Bacterial symbionts may influence the fitness of their herbivore hosts, but such effects have been poorly studied across most invertebrate groups. The spider mite, Tetranychus truncatus, is a polyphagous agricultural pest harboring various bacterial symbionts whose function is largely unknown. Here, by using a high‐throughput 16S rRNA amplicon sequencing approach, we characterized the bacterial diversity and community composition of spider mites fed on five host plants after communities were modified following tetracycline exposure. We demonstrated that spider mite bacterial diversity and community composition were significantly affected by host plants and antibiotics. In particular, the abundance of the maternally inherited endosymbionts Wolbachia and Spiroplasma significantly differed among spider mites that were reared on different plant species and were completely removed by antibiotics. There was an overall tendency for daily fecundity to be lower in the mites with reduced bacterial diversity following the antibiotic treatment. Our data suggest that host plants and antibiotics can shape spider mite bacterial communities and that bacterial symbionts improve mite performance.

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.

Wolbachia and Spiroplasma are intracellular bacteria that are of great interest to entomologists, because of their ability to alter insect host biology in multiple ways. In the spider mite Tetranychus truncatus, co‐infection of Wolbachia and Spiroplasma can induce cytoplasmic incompatibility (CI) and fitness costs; however, little is known about the effect of co‐infection at the genetic level and the molecular mechanisms underlying CI. In this study, we explored the influence of the two symbionts on male mite host fitness and used RNA sequencing to generate the transcriptomes of T. truncatus with four different types of infection. In total, we found symbiont‐infected lines had a higher hatch proportion than the uninfected line, and the development time of the uninfected line was longer than that of the other lines. Co‐infection changed the expression of many genes related to digestion detoxification, reproduction, immunity and oxidation reduction. Our results indicate that co‐infection of Wolbachia and Spiroplasma confers multiple effects on their hosts, and helps illuminate the complex interactions between endosymbionts and arthropods.

Most arthropod-associated bacterial communities play a crucial role in host functional traits, whose structure could be dominated by endosymbionts. The spider mite Tetranychus truncatus is a notorious agricultural pest harboring various endosymbionts, yet the effects of endosymbionts on spider mite microbiota remain largely unknown. Here, using deep sequencing of the 16S rRNA gene, we characterized the microbiota of male and female T. truncatus with different endosymbionts (Wolbachia and Spiroplasma) across different developmental stages. Although the spider mite microbiota composition varied across the different developmental stages, Proteobacteria were the most dominant bacteria harbored in all samples. Positive relationships among related operational taxonomic units dominated the significant coassociation networks among bacteria. Moreover, the spider mites coinfected with Wolbachia and Spiroplasma had a significantly higher daily fecundity and juvenile survival rate than the singly infected or uninfected spider mites. The possible function of spider-mite associated bacteria was discussed. Our results highlight the dynamics of spider mite microbiotas across different life stages, and the potential role of endosymbionts in shaping the microbiota of spider mites and improving host fitness.

Colouration in spider mites is due to the presence of carotenoids with diverse colours, including yellows, oranges and reds. Tetranychus urticae has two main colour forms, red and green. Although a ketolase has been implicated in determining the colour, the underlying genetic basis of body colour divergence between the two forms has remained unclear. Based on a combination of comparative transcriptomes and RNA interference, we found that a gene encoding a cytochrome P450 enzyme of the CYP4 clan (CYP389B1) had remarkably high expression in adult females of the red T. urticae, as well as in hybrids obtained by crossing the red and green forms. Down‐regulation of this gene by RNA interference resulted in decreased accumulation of red pigment. Up‐regulation of the expressions of a scavenger receptor gene (SCARB1) and a mitochondrial glycine transporter (SLC25A38) also strongly contributed to red colour development in adult females. Suppressing the mRNA levels of these genes also resulted in reduced accumulation of red pigment in the three other spider mites with red body colour. Our results provide evidence that the body colour divergence between the two forms is caused by different expressions of pigmentation‐related genes, and point to a possible role of a novel cytochrome P450 gene (CYP389B1) in regulating red‐orange body colour. These findings expand the number of candidate cytochrome P450 genes involved in endogenous pigmentation and will help to understand their roles in determining colour patterns in mites and other species.