Bombyx Mori) and Wild Mulberry-Feeding Relatives
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The ISME Journal (2018) 12:2252–2262 https://doi.org/10.1038/s41396-018-0174-1 ARTICLE Gut bacterial and fungal communities of the domesticated silkworm (Bombyx mori) and wild mulberry-feeding relatives 1 1 2 1 1 3 Bosheng Chen ● Kaiqian Du ● Chao Sun ● Arunprasanna Vimalanathan ● Xili Liang ● Yong Li ● 4 1 4 1,5 Baohong Wang ● Xingmeng Lu ● Lanjuan Li ● Yongqi Shao Received: 5 May 2017 / Revised: 2 February 2018 / Accepted: 20 March 2018 / Published online: 12 June 2018 © The Author(s) 2018. This article is published with open access Abstract Bombyx mori, the domesticated silkworm, is of great importance as a silk producer and as a powerful experimental model for the basic and applied research. Similar to other animals, abundant microorganisms live inside the silkworm gut; however, surprisingly, the microbiota of this model insect has not been well characterized to date. Here, we comprehensively characterized the gut microbiota of the domesticated silkworm and its wild relatives. Comparative analyses with the mulberry-feeding moths Acronicta major and Diaphania pyloalis revealed a highly diverse but distinctive silkworm gut microbiota despite thousands of years of domestication, and stage-specific signatures in both total (DNA-based) and active 1234567890();,: 1234567890();,: (RNA-based) bacterial populations, dominated by the phyla Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. Most fungal sequences were assigned to the phyla Ascomycota and Basidiomycota. Environmental factors, including diet and human manipulation (egg production), likely influence the silkworm gut composition. Despite a lack of spatial variation along the gut, microbial community shifts were apparent between early instars and late instars, in concert with host developmental changes. Our results demonstrate that the gut microbiota of silkworms assembles into increasingly identical community throughout development, which differs greatly from those of other mulberry-feeding lepidopterans from the same niche, highlighting host-specific effects on microbial associations and the potential roles these communities play in host biology. Introduction The silkworm Bombyx mori, a specialist herbivore that Electronic supplementary material The online version of this article (https://doi.org/10.1038/s41396-018-0174-1) contains supplementary feeds on mulberry (Morus alba) leaves, is an economically material, which is available to authorized users. important insect that has been domesticated for thousands of years to maximize silk fiber productivity [1, 2]. The * Yongqi Shao importance of the silkworm is also reflected by its use as a [email protected] powerful insect model for research due to its relatively large 1 Institute of Sericulture and Apiculture, College of Animal size and ease of rearing [3]. Especially the silkworm gen- Sciences, Zhejiang University, Hangzhou, China ome sequence was the first one available for Lepidoptera 2 Analysis Center of Agrobiology and Environmental Sciences, [4], the second largest insect order, which includes the most Zhejiang University, Hangzhou, China disruptive agricultural pests. These genome-based studies 3 Institute of Soil and Water Resources and Environmental Science, have facilitated our understanding of this species. For College of Environmental and Resource Sciences, Zhejiang instance, genetic evidence has indicated that massive hor- University, Hangzhou, China izontal gene transfers have occurred from bacteria and fungi 4 National Collaborative Innovation Center for Diagnosis and to B. mori, possibly increasing silkworm survival and Treatment of Infectious Diseases, State Key Laboratory for fecundity [5] and suggesting the important role of micro- Diagnosis and Treatment of Infectious Diseases, the First Affiliated Hospital, School of Medicine, Zhejiang University, organisms in the evolution of B. mori Hangzhou, China With recent developments in the next-generation 5 Key Laboratory for Molecular Animal Nutrition, Ministry of sequencing technology, a growing number of studies have Education, Beijing, China cataloged and characterized microbial communities that live Gut bacterial and fungal communities of the domesticated silkworm (Bombyx mori) and wild. 2253 in diverse animal species, ranging from invertebrates to rRNA-Seq to characterize the metabolically active fractions vertebrates. For example, a simple and distinctive gut of the community. This method has been employed to study microbiota, consisting of eight bacterial phylotypes, has microbial communities from diverse environments [15–17], been found in honey bees [6]. Three proposed enterotypes such as the larval gut of the wood-feeding beetle [18]. were identified by their enrichment in the human gut Together, these comparative and systematic analyses microbiome and were unrelated to nationality or host establish a framework for comprehensive studies of the B. characteristics such as age or gender [7]. Furthermore, it is mori gut microbiome and yield insights into the influence of increasingly being recognized that these gut-inhabiting microbial symbionts in this economically important and microbes are of crucial importance for the host, with model insect. effects that range from enhancing host metabolism to shaping the immune system [8–10]. For example, foregut bacteria in the herbivorous white-throated woodrat (Neo- Materials and methods toma albigula) can degrade defensive plant secondary compounds, conferring detoxification potential to its host Sample collection and rearing [11]. Like most animals, silkworms are associated with large The Bombyx mori inbred strain p50 was provided by the consortia of nonpathogenic microbes [12]. Despite the Silkworm Germplasm Bank at the College of Animal Sci- existence of a large body of information on their biology, ences, Zhejiang University, China. The large dagger moth few studies have investigated the silkworm gut microbiota. Acronicta major Bremer and the mulberry pyralid Dia- Using 16S PCR-DGGE and clone library analyses, Xiang phania pyloalis Walker were collected from a mulberry- et al. estimated the midgut microbiota of two silkworm planting field (30°18′N, 120°04′E). All insects were reared strains and identified 41 bacterial phylotypes [13]. By cul- under standard conditions (25 ± 1 °C, 70 ± 5% humidity turing methods, 11 isolates were obtained from the gut of with a photoperiod of 14 h of light and 10 h of dark) using the B. mori 5th-instar larvae, most of which have the cap- fresh mulberry leaves collected from a greenhouse. ability of degrading various otherwise host-inaccessible polysaccharides from the diet [12], suggesting the potential Total nucleic acid extraction and reverse importance of microbiota members for host nutrition and transcription growth. These previous studies mainly yielded snapshots of the silkworm gut microbiome, as they were limited to The gut tissue dissected from freeze-killed insects was relatively few sequences from samples pooled from multi- weighed, directly transferred into a 2 mL sterile cryogenic ple individuals or to a specific host life stage. Thus, the vial sitting on ice. Tissues were homogenized in a Precellys- extent of the variation in this microbiota within individual 24 homogenizer (Bertin Technologies, Aixen, France) by silkworms, and the community dynamics across silkworm 5000 rpm for 20 s. The egg sample comprised 100 eggs due development, has been unclear. Furthermore, compared to to the limited material of a single egg. Foliage was collected its wild relatives that also exploit mulberry leaves as a food from a mulberry-planting field, cut into 2–3 cm squares with source, how the domestication process shapes gut microbial sterilized instruments, and washed in the buffer (10 mM communities of mulberry silkworms is still unknown. EDTA, 20 mM Tris, 0.012% Triton X-100, pH 7.8) for 30 To establish an ecological and comparative baseline for min on a shaker. Phyllosphere microbes were collected by experimental studies on the silkworm gut microbiome, here centrifuge. The total nucleic acid was extracted using the by using both culture-dependent techniques and culture- MasterPure™ Complete DNA and RNA Purification Kit independent high-throughput sequencing of 16S and inter- (Epicentre, Madison, USA). Specifically, template-free nal transcribed spacer (ITS) regions of the ribosomal RNA water “blanks” were processed with the same DNA gene, we conducted a systematic investigation of the extraction and PCR amplification kits as the negative con- microbial communities associated with the domesticated trols to avoid reagent contamination. To ensure efficient silkworm B. mori (the standard strain p50) and related extraction, lysozyme (50 mg/mL) was added into the tissue genera (the large dagger moth Acronicta major Bremer and and cell lysis solution, and gut homogenates or leaf washing the mulberry pyralid Diaphania pyloalis Walker) collected were incubated at 37 °C for 40 min. The amount and quality from a mulberry-planting field. As most studies on the gut of the total nucleic acid were measured by 2100 Bioanalyser microbiota have relied on DNA sequencing, it is unknown (Agilent, Palo Alto, USA). The extracted RNA was further whether these identified organisms were truly active inside reverse transcribed to cDNA for sequencing according to the host or were dormant or dead cells resulting from harsh the manufacturer’s instruments using random primers gut conditions, such as the extremely alkaline gut pH in (GoScript™ Reverse Transcription System, Promega, lepidopteran larvae [14].