosymbionts in arthropods, and they can co-infect the same species. However, how they interact with each other in the same host is not clear. Here we investigate a co-infected Tetranychus truncatus spider mite strain that shares the same genetic background with singly infected and uninfected strains to detect the impacts of the two symbionts on their host. We found that Wolbachia-infected and Spiroplasma-infected mites can suffer significant fitness costs involving decreased fecundity, although with no effect on lifespan or development. Wolbachia induced incomplete cytoplasmic incompatibility in T. truncatus both in singly infected and doubly infected strains, resulting in female killing. In both females and males of the co-infected spider mite strain, Wolbachia density was higher than Spiroplasma density. Transcriptome analysis of female adults showed that the most differentially expressed genes were found between the co-infected strain and both the singly infected Spiroplasma strain and uninfected strain. The Wolbachia strain had the fewest differentially expressed genes compared with the co-infected strain, consistent with the higher density of Wolbachia in the co-infected strain. Wolbachia, therefore, appears to have a competitive advantage in host mites over Spiroplasma and is likely maintained in populations by cytoplasmic incompatibility despite having deleterious fitness effects.

Infection pattern and negative effects of a facultative endosymbiont on its insect host are environment-dependent

Regiella insecticola is a bacterial endosymbiont in insects that exhibits a negative effect on the fitness of hosts. Thus, it is not clear why this costly endosymbiont can persist in host populations. Here, we tested a hypothesis that the infection pattern and negative roles of the endosymbiont were not constant but environmentally dependent. The grain aphids Sitobion avenae, belonging to different genotypes and infected with Regiella or not, were used in this study. We found that S. avenae populations were infected with Regiella, Hamiltonella defensa, Serratia symbiotica and Rickettsia. The predominant endosymbionts in the aphid populations varied with season. Serratia and Rickettsia were predominant from December to February while Regiella predominated from March to May. The vertical transmission of Regiella was poorer at high temperature, but following conditioning for seven generations, the transmission rate improved. Regiella inhibited the production of winged aphids at 25 °C, but it did not affect winged morph production at the higher temperatures of 28 °C and 31 °C. Regiella infection decreased the intrinsic rate of increase (r_m) of aphids at 25 °C and 28 °C. However, at 31 °C, the effect of Regiella on the r_m varied depending on the aphid genotype and density. Thus, the negative effects of this endosymbiont on its host were environmentally dependent.

Wolbachia infection may improve learning and memory capacity of Drosophila by altering host gene expression through microRNA

Wolbachia are endosymbiotic bacteria present in a wide range of invertebrates. Although their dramatic effects on host reproductive biology have been well studied, little is known about the effects of Wolbachia on the learning and memory capacity (LMC) of hosts, despite their distribution in the host nervous system, including brain. In this study, we found that Wolbachia infection significantly enhanced LMC in both Drosophila melanogaster and D. simulans. Expression of LMC-related genes was significantly increased in the head of D. melanogaster infected with the wMel strain, and among these genes, crebA was up-regulated the most. Knockdown of crebA in Wolbachia-infected flies significantly decreased LMC, while overexpression of crebA in Wolbachia-free flies significantly enhanced the LMC of flies. More importantly, a microRNA (miRNA), dme-miR-92b, was identified to be complementary to the 3'UTR of crebA. Wolbachia infection was correlated with reduced expression of dme-miR-92b in D. melanogaster, and dme-miR-92b negatively regulated crebA through binding to its 3'UTR region. Overexpression of dme-miR-92b in Wolbachia-infected flies by microinjection of agomirs caused a significant decrease in crebA expression and LMC, while inhibition of dme-miR-92b in Wolbachia-free flies by microinjection of antagomirs resulted in a significant increase in crebA expression and LMC. These results suggest that Wolbachia may improve LMC in Drosophila by altering host gene expression through a miRNA-target pathway. Our findings help better understand the host-endosymbiont interactions and, in particular, the impact of Wolbachia on cognitive processes in invertebrate hosts.