Microbiome-assisted carrion preservation aids larval development in a burying beetle

Abstract

The ability to feed on a wide range of diets has enabled insects to diversify and colonize specialized niches. Carrion, for example, is highly susceptible to microbial decomposers, but is kept palatable several days after an animal's death by carrion-feeding insects. Here we show that the burying beetle Nicrophorus vespilloides preserves carrion by preventing the microbial succession associated with carrion decomposition, thus ensuring a high-quality resource for their developing larvae. Beetle-tended carcasses showed no signs of degradation and hosted a microbial community containing the beetles' gut microbiota, including the yeast Yarrowia In contrast, untended carcasses showed visual and olfactory signs of putrefaction, and their microbial community consisted of endogenous and soil-originating microbial decomposers. This regulation of the carcass' bacterial and fungal community and transcriptomic profile was associated with lower concentrations of putrescine and cadaverine (toxic polyamines associated with carcass putrefaction) and altered levels of proteases, lipases, and free amino acids. Beetle-tended carcasses develop a biofilm-like matrix housing the yeast, which, when experimentally removed, leads to reduced larval growth. Thus, tended carcasses hosted a mutualistic microbial community that promotes optimal larval development, likely through symbiont-mediated extraintestinal digestion and detoxification of carrion nutrients. The adaptive preservation of carrion coordinated by the beetles and their symbionts demonstrates a specialized resource-management strategy through which insects modify their habitats to enhance fitness.

Keywords: fungus; gut microbiota; insect nutrition; resource competition; symbiosis.

Fig. 1.

Comparison of untended and N. vespilloides-tended carcasses. (A) Untended carcasses showed overt signs of carrion degradation, tissue liquefaction, and fungal (Mucor) overgrowth. Tended carcasses, however, did not show signs of carrion deterioration or fungal growth. (B) Burying beetles prepare carcasses by creating a feeding cavity (indicated by the arrow) that houses the larvae. We test the hypothesis that the microbiota present in the biofilm-like matrix acts as an interface between the carcass and the beetles and promotes carrion preservation and larval development.

Fig. 2.

Beetles alter the microbial communities of tended carcasses. (A) Principal coordinates analysis using three separate distances showed no overlap between the bacterial communities of untended carcasses, tended carcasses, and soil. (B) Relative proportion of bacterial taxa (summarized at the genus level) for untended carcasses (UC), tended carcasses (TC), and soil (S) plotted as a heatmap. (C) Fungal communities (characterized using ITS sequences) also separated in principal coordinate space, indicating that the beetles altered the fungal community of tended carcasses.

Fig. 3.

Metatranscriptomic analysis of carcasses. (A) Tended carcasses supported a highly divergent eukaryotic and prokaryotic community compared with the decomposing, untended carcasses (multidimensional scaling based on trimmed mean of M-value–normalized transcript abundances). (B) Fungal (blue bars) and bacterial (red bars) community distribution based on top hits of differentially abundant RNA transcripts that are significantly overrepresented in untended and tended carcasses, representing percentage distribution for each genus. Transcripts assigned to Yarrowia were highly overrepresented in tended carcasses.

Fig. 4.

Preservation of beetle-tended carcasses is associated with changes in their biochemical properties. (A) Tended carcasses had significantly lower concentrations of putrescine and cadaverine compared with untended carcasses. Concentrations are expressed per gram of carcass-matrix biomass. (B) Tended carcasses also showed reduced levels of free amino acids valine (val), isoleucine (ile), leucine (leu), tyrosine (tyr), phenylalanine (phe), and tryptophan (trp), indicating suppression of protein degradation in tended carcasses. For each amino acid, peak areas per gram of carcass matrix are plotted. (C) Protease concentrations (ng/g carcass matrix) were higher in untended carcasses, indicating higher proteolytic degradation in untended carcasses. TC, tended carcasses; UC, untended carcasses. Different letters indicate significant pairwise differences, P < 0.05.

Fig. 5.

Effect of the carcass matrix on larval development. (A) Matrix-removed carcasses produced significantly smaller larvae for every gram of carcass tissue consumed. Average larval weights per gram of carcass were compared between matrix-control (n = 15) and matrix-removed (n = 17) carcasses. (B) The maximum larval weight per brood was lower in matrix-removed carcasses compared with the matrix-control carcasses. (C) There was no difference in the average number of larvae per brood per gram of carcass tissue consumed between the two groups. Bar plots show average with SE values. Different letters indicate significant differences using a t test; alpha = 0.05.