Diet is the main source of microbiota in predatory mite hosts
Many studies have found dietary patterns can introduce different bacteria to the hosts [1, 7, 40], thus diets might most likely determine the host-specific intestinal bacterial community. In our study, diets also shape the microbiota in predatory mites from the results of microbial source track and microbiota comparisons among different mites. The comparison of microbiota source indicated about half of microbes from their prey T. urticae regardless of the types of predatory mites. In our hypothesis, we expected to see exactly independent microbial profiles in specialist mite. Whereas only one distinct ZOTU was found in comparison with generalist mites, suggesting large part of microbiota was provided by diet which is the same feeding to other mites. Although some microbes are reported to adjust host’s diet adaptation [20, 43], we did not find any signal of diet-associated microbes in response to specific diet in predatory mites. Generally, little relationship occurred between mite feeding types and their overall microbial species in predatory mites. Theoretically, impacts of the microbes on their hosts may range along a continuum from strong to weak dependence until to no association. Fruit flies and stick insects was found less dependency on gut microbes for digestion [11, 37], indicating that not all animals need a microbiome [15]. However, different core microbiota from diets is worth exploring whether some unique microbes could regulate the diet metabolism in predatory mites in further study.
Host species resulted in microbiota differentiation among generalist predators
In our study, we found that microbiota in polyphagous predators was only stable in A. orientalis and N. barkeri rather than A. swirskii. During the interaction of microbiota and hosts, various factors including stochastic, neutral, or selective processes could determine microbial community compositions of the host [7]. Among them, hosts deterministic selection for specific diet-acquired microbiota indicate functional association and strong dependence [24]. In our study, we also highlighted the host-imposed selection to the microbiota of predatory mites that different host species affected the core microbiota and network structure. For example, microbiota of A. swirskii was strikingly affected by their diet, suggesting fast replacement of microbiome in response to diet alteration. In predatory insects, dragonfly also do not have consistent diet-specific or stage-specific associations with gut bacterial communities, and dietary specialization and spatial variation in bacterial communities suggest the passive process [7]. Although microbiota in A. swirskii could be changed with the diets, host species itself is determinative to fluctuation of microbial community after prey switching. Different predator species had various adaptations to alternative preys. The niche width of A. swirskii was lower than other predators. Niche width refers to the sum of different resources used by a population in a community. In the case of less available resources, the species with higher niche width would generally enable to get sufficient resources. However, rapid changes in microbiome may help A. swirskii to obtain sufficient resources in disadvantageous environment [12].
Unknown environment influences bacterial community of the predators
In addition to dietary effect, environmental factor cannot be ignored in shaping the host microbial community. Studies from animals have shown that an important part of the microbiome is originated from non-dietary sources [35], such as herbivorous insects take up specific symbiont bacteria from the environment or directly from the soil [25]. Interestingly, caterpillars acquire a large part of their microbiome directly from the soil, rather than from the plants they are feeding [16]. In predatory mites, the source of a part of bacteria was unknown, indicating environmental and stochastic factors in microbiota construction. This “unknow” part was variable in predators, equivalent to 14% in A. orientalis, 27% in N. barkeri and 34% in A. swirski. It suggested the stochastic role of the environment played in microbiota construction of predatory mites.
Core microbiota of predatory mites
Even though host-associated microbial communities generally present substantial fluctuations with different conditions [17, 18, 33], hosts often associate with a specific set of microbes as core microbiota [13, 21, 44]. The taxa and abundance of these core microbes are often stable and conserved. It can be considered as a host species-, genus- or family-specific trait that may play key roles in determining host fitness and evolutionary potential [5, 19, 42]. The relationship between core microbiota and host diet width has been studied in insects. Moraxellaceae, Enterobacteriaceae, and Pseudomonadaceae were highly prevalent in specialist beetle species, while Rickettsiaceae associated exclusively with generalist beetles [3]. In our study, Clostridia was abundant and consistent in polyphagous predatory mite species, no matter changed diet or not. On the other hand, Gammaproteobacteria was abundant in monophagous predator, suggesting a possible relationship between diet width and these microbes. The high classification level of Gammaproteobacteria and Clostridia reveals a long-term and stable evolutionary relationship between microbiota and host diet width.
In addition to stable core microbiota, some typical bacterial taxa are enriched or depleted in response to different preys [23]. The abundance change of host bacterial taxa may due to the alteration of microbiota content in different preys, or the proliferation of some bacteria caused by different food nutrients. In mammals, different microbial groups are believed to specialize in the utilization of specific dietary substrates, in part because they tend to increase in abundance when these substrates are enriched in the host’s diet. For example, Bacteroidetes are associated with high-protein diets, while Firmicutes are associated with high-fiber diets [22]. Typical bacterial taxa were also enriched or depleted in predatory mites when they feeding on different preys. When feeding on T. urticae, bacterial species increased in predatory mites were most of plant endophytic bacteria, indicating the microbial transferences in tritrophic relationship among plant, spider mites and predatory mites.
In short, our research revealed that both host species and dietary variation modify the microbial composition of predatory mites. Predator feeding width shapes its core microbiota, but not overall microbiome. Although predatory mites had stable core microbiota, fluctuation of microbiome indicated that predatory mites have weak dependence on their microbial communities which were greatly influenced by diet. The association is closely related to predatory mite species.