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

Bin Hirota,Ryuichi Koga,Minoru Moriyama,Naruo Nikoh,Xian-Ying Meng,Takema Fukatsu,Ryo Futahashi,Genta Okude,Hisashi Anbutsu,Edward G Ruby

doi:10.1128/mbio.01482-17

Abstract

ABSTRACTThe 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.

Highlights

The saw-toothed grain beetle, Oryzaephilus surinamensis (Silvanidae), is a cosmopolitan stored-product pest
We identified the symbiont as a novel bacterial lineage belonging to the Bacteroidetes, confirmed the histological descriptions in old literature, including endosymbiotic constitution with four oval bacteriomes and transovarial symbiont transmission to eggs, and newly discovered that the symbiont cells are extraordinarily elongated in shape and that the symbiont is certainly not essential for the host’s growth and reproduction but contributes to the host’s cuticle formation
When we dissected adult insects of the laboratory strain of O. surinamensis, translucent oval tissues were found in the abdomen (Fig. 1B), which were closely associated with Malpighian tubules, fat bodies, and tracheae (Fig. 1C)

Summary

Introduction

The saw-toothed grain beetle, Oryzaephilus surinamensis (Silvanidae), is a cosmopolitan stored-product pest. Relevant to their peculiar ecology of continuously living on monotonous and non-fresh food sources, stored-product pest beetles are often associated with symbiotic microorganisms, including the Sitophilus grain weevils (Curculionidae) with bacteriomeharbored gammaproteobacterium Sodalis pierantonius [26, 27], the cigarette beetle Lasioderma serricorne and the drugstore beetle Stegobium paniceum (Anobiidae) with yeast-like fungal symbionts Symbiotaphrina spp. in the gut-associated symbiotic organs both endocellularly and extracellularly [39,40,41], and the lesser grain borer, Rhyzopertha dominica (Bostrichidae), whose bacteriome-associated endosymbiont was recently identified as belonging to the Bacteroidetes [42] Considering their easy rearing, experimental tractability, and economic importance, identification and understanding of biological roles of the bacteriome-associated endosymbionts in such stored-product pests are of general relevance to both basic and applied aspects of entomology and microbiology. The bacteriome-associated endosymbiosis in O. surinamensis has been totally untouched, leaving the understanding of the microbiological nature of the symbiont still elusive

Methods

Results

Conclusion