Aedes aegypti microbiome composition covaries with the density of Wolbachia infection.

BACKGROUND: Wolbachia is a widespread bacterial endosymbiont that can inhibit vector competency when stably transinfected into the mosquito, Aedes aegypti, a primary vector of the dengue virus (DENV) and other arboviruses. Although a complete mechanistic understanding of pathogen blocking is lacking, it is likely to involve host immunity induction and resource competition between Wolbachia and DENV, both of which may be impacted by microbiome composition. The potential impact of Wolbachia transinfection on host fitness is also of importance given the widespread release of mosquitos infected with the Drosophila melanogaster strain of Wolbachia (wMel) in wild populations. Here, population-level genomic data from Ae. aegypti was surveyed to establish the relationship between the density of wMel infection and the composition of the host microbiome. RESULTS: Analysis of genomic data from 172 Ae. aegypti females across six populations resulted in an expanded and quantitatively refined, species-level characterization of the bacterial, archaeal, and fungal microbiome. This included 844 species of bacteria across 23 phyla, of which 54 species were found to be ubiquitous microbiome members across these populations. The density of wMel infection was highly variable between individuals and negatively correlated with microbiome diversity. Network analyses revealed wMel as a hub comprised solely of negative interactions with other bacterial species. This contrasted with the large and highly interconnected network of other microbiome species that may represent members of the midgut microbiome community in this population. CONCLUSION: Our bioinformatic survey provided a species-level characterization of Ae. aegypti microbiome composition and variation. wMel load varied substantially across populations and individuals and, importantly, wMel was a major hub of a negative interactions across the microbiome. These interactions may be an inherent consequence of heightened pathogen blocking in densely infected individuals or, alternatively, may result from antagonistic Wolbachia-incompatible bacteria that could impede the efficacy of wMel as a biological control agent in future applications. The relationship between wMel infection variation and the microbiome warrants further investigation in the context of developing wMel as a multivalent control agent against other arboviruses. Video Abstract.

Mitochondrial effects on fertility and longevity in Tigriopus californicus contradict predictions of the mother's curse hypothesis.

Strict maternal inheritance of mitochondria favours the evolutionary accumulation of sex-biased fitness effects, as mitochondrial evolution occurs exclusively in female lineages. The 'mother's curse' hypothesis proposes that male-harming mutations should accumulate in mitochondrial genomes when they have neutral or beneficial effects on female fitness. Rigorous empirical tests have largely focused on Drosophila, where support for the predictions of mother's curse has been mixed. We investigated the impact of mother's curse mutations in Tigriopus californicus, a minute crustacean. Using non-recombinant backcrosses, we introgressed four divergent mitochondrial haplotypes into two nuclear backgrounds and recorded measures of fertility and longevity. We found that the phenotypic effects of mitochondrial mutations were context dependent, being influenced by the nuclear background in which they were expressed, as well as the sex of the individual and rearing temperature. Mitochondrial haplotype effects were greater for fertility than longevity, and temperature effects were greater for longevity. However, in opposition to mother's curse expectations, females had higher mitochondrial genetic variance than males for fertility and longevity, little evidence of sexual antagonism favouring females was found, and the impacts of mitonuclear mismatch harmed females but not males. Together, this indicates that selection on mitochondrial variation has not resulted in the accumulation of male mutation load in Tigriopus californicus.

Evolutionary transition to XY sex chromosomes associated with Y-linked duplication of a male hormone gene in a terrestrial isopod.

Sex chromosomes are highly variable in some taxonomic groups, but the evolutionary mechanisms underlying this diversity are not well understood. In terrestrial isopod crustaceans, evolutionary turnovers in sex chromosomes are frequent, possibly caused by Wolbachia, a vertically-transmitted endosymbiont causing male-to-female sex reversal. Here, we use surgical manipulations and genetic crosses, plus genome sequencing, to examine sex chromosomes in the terrestrial isopod Trachelipus rathkei. Although an earlier cytogenetics study suggested a ZZ/ZW sex chromosome system in this species, we surprisingly find multiple lines of evidence that in our study population, sex is determined by an XX/XY system. Consistent with a recent evolutionary origin for this XX/XY system, the putative male-specific region of the genome is small. The genome shows evidence of Y-linked duplications of the gene encoding the androgenic gland hormone, a major component of male sexual differentiation in isopods. Our analyses also uncover sequences horizontally acquired from past Wolbachia infections, consistent with the hypothesis that Wolbachia may have interfered with the evolution of sex determination in T. rathkei. Overall, these results provide evidence for the co-occurrence of multiple sex chromosome systems within T. rathkei, further highlighting the relevance of terrestrial isopods as models for the study of sex chromosome evolution.

A bioinformatics approach to identifying Wolbachia infections in arthropods.

Wolbachia is the most widespread endosymbiont, infecting >20% of arthropod species, and capable of drastically manipulating the host's reproductive mechanisms. Conventionally, diagnosis has relied on PCR amplification; however, PCR is not always a reliable diagnostic technique due to primer specificity, strain diversity, degree of infection and/or tissue sampled. Here, we look for evidence of Wolbachia infection across a wide array of arthropod species using a bioinformatic approach to detect the Wolbachia genes ftsZ, wsp, and the groE operon in next-generation sequencing samples available through the NCBI Sequence Read Archive. For samples showing signs of infection, we attempted to assemble entire Wolbachia genomes, and in order to better understand the relationships between hosts and symbionts, phylogenies were constructed using the assembled gene sequences. Out of the 34 species with positively identified infections, eight species of arthropod had not previously been recorded to harbor Wolbachia infection. All putative infections cluster with known representative strains belonging to supergroup A or B, which are known to only infect arthropods. This study presents an efficient bioinformatic approach for post-sequencing diagnosis and analysis of Wolbachia infection in arthropods.