The red flour beetle Tribolium castaneum: A model for host-microbiome interactions

Abstract

A large body of ongoing research focuses on understanding the mechanisms and processes underlying host-microbiome interactions, and predicting their ecological and evolutionary outcomes. To draw general conclusions about such interactions and understand how they are established, we must synthesize information from a diverse set of species. We analysed the microbiome of an important insect model-the red flour beetle Tribolium castaneumwhich is a widespread generalist pest of stored cereals. The beetles complete their entire life cycle in flour, which thus serves multiple functions: habitat, food, and a source of microbes. We determined key factors that shape the T. castaneum microbiome, established protocols to manipulate it, and tested its consequences for host fitness. We show that the T. castaneum microbiome is derived from flour-acquired microbes, and varies as a function of (flour) resource and beetle density. Beetles gain multiple fitness benefits from their microbiome, such as higher fecundity, egg survival, and lifespan; and reduced cannibalism. In contrast, the microbiome has a limited effect on development rate, and does not enhance pathogen resistance. Importantly, the benefits are derived only from microbes in the ancestral resource (wheat flour), and not from novel resources such as finger millet, sorghum, and corn. Notably, the microbiome is not essential for beetle survival and development under any of the tested conditions. Thus, the red flour beetle is a tractable model system to understand the ecology, evolution and mechanisms of host-microbiome interactions, while closely mimicking the host species' natural niche.

Oral Immune Priming Treatment Alters Microbiome Composition in the Red Flour Beetle Tribolium castaneum

“…The most abundant bacterial taxa in whole body larvae across all treatments belong to the phyla Proteobacteria, Firmicutes, and Actinobacteria, and the genera Bacillus, Acinetobacter, Pseudomonas , and Escherichia . These microbiome compositions are consistent with the previous studies in insects in general (e.g., Yun et al, 2014 ), as well as the studies in T. castaneum ( Kumari et al, 2011 ; Agarwal and Agashe, 2020 ). However, while Enterobacteriaceae were the most common and dominant bacteria in adults, larvae, and pupae of flour beetles in Agarwal and Agashe (2020) , the genus Massilia , which has so far not been described for T. castaneum , was most prevalent in most of our treatments.…”

“…These microbiome compositions are consistent with the previous studies in insects in general (e.g., Yun et al, 2014 ), as well as the studies in T. castaneum ( Kumari et al, 2011 ; Agarwal and Agashe, 2020 ). However, while Enterobacteriaceae were the most common and dominant bacteria in adults, larvae, and pupae of flour beetles in Agarwal and Agashe (2020) , the genus Massilia , which has so far not been described for T. castaneum , was most prevalent in most of our treatments. Massilia is considered to be a part of the soil microbiome ( Ofek et al, 2012 ) and has been reported in black ants ( Osimani et al, 2018 ) and longhorn beetles ( Rizzi et al, 2013 ).…”

“…It is thus most likely that Bacillus species that were previously present in the larvae expand in response to the priming treatment. Indeed, Bacillus has been previously reported in T. castaneum ( Kumari et al, 2011 ; Agarwal and Agashe, 2020 ) and also in the closely related Tenebrio molitor ( Osimani and Aquilanti, 2021 ). Furthermore, Bacillus was present in all our treatments, not only the orally primed ones.…”

Disruption of flour beetle microbiota limits experimentally evolved immune priming response, but not pathogen resistance

“…Previous work shows that the beetle microbiome is most likely acquired from the flour that the beetles inhabit and consume, and in which they also defecate and reproduce (Agarwal and Agashe, 2020). Beetles also derived significant fitness benefits from flour-acquired microbes, including higher fecundity and lifespan (Agarwal and Agashe, 2020). Thus, the easiest way to manipulate the beetle microbiome is to deplete the flour-associated microbial flora.…”

“…Thus, the easiest way to manipulate the beetle microbiome is to deplete the flour-associated microbial flora. We followed a previously published protocol in the lab (Agarwal and Agashe, 2020), where thin layers of wheat flour were exposed to UV radiation (UV -C ~254nm) in a laminar airflow for 2h. This treatment significantly alters flour microbiome with drastic depletion of the dominant bacterial taxa (also see Fig.…”