Mosquito Microbiome Diversity Varies Along a Landscape-Scale Moisture Gradient

Intrinsic factors driving mosquito vector competence and viral evolution: a review

Mosquitoes are responsible for the transmission of numerous viruses of global health significance. The term "vector competence" describes the intrinsic ability of an arthropod vector to transmit an infectious agent. Prior to transmission, the mosquito itself presents a complex and hostile environment through which a virus must transit to ensure propagation and transmission to the next host. Viruses imbibed in an infectious blood meal must pass in and out of the mosquito midgut, traffic through the body cavity or hemocoel, invade the salivary glands, and be expelled with the saliva when the vector takes a subsequent blood meal. Viruses encounter physical, cellular, microbial, and immunological barriers, which are influenced by the genetic background of the mosquito vector as well as environmental conditions. Collectively, these factors place significant selective pressure on the virus that impact its evolution and transmission. Here, we provide an overview of the current state of the field in understanding the mosquito-specific factors that underpin vector competence and how each of these mechanisms may influence virus evolution.

Filtration of environmentally sourced aquatic media impacts laboratory-colonised Aedes albopictus early development and adult bacteriome composition

Microorganisms form close associations with metazoan hosts forming symbiotic communities, known as microbiomes, that modulate host physiological processes. Mosquitoes are of special interest in exploring microbe-modulated host processes due to their oversized impact on human health. However, most mosquito work is done under controlled laboratory conditions where natural microbiomes are not present and inferences from these studies may not extend to natural populations. Here we attempt to assemble a wild-resembling bacteriome under laboratory conditions in an established laboratory colony of Aedes albopictus using aquatic media from environmentally-exposed and differentially filtered larval habitats. While we did not successfully replicate a wild bacteriome using these filtrations, we show that these manipulations alter the bacteriomes of mosquitoes, generating a unique composition not seen in wild populations collected from and near our source water or in our laboratory colony. We also demonstrate that our filtration regimens impact larval development times, as well as impact adult survival on different carbohydrate diets.

Three species of axenic mosquito larvae recruit a shared core of bacteria in a common garden experiment

In this study, we describe the generation of two new species of axenic mosquito, Aedes albopictus and Aedes triseriatus. Along with Aedes aegypti, axenic larvae of these three species were exposed to an environmental water source to document the assembly of the microbiome in a common garden experiment. Additionally, the larvae were reared either individually or combinatorially with the other species to characterize the effects of co-rearing on the composition of the microbiome. We found that the microbiome of the larvae was composed of a relatively low-diversity collection of bacteria from the colonizing water. The abundance of bacteria in the water was a poor predictor of their abundance in the larvae, suggesting the larval microbiome is made up of a subset of relatively rare aquatic bacteria. We found 11 bacterial 16S rRNA gene amplicon sequence variants (ASVs) that were conserved among ≥90% of the mosquitoes sampled, including 2 found in 100% of the larvae, pointing to a conserved core of bacteria capable of colonizing all three species of mosquito. Yet, the abundance of these ASVs varied widely between larvae, suggesting individuals harbored largely unique microbiome structures, even if they overlapped in membership. Finally, larvae reared in a tripartite mix of the host-species consistently showed a convergence in the structure of their microbiome, indicating that multi-species interactions between hosts potentially lead to shifts in the composition of their respective microbiomes. IMPORTANCE This study is the first report of the axenic (free of external microbes) rearing of two species of mosquito, Aedes albopictus and Aedes triseriatus. Our previous report of axenic Aedes aegypti brings the number of axenic species to three. We designed a method to perform a common garden experiment to characterize the bacteria the three species of axenic larvae assemble from their surroundings. Furthermore, species could be reared in isolation or in multi-species combinations to assess how host-species interactions influence the composition of the microbiome. We found all three species recruited a common core of bacteria from their rearing water, with a large contingent of rare and sporadically detected bacteria. Finally, we also show that co-rearing of mosquito larvae leads to a coalescence in the composition of their microbiome, indicating that host-species interactions potentially influence the composition of the microbiome.

Aedes albopictus microbiome derives from environmental sources and partitions across distinct host tissues

The mosquito microbiome consists of a consortium of interacting microorganisms that reside on and within culicid hosts. Mosquitoes acquire most of their microbial diversity from the environment over their life cycle. Once present within the mosquito host, the microbes colonize distinct tissues, and these symbiotic relationships are maintained by immune-related mechanisms, environmental filtering, and trait selection. The processes that govern how environmental microbes assemble across the tissues within mosquitoes remain poorly resolved. We use ecological network analyses to examine how environmental bacteria assemble to form bacteriomes among Aedes albopictus host tissues. Mosquitoes, water, soil, and plant nectar were collected from 20 sites in the Mānoa Valley, Oahu. DNA was extracted and associated bacteriomes were inventoried using Earth Microbiome Project protocols. We find that the bacteriomes of A. albopictus tissues were compositional taxonomic subsets of environmental bacteriomes and suggest that the environmental microbiome serves as a source pool that supports mosquito microbiome diversity. Within the mosquito, the microbiomes of the crop, midgut, Malpighian tubules, and ovaries differed in composition. This microbial diversity partitioned among host tissues formed two specialized modules: one in the crop and midgut, and another in the Malpighian tubules and ovaries. The specialized modules may form based on microbe niche preferences and/or selection of mosquito tissues for specific microbes that aid unique biological functions of the tissue types. A strong niche-driven assembly of tissue-specific microbiotas from the environmental species pool suggests that each tissue has specialized associations with microbes, which derive from host-mediated microbe selection.

Gut Bacterial Diversity of Field and Laboratory-Reared Aedes albopictus Populations of Rio de Janeiro, Brazil

BACKGROUND

The mosquito microbiota impacts different parameters in host biology, such as development, metabolism, immune response and vector competence to pathogens. As the environment is an important source of acquisition of host associate microbes, we described the microbiota and the vector competence to Zika virus (ZIKV) of Aedes albopictus from three areas with distinct landscapes.

METHODS

Adult females were collected during two different seasons, while eggs were used to rear F1 colonies. Midgut bacterial communities were described in field and F1 mosquitoes as well as in insects from a laboratory colony (>30 generations, LAB) using 16S rRNA gene sequencing. F1 mosquitoes were infected with ZIKV to determine virus infection rates (IRs) and dissemination rates (DRs). Collection season significantly affected the bacterial microbiota diversity and composition, e.g., diversity levels decreased from the wet to the dry season. Field-collected and LAB mosquitoes' microbiota had similar diversity levels, which were higher compared to F1 mosquitoes. However, the gut microbiota composition of field mosquitoes was distinct from that of laboratory-reared mosquitoes (LAB and F1), regardless of the collection season and location. A possible negative correlation was detected between Acetobacteraceae and Wolbachia, with the former dominating the gut microbiota of F1 Ae. albopictus, while the latter was absent/undetectable. Furthermore, we detected significant differences in infection and dissemination rates (but not in the viral load) between the mosquito populations, but it does not seem to be related to gut microbiota composition, as it was similar between F1 mosquitoes regardless of their population.

CONCLUSIONS

Our results indicate that the environment and the collection season play a significant role in shaping mosquitoes' bacterial microbiota.