Abstract
The research field of animal and plant symbioses is advancing from studying interactions between two species to whole communities of associates. High-throughput sequencing of microbial communities supports multiplexed sampling for statistically robust tests of hypotheses about symbiotic associations. We focus on ambrosia beetles, the increasingly damaging insects primarily associated with fungal symbionts, which have also been reported to support bacteria. To analyze the diversity, composition, and specificity of the beetles' prokaryotic associates, we combine global sampling, insect anatomy, 454 sequencing of bacterial rDNA, and multivariate statistics to analyze prokaryotic communities in ambrosia beetle mycangia, organs mostly known for transporting symbiotic fungi. We analyze six beetle species that represent three types of mycangia and include several globally distributed species, some with major economic importance (Dendroctonus frontalis, Xyleborus affinis, Xyleborus bispinatus-ferrugineus, Xyleborus glabratus, Xylosandrus crassiusculus, and Xylosandrus germanus). Ninety-six beetle mycangia yielded 1,546 bacterial phylotypes. Several phylotypes appear to form the core microbiome of the mycangium. Three Mycoplasma (originally thought restricted to vertebrates), two Burkholderiales, and two Pseudomonadales are repeatedly present worldwide in multiple beetle species. However, no bacterial phylotypes were universally present, suggesting that ambrosia beetles are not obligately dependent on bacterial symbionts. The composition of bacterial communities is structured by the host beetle species more than by the locality of origin, which suggests that more bacteria are vertically transmitted than acquired from the environment. The invasive X. glabratus and the globally distributed X. crassiusculus have unique sets of bacteria, different from species native to North America. We conclude that the mycangium hosts in multiple vertically transmitted bacteria such as Mycoplasma, most of which are likely facultative commensals or parasites.
Highlights
The study of symbioses, whether between humans and their microbiome or insects and their symbionts, has begun to see a shift from research focusing on pairs of species toward the study of symbiotic communities
Studies of the ambrosia symbiosis have focused on insect interactions with the main fungal symbiont, and few addressed the role of other fungi, and even fewer studies exist on the presence and role of bacteria in this system
We addressed four questions: 1. Do ambrosia beetle mycangia, organs originally evolved to host fungi, contain a significant diversity of bacteria?
Summary
The study of symbioses, whether between humans and their microbiome or insects and their symbionts, has begun to see a shift from research focusing on pairs of species toward the study of symbiotic communities This is in part because new methods of environmental DNA sampling are revealing many previously unnoticed bacterial and fungal associates. Both coevolved symbionts and incidental associates may play important but different roles One such example is the ambrosia symbiosis between wood-boring beetles and diverse fungal and microbial associates. The beetles are often cited as a textbook example of tree disease vectors [1], yet in some cases, the disease agents may be incidental opportunists, rather than the primary symbionts of the beetles [2, 3] Such complexity of interactions highlights the need to survey the full diversity of microbial associates of ambrosia beetles. We combine the approaches of culture-independent microbiology, insect anatomy, and multivariate statistics to analyze bacterial communities inside one of the most frequently evolved organs mediating a symbiosis—the mycangia of ambrosia beetles
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