A Novel, Extremely Elongated, and Endocellular Bacterial Symbiont Supports Cuticle Formation of a Grain Pest Beetle

Abstract

The saw-toothed grain beetle, Oryzaephilus surinamensis (Silvanidae), is a cosmopolitan stored-product pest. Early studies on O. surinamensis in the 1930s described the presence of peculiar bacteriomes harboring endosymbiotic bacteria in the abdomen. Since then, however, the microbiological nature of the symbiont has been elusive. Here we investigated the endosymbiotic system of O. surinamensis in detail. In the abdomen of adults, pupae, and larvae, four oval bacteriomes were consistently identified, whose cytoplasm was full of extremely elongated tubular bacterial cells several micrometers wide and several hundred micrometers long. Molecular phylogenetic analysis identified the symbiont as a member of the Bacteroidetes, in which the symbiont was the most closely related to the endosymbiont of a grain pest beetle, Rhyzopertha dominica (Bostrichidae). The symbiont was detected in developing embryos, corroborating vertical symbiont transmission through host generations. The symbiont gene showed AT-biased nucleotide composition and accelerated molecular evolution, plausibly reflecting degenerative evolution of the symbiont genome. When the symbiont infection was experimentally removed, the aposymbiotic insects grew and reproduced normally, but exhibited a slightly but significantly more reddish cuticle and lighter body mass. These results indicate that the symbiont of O. surinamensis is not essential for the host’s growth and reproduction but contributes to the host’s cuticle formation. Symbiont genome sequencing and detailed comparison of fitness parameters between symbiotic and aposymbiotic insects under various environmental conditions will provide further insights into the symbiont’s biological roles for the stored-product pest.

Metabolic Cost of a Nutritional Symbiont Manifests in Delayed Reproduction in a Grain Pest Beetle

“…The sawtoothed grain beetle O. surinamensis engages in an ancient symbiosis with cuticle supplementing Bacteroidetes bacteria [ 43 , 44 ]. Symbiotic beetles exhibited a context-dependent higher desiccation tolerance, especially a lower mortality during larval development [ 44 ].…”

“…We previously observed an important symbiont contribution to larval survival, most likely through supported cuticle synthesis. However, the need for such support is probably highest for O. surinamensis , but holometabolous insects in general, during metamorphosis, when the entire insect body is reshaped, and as young adults, when the initially soft and weak cuticle is strengthened via sclerotization and melanization [ 22 , 43 , 49 ]. This would correlate with a control, at least partial, of the symbiont contribution via symbiont titer, but also means that the highest investment of the host is required during metamorphosis, when nutrients must be also invested into the reshaping of its own body, and no novel nutrients can be acquired until imagines hatch and are fully hardened.…”

“…Few model systems have so far been established in which microbial symbionts provide nutrients, but are not obligate for their host, either in laboratory conditions or, in some cases, where natural aposymbiotic host populations have been reported [ 23 , 24 , 41 ]. We recently demonstrated that a grain pest beetle, the sawtoothed grain beetle Oryzaephilus surinamensis , harbors Bacteroidetes symbionts that support their host by hardening the cuticle [ 42 , 43 , 44 ]. O. surinamensis is able to survive and reproduce in a quite broad range of environmental humidity conditions (at least from 12% to 90%) [ 45 , 46 ], partially supported by this mutualistic association—symbiotic beetles exhibited a humidity-dependent benefit in terms of lower larval mortality and stronger population growth compared to aposymbiotic beetles [ 44 ].…”

What can a weevil teach a fly, and reciprocally? Interaction of host immune systems with endosymbionts in Glossina and Sitophilus

“…The building of a strong cuticle, a characteristic feature of coleopterans, requires a huge amount of tyrosine and phenylalanine amino acids, which are the precursors of the 3,4-dihydroxyphénylalanine (DOPA) amino acid involved in the cuticle melanization and sclerotization (see below). Endosymbiont contribution to host cuticle synthesis through aromatic amino acid supply appears to be a convergent strategy among beetles [ 52 – 54 ].…”

Bacteriome-Associated Endosymbiotic Bacteria of Nosodendron Tree Sap Beetles (Coleoptera: Nosodendridae)

“…Besides weevils, early histological studies described the presence of bacteriome-associated endosymbiotic bacteria in horned powderpost beetles and grain borers (Bostrichidae) (Mansour, 1934;Buchner, 1954;Huger, 1956), saw-toothed grain beetles (Silvanidae) (Koch, 1931(Koch, , 1936Huger, 1956), soft-winged flower beetles (Dasytidae) (Stammer, 1933), false click beetles (Throscidae) (Stammer, 1933), and tree sap beetles (Nosodendridae) (Stammer, 1933). For decades, the microbiological nature of these endosymbiotic bacteria associated with the tiny minor beetles has been elusive, but recent molecular phylogenetic and genomic studies identified the bostrichid endosymbionts as belonging to the Bacteroidetes (Okude et al, 2017;Engl et al, 2018), the silvanid endosymbionts as allied to the bostrichid endosymbionts in the Bacteroidetes (Hirota et al, 2017;Engl et al, 2018), and the dasytid endosymbionts belonging to the γ-Proteobacteria (Weiss and Kaltenpoth, 2016). Meanwhile, the throscid endosymbionts and the nosodendrid endosymbionts are still to be characterized.…”