Body Shape and Coloration of Silkworm Larvae Are Influenced by a Novel Cuticular Protein
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HIGHLIGHTED ARTICLE | INVESTIGATION Body Shape and Coloration of Silkworm Larvae Are Influenced by a Novel Cuticular Protein Gao Xiong,*,1 Xiaoling Tong,*,†,1 Tingting Gai,* Chunlin Li,* Liang Qiao,‡ Antónia Monteiro,§,** Hai Hu,*,† Minjin Han,*,† Xin Ding,* Songyuan Wu,* Zhonghuai Xiang,*,† Cheng Lu,*,† and Fangyin Dai*,†,2 *State Key Laboratory of Silkworm Genome Biology and †Key Laboratory of Sericultural Biology and Genetic Breeding, Ministry of Agriculture, College of Biotechnology, Southwest University, Chongqing 400715, China, ‡Institute of Entomology and Molecular Biology, College of Life Sciences, Chongqing Normal University, 401331, China, §Department of Biological Sciences, National University of Singapore, 117543 Singapore, and **Yale-NUS College, 138614 Singapore ABSTRACT The genetic basis of body shape and coloration patterns on caterpillars is often assumed to be regulated separately, but it is possible that common molecules affect both types of trait simultaneously. Here we examine the genetic basis of a spontaneous cuticle defect in silkworm, where larvae exhibit a bamboo-like body shape and decreased pigmentation. We performed linkage mapping and mutation screening to determine the gene product that affects body shape and coloration simultaneously. In these mutant larvae we identified a null mutation in BmorCPH24, a gene encoding a cuticular protein with low complexity sequence. Spatiotemporal expression analyses showed that BmorCPH24 is expressed in the larval epidermis postecdysis. RNAi-mediated knock- down and CRISPR/Cas9-mediated knockout of BmorCPH24 produced the abnormal body shape and the inhibited pigment typical of the mutant phenotype. In addition, our results showed that BmorCPH24 may be involved in the synthesis of endocuticle and its disruption-induced apoptosis of epidermal cells that accompanied the reduced expression of R&R-type larval cuticle protein genes and pigmentation gene Wnt1. Strikingly, BmorCPH24, a fast-evolving gene, has evolved a new function responsible for the assembly of silkworm larval cuticle and has evolved to be an indispensable factor maintaining the larval body shape and its coloration pattern. This is the first study to identify a molecule whose pleiotropic function affects the development of body shape and color patterns in insect larvae. KEYWORDS body shape; color patterns; cuticular protein; apoptosis; novel gene NSECTS exhibit body shape and color patterns in the adult which involves the larva rolling away from predators in a Iand larval stages to either disguise them against the back- doughnut shape, where the dorsal side of each segment ground, and allow them to approach unsuspecting prey stretches to produce a perfectly smooth curved shape (O’Hanlon et al. 2014), or to protect themselves from their (Brackenbury 1997). On the other hand, the eyespot color own predators (Joron et al. 2011; Dasmahapatra et al. 2012; markings of some large lepidopteran larvae make them less Hossie et al. 2015; Prudic et al. 2015). Larval adaptations can susceptible to be attacked (Hossie et al. 2015). Thus, the high be especially intriguing. For example, the dorsally bulging diversity of both body shape and color patterns make insect segments of the caterpillar of the mother of pearl moth, larvae outstanding models for studies of adaptation and of its Patania ruralis, facilitate its high-speed escape method, underlying molecular basis (Ronshaugen et al. 2002; Suzuki and Nijhout 2006; Futahashi and Fujiwara 2008; Shirataki Copyright © 2017 by the Genetics Society of America et al. 2010). doi: https://doi.org/10.1534/genetics.117.300300 Manuscript received August 20, 2017; accepted for publication September 15, 2017; One of the key structures that determines body shape and published Early Online September 18, 2017. color patterns in insect larvae is the cuticle. The insect cuticle is Supplemental material is available online at www.genetics.org/lookup/suppl/doi:10. an extracellular matrix (ECM) covering the entire body. It 1534/genetics.117.300300/-/DC1. 1 fi These authors contributed equally to this work. generally consists of chitin bers, structural cuticular pro- 2Corresponding author: State Key Laboratory of Silkworm Genome Biology, Key teins (CPs), and some catecholamines and lipids (Moussian Laboratory of Sericultural Biology and Genetic Breeding, Ministry of Agriculture, College of Biotechnology, Southwest University, No. 1 Tiansheng Rd., Beibei 2010). Previous reports showed that disruption of chitin District, Chongqing 400715, China. E-mail: [email protected] fiber synthesis, decomposition, or modification in Drosophila Genetics, Vol. 207, 1053–1066 November 2017 1053 melanogaster (Moussian et al. 2005, 2006) and Tribolium hemocyte, trachea, ventral nerve cord, and wing discs. The castaneum (Zhu et al. 2008; Chaudhari et al. 2011, 2013), tissues were powdered in liquid nitrogen, and total RNA was and mutations of CPs in Bombyx mori (Qiao et al. 2014), led extracted and purified using TRIzol (Invitrogen, Carlsbad, to defects in cuticle assembly and ultimately caused abnor- CA) according to the manufacturer’s protocol. First cDNA was mal body shapes. On the other hand, the diversity of larval generated by reverse transcription using the PrimeScript RT color patterns depends on the content and distribution of reagent Kit with gDNA Eraser according to the supplier’s in- pigments in the cuticle, which are regulated by transcription structions and was used for quantitative and qualitative factors and pigment biosynthesis pathway genes expressed in RT-PCR analyses. the underlying epidermis (Shirataki et al. 2010; Osanai-Futa- Quantitative RT- PCR hashi et al. 2012; Yoda et al. 2014). Although these studies have advanced our understanding of the diversity of body To quantify the expression of BmorCPH24 and other genes, shapes and color patterns, they focused on the formation of we used the Bio-rad CFX96 sequence detection system with ’ each trait independently. Since the two larval traits coexist in an iTaqSYBRGreen (Bio-rad) according to the manufacturer s fi the ECM, it is possible that a common factor may cocontrol protocol. The primers designed for ampli cation are listed in body shape and color pattern simultaneously. The molecular Table S2 in File S1. The B. mori ribosomal protein L3 (rpL3, characterization of such factors, however, remains elusive. GenBank: AY769270.1) was used as the internal control. All In this work, we characterize a mutation in the silkworm, assessments were performed on three or four biological rep- B. mori, named Bamboo (Bo, chromosome 11–28.8 cM) that licates per sample. Gene expression levels were normalized causes variation in both body shape and pigmentation by those of a random sample, which were set to 1.0. Statis- ’ (Kanekatsu et al. 1988). Using positional cloning followed tical comparisons were made by Student s t-test or paired ’ by functional validation, we identified a low complexity CP Student s t-test. gene, BmorCPH24, that is responsible for the Bo phenotype. Identifying the Bo mutation site In addition, our findings indicate that novel or fast-evolving Primers were designed based on the sequence of genes can have key functions in development of larval body BGIBMGA011765 and BGIBMGA011766 mRNA and are shape and color patterns. shown in Table S1 in File S1. Total RNA was isolated from the wild-type and Bo strains using TRIzol reagent (Invitro- Materials and Methods gen), and cDNA libraries were established following the instructions of the Reverse Transcript Kit (No. RR037A, Insects Takara). PCR products were cloned into a PMD19-T vector The wild-type domesticated silkworm strains (Dazao, (Takara) and sequenced. For multi-strain analyses, PCR +Bo/+Bo from Bo/+Bo inbreeding), Bo mutant strains was performed on genomic DNA, and the amplified PCR (Bo/+Bo, Bo/Bo) and other strains without Bo phenotype, product was sequenced. were obtained from the Silkworm Gene Bank of Southwest In situ hybridization University, China. B. mandarina, a wild relative of B. mori, was collected in a field in Chongqing, China. All domesticated The digoxigenin-labeled RNA probes for antisense and silkworm strains were reared on mulberry leaves under a sense strands of BmorCPH24 were synthesized using the T7 12 hr/12 hr light/dark photoperiod at 24°. RiboMAXTM Express RNAi System (Promega, Madison, WI). A whole dorsal epidermis from the sixth to the ninth segment Positional cloning of day 2, fifth instar larvae were carefully dissected to protect The Bo mutant and Dazao strains were used as the parental the epidermal cells from falling off. The muscles and fat body strains for genetic mapping. F1 heterozygous males were pro- were retained on the epidermis until the staining was re- duced from a cross between a female Bo and a male Dazao. moved. The materials were then fixed immediately in 4% One thousand and forty-five BC1M progeny from the back- paraformaldehyde in PBS. In situ hybridization was performed cross Dazao ♀3 (Bo 3 Dazao) ♂ were used for recombina- according to a previously described protocol (Futahashi and tion analysis. For fine mapping, we used nine polymorphic Fujiwara 2005). fi PCR markers identi ed among the parents, and these were RNAi experiment assessed in BC1M individuals. Primers used for genotyping are listed in Supplemental Material, Table S1 in File S1. The double-stranded RNA (dsRNA) targeting BmorCPH24 and red fluorescence protein (dsRed, the negative control) Complementary DNA (cDNA) synthesis from the larval were synthesized using the RiboMAX Large