Brood Ball-Mediated Transmission of Microbiome Members in the Dung Beetle, Onthophagus taurus (Coleoptera: Scarabaeidae)

Abstract

Insects feeding on plant sap, blood, and other nutritionally incomplete diets are typically associated with mutualistic bacteria that supplement missing nutrients. Herbivorous mammal dung contains more than 86% cellulose and lacks amino acids essential for insect development and reproduction. Yet one of the most ecologically necessary and evolutionarily successful groups of beetles, the dung beetles (Scarabaeinae) feeds primarily, or exclusively, on dung. These associations suggest that dung beetles may benefit from mutualistic bacteria that provide nutrients missing from dung. The nesting behaviors of the female parent and the feeding behaviors of the larvae suggest that a microbiome could be vertically transmitted from the parental female to her offspring through the brood ball. Using sterile rearing and a combination of molecular and culture-based techniques, we examine transmission of the microbiome in the bull-headed dung beetle, Onthophagus taurus. Beetles were reared on autoclaved dung and the microbiome was characterized across development. A ~1425 bp region of the 16S rRNA identified Pseudomonadaceae, Enterobacteriaceae, and Comamonadaceae as the most common bacterial families across all life stages and populations, including cultured isolates from the 3rd instar digestive system. Finer level phylotyping analyses based on lepA and gyrB amplicons of cultured isolates placed the isolates closest to Enterobacter cloacae, Providencia stuartii, Pusillimonas sp., Pedobacter heparinus, and Lysinibacillus sphaericus. Scanning electron micrographs of brood balls constructed from sterile dung reveals secretions and microbes only in the chamber the female prepares for the egg. The use of autoclaved dung for rearing, the presence of microbes in the brood ball and offspring, and identical 16S rRNA sequences in both parent and offspring suggests that the O. taurus female parent transmits specific microbiome members to her offspring through the brood chamber. The transmission of the dung beetle microbiome highlights the maintenance and likely importance of this newly-characterized bacterial community.

Gut microbiota of dung beetles correspond to dietary specializations of adults and larvae

“…Female transmission of microbiota through the brood ball has also been shown in Onthophagus dung beetles (Estes et al . ), although the contribution of dung, and the differences in gut communities between life stages has not been shown for Onthophagus and other dung beetles. The bacterial community associated with E. intermedius and E. triangulatus eggs was highly similar to that of the female gut, the maternal gift and the brood ball (Figs , a, c, S4 and S5, Supporting information), and the eggs did not appear to contain any specialized or distinctive bacteria.…”

“…Estes et al . () used sterile rearing and a combination of molecular and culture‐based techniques to analyse the transmission of the microbiota of the dung beetle, Onthophagus taurus . This study provided strong evidence that the female parent transmits specific microbiota to her offspring via the maternal brood chamber, and supported the hypothesis that gut microbiota are important for development.…”

Gut Microbiota Colonization and Transmission in the Burying Beetle Nicrophorus vespilloides throughout Development

“…Although different factors may underlie the divergent influences of bacterial symbionts on insect hosts, one key component is the way that bacteria are transmitted between insect generations (12). Whereas obligate symbionts are always transmitted vertically, often via direct passage through eggs, more transient associations, typical of the gut microbiota, involve an external stage where bacteria are reacquired horizontally each generation via ingestion (13,14).…”

Developmental bias in horned dung beetles and its contributions to innovation, adaptation, and resilience

“…These contributions first begin when mothers engage in niche construction by digging deep tunnels underneath cow dung pats, within which they construct brood balls. Moreover, each brood ball is further endowed with a maternal fecal pedestal onto which a single egg is oviposited (Estes et al, 2013). Moreover, each brood ball is further endowed with a maternal fecal pedestal onto which a single egg is oviposited (Estes et al, 2013).…”

“…Specifically, recent work shows that the exchange of pedestals between two dung beetle species results in pronounced negative survival outcomes for one host species, while the other species demonstrates modest developmental delays with no significant effect on survival (Figure 3b). Immediately following hatching, larvae consume the pedestal and begin engaging in niche constructing behaviors (b'), which will continue through pupation and into adulthood (c') (image modified after Estes et al, 2013). For instance, larvae mechanically manipulate surrounding dung to alter the physical composition of the brood ball throughout their growth period, repairing the brood ball where maternal construction is inadequate, and eventually constructing a complex pupation chamber from dung fibers and the beetle's own feces shortly before the metamorphic molt.…”