Endosymbiont diversity in natural populations of Tetranychus mites is rapidly lost under laboratory conditions

Egg provisioning explains the penetrance of symbiont-mediated sex allocation distortion in haplodiploids

Maternally transmitted symbionts such as Wolbachia can alter sex allocation in haplodiploid arthropods. By biasing population sex ratios towards females, these changes in sex allocation may facilitate the spread of symbionts. In contrast to symbiont-induced cytoplasmic incompatibility (CI), the mechanisms that underpin sex allocation distortion remain poorly understood. Using a nuclear genotype reference panel of the haplodiploid mite Tetranychus urticae and a single Wolbachia variant that is able to simultaneously induce sex allocation distortion and CI, we unraveled the mechanistic basis of Wolbachia-mediated sex allocation distortion. Host genotype was an important determinant for the strength of sex allocation distortion. We further show that sex allocation distortion by Wolbachia in haplodiploid mites is driven by increasing egg size, hereby promoting egg fertilization. This change in reproductive physiology was also coupled to increased male and female adult size. Our results echo previous work on Cardinium symbionts, suggesting that sex allocation distortion by regulating host investment in egg size is a common strategy among symbionts that infect haplodiploids. To better understand the relevance that sex allocation distortion may have for the spread of Wolbachia in natural haplodiploid populations, we parametrized a model based on generated phenotypic data. Our simulations show that empirically derived levels of sex allocation distortion can be sufficient to remove invasion thresholds, allowing CI to drive the spread of Wolbachia independently of the initial infection frequency. Our findings help elucidate the mechanisms that underlie the widespread occurrence of symbionts in haplodiploid arthropods and the evolution of sex allocation.

Age-specific mortality and fecundity of a spider mite under diet restriction and delayed mating

Numerous experimental life-history studies on aging are mainly baised on two classical models-fruit fly Drosophila melanogaster (Meigen) and nematode Caenorhabditis elegans (Maupas)-with relatively little attention given to other organisms with different life-history characters. Two-spotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae) differs from many other arthropods in that the females continue their growth in the early adult stage and can reproduce sexually and asexually. In this study, the influences of dietary restriction and delayed mating on the aging patterns of the spider mite were examined with the prevailing survival and reproduction trade-off hypothesis of aging being tested. Significant sex-specific responses of the spider mites were found. The females showed longevity extension on diet restriction (fasting for 2 days in every 4 days) compared with their counterparts being fed ad libitum, and after delayed mating for 9 days, while the males displayed a decrease in lifespan when experiencing diet restriction but were not significantly influenced by delayed mating. Path analysis was used to investigate the relationship between mite survival and reproduction traits, including longevity, female lifetime reproduction, age at first reproduction, early reproductive efforts and late reproductive efforts, yielding no evidence for trade-offs between these life-history traits. The additive effects of dietary restriction and delayed mating in lifespan extension of female spider mites were confirmed, proving that diet restriction is a robust anti-aging intervention, and that later onset of reproduction can prolong adult lifespan in females.

Interacting host modifier systems control Wolbachia ‐induced cytoplasmic incompatibility in a haplodiploid mite

Reproductive parasites such as Wolbachia spread within host populations by inducing cytoplasmic incompatibility (CI). CI occurs when parasite‐modified sperm fertilizes uninfected eggs and is typified by great variation in strength across biological systems. In haplodiploid hosts, CI has different phenotypic outcomes depending on whether the fertilized eggs die or develop into males. Genetic conflict theories predict the evolution of host modulation of CI, which in turn influences the stability of reproductive parasitism. However, despite the ubiquity of CI‐inducing parasites in nature, there is scarce evidence for intraspecific host modulation of CI strength and phenotype. Here, we tested for intraspecific host modulation of Wolbachia‐induced CI in haplodiploid Tetranychus urticae mites. Using a single CI‐inducing Wolbachia variant and mitochondrion, a nuclear panel was created that consisted of infected and cured near‐isogenic lines. We performed a highly replicated age‐synchronized full diallel cross composed of incompatible and compatible control crosses. We uncovered host modifier systems that cause striking variation in CI strength when carried by infected T. urticae males. We observed a continuum of CI phenotypes in our crosses and identified strong intraspecific female modulation of the CI phenotype. Crosses established a recessive genetic basis for the maternal effect and were consistent with polygenic Mendelian inheritance. Both male and female modulation interacted with the genotype of the mating partner. Our findings identify spermatogenesis as an important target of selection for host modulation of CI strength and underscore the importance of maternal genetic effects for the CI phenotype. Our findings reveal that intraspecific host modulation of CI is underpinned by complex genetic architectures and confirm that the evolution of reproductive parasitism is contingent on host genetics.

Wolbachia and host intrinsic reproductive barriers contribute additively to postmating isolation in spider mites

Wolbachia are maternally-inherited bacteria that induce cytoplasmic incompatibility in many arthropod species. However, the ubiquity of this isolation mechanism for host speciation processes remains elusive, as only few studies have examined Wolbachia-induced incompatibilities when host populations are not genetically compatible. Here, we used three populations of two genetically differentiated colour forms of the haplodiploid spider mite Tetranychus urticae to dissect the interaction between Wolbachia-induced and host-associated incompatibilities, and their relative contribution to postmating isolation. We found that these two sources of incompatibility act through different mechanisms in an additive fashion. Host-associated incompatibility contributes 1.5 times more than Wolbachia-induced incompatibility in reducing hybrid production, the former through an overproduction of haploid sons at the expense of diploid daughters (ca. 75% decrease) and the latter by increasing the embryonic mortality of daughters (by ca. 49%). Furthermore, regardless of cross direction, we observed near-complete F1 hybrid sterility and complete F2 hybrid breakdown between populations of the two forms, but Wolbachia did not contribute to this outcome. We thus show mechanistic independence and an additive nature of host-intrinsic and Wolbachia-induced sources of isolation. Wolbachia may contribute to reproductive isolation in this system, thereby potentially affecting host differentiation and distribution in the field.

Wolbachia and Spiroplasma could influence bacterial communities of the spider mite Tetranychus truncatus

The structures of arthropod bacterial communities are complex. These microbiotas usually provide many beneficial services to their hosts, whereas occasionally they may be parasitical. To date, little is known about the bacterial communities of Tetranychus truncatus and the factors contributing to the structure of its bacterial communities are unexplored yet. Here, we used four symbiont-infected T. truncatus strains-including one Wolbachia and Spiroplasma co-infected strain, two symbiont singly-infected strains and one symbiont uninfected strain-to investigate the influence of endosymbionts on the structure of the host mites' microbiota. Based on 16S rRNA genes sequencing analysis, we found Wolbachia and Spiroplasma were the two most abundant bacteria in T. truncatus and the presence of both symbionts could not change the diversity of bacterial communities (based on alpha-diversity indexes such as ACE, Chao1, Shannon and Simpson diversity index). Symbiont infection did alter the abundance of many other bacterial genera, such as Megamonas and Bacteroides. The structures of bacterial communities differed significantly among symbiont-infected strains. These results suggested a prominent effect of Wolbachia and Spiroplasma on bacterial communities of the host T. truncatus. These findings advance our understanding of T. truncatus microbiota and will be helpful for further study on bacterial communities of spider mites.

Consequences of population structure for sex allocation and sexual conflict

Both sex allocation and sexual conflict can be modulated by spatial structure. However, how the interplay between the type of dispersal and the scale of competition simultaneously affects these traits in sub-divided populations is rarely considered. We investigated sex allocation and sexual conflict evolution in meta-populations of the spider mite Tetranychus urticae evolving under budding (pairing females from the same patch) or random (pairing females from different patches) dispersal and either local (fixed sampling from each subpopulation) or global (sampling as a function of subpopulation productivity) competition. Females evolving under budding dispersal produced less female-biased offspring sex ratios than those from the random dispersal selection regimes, contradicting theoretical predictions. In contrast, the scale of competition did not strongly affect sex allocation. Offspring sex ratio and female fecundity were unaffected by the number of mates, but female fecundity was highest when their mates evolved under budding dispersal, suggesting these males inflict less harm than those evolving under random dispersal. This work highlights that population structure can impact the evolution of sex allocation and sexual conflict. Moreover, selection on either trait may reciprocally affect the evolution of the other, for example via effects on fecundity.