Ethical Approval statement
H. cumingii were treated according to animal care and use guidelines for scientific purposes
established by the Institutional Animal Care and Use Committee of Shanghai Ocean University, Shanghai, China.
Experimental materials
Two-year-old healthy H. cumingii mussels (average shell length of 10 cm) with purple and white inner-shell colors were obtained from Weimin Aquaculture Base, Jinhua City, Zhejiang Province, China(Fig. 5). Before the experiment, the mussels were placed in a laboratory water tank for oxygenation for about a week, and then fresh mantle samples were stored at −80°C for later use.
Experimental method
Total RNA extraction and cloning of the full-length HcPKC
The TRIzol method was used to extract total RNA from healthy mantle tissue samples. The SMARTer RACE 5′/3′ kit was used to synthesize RACE-Ready cDNA as a gene cloning template. The PKC gene fragment was obtained from a mantle transcriptome library of H. cumingii (Table 5), and the specific primers were designed by Primer 5.0 to perform PCR amplification and verify the sequence. According to the SMARTer RACE 5′/3′ kit instructions, 5′-RACE and 3′-RACE specific primers were designed, RACE cloning was performed, and the DNA was sequenced by Sangon (Shanghai, China) to obtain the full-length PKC gene.
Table 5 Primers used in the study
primers
|
(5′-3′)sequence of primers
|
purpose
|
HcPKC-F
|
GCTTGTTTCCAGACTGACGA
|
Partial fragment amplifification of HcPKC
|
HcPKC--R
|
CGTCAGTCTGGAAACAAGCAA
|
Partial fragment amplifification of HcPKC
|
HcPKC-3’
|
CCATCCATTCCTTGTAAACCTG
|
3’RACE
|
HcPKC-5’
|
GTCAGTCTGGAAACAAGCAAAC
|
5’RACE
|
HcPKC-RT-F
|
AGTGAACGTGATGCAGAGGA
|
qPCR
|
HcPKC-RT-R
|
GGTGTCAACACTGGCTTCTC
|
qPCR
|
HcPKC-Y-F
|
AGTGAACGTGATGCAGAGGA
|
In situ hybridization
|
HcPKC-Y-R
|
TAATACGACTCACTATAGGGGGTGTCAACACTGGCTTCTC
|
In situ hybridization
|
EF1α-F
|
GGAACTTCCCAGGCAGACTGTGC
|
qPCR internal control
|
EF1α-R
|
TCAAAACGGGCCGCAGAGAAT
|
qPCR internal control
|
Gene sequence analysis
ORF Finder (https://www.ncbi.nlm.nih.gov/orffinder/) was used to predict the ORF (open reading frame) of the HcPKC gene sequence and the encoded amino acid sequence[35]. Smart Blast was used to predict amino acid sequence homology analysis[36]. The amino acid inclusion domains were analyzed by Simple Modular Architecture Research Tool SMART (http://smart.embl-heidelberg.de/). The Protparam online tool (https://www.expasy.org/) was used to obtain information on physical parameters such as amino acid sequence composition, molecular weight, isoelectric point, etc[37]. ClustalX software was used for multiple sequence alignment analysis[38] and MEGA 5.2 (Arizona State University, USA) was used to construct a phylogenetic tree[39].
Tissue-specific expression analysis of the HcPKC gene
Hepatopancreas, gill, adductor muscle, foot, marginal membrane, central membrane samples were taken from six healthy H. cumingii individuals and were used for RNA extraction. The RNA was then reverse-transcribed to cDNA by using SYBR®Premix Ex Taq II (TliRNaseH Plus, TaKaRa). Bio-Rad-CFX-96 (Bio-Rad, USA) was used for fluorescence quantitative PCR. The PCR reaction mixture was as follows: SYBR®Premix Ex Taq II (TliRNaseH Plus), 10 μL; upstream and downstream primers, 0.8 μL; ddH2O, 6.8 μL and cDNA template 1.6 µL. Each reaction was performed in three replicates. The reaction parameters were: pre-denaturation at 95 °C for 30 s; followed by 40 cycles of 95 °C for 5 s; 56 °C for 35 s ; and 72 °C for 30 s. Referring to the previous research results of our laboratory, EF1α was used as an internal reference gene[40](Table 5).
In situ hybridization
Specific primers were designed and the T7 promoter sequence TAATACGACTCACTATAGGG (Table 5) was added at the 5′ end. The target fragment was obtained after PCR amplification and product purification, and in vitro transcription was performed using a Complete Gold in vitro transcription kit. The fresh mantle tissue of the mussel was placed in 4% paraformaldehyde to fix and dehydrate for 4 hours (in a 4°C refrigerator), then placed in 25% sucrose solution at 4°C overnight. The tissue was cut into ~10 μm sections. They were marked and stored on glass slides at −80°C for later use. Follow-up in situ hybridization experiments were performed later.
Extraction of genomic DNA
For SNP experiments, 70 white mussels and 70 purple mussels were selected randomly. The genomic DNA of the experimental samples was extracted using a TIANamp Marine Animals DNA Kit and coagulated with 1% agarose. The quality of DNA was detected by gel electrophoresis and a NanoDrop 2000C spectrophotometer, and the samples were placed in a −20°C refrigerator for later use.
Data measurement
Using a Lovibond-RT200 surface colorimeter to measure the inner shell color of purple and white experimental mussels, and according to the uniform color space determined by the International Commission on Illumination (CIE), L* represents the brightness. L* > 0 indicated that the color was bright, L* <0, darker color; a* >0, redder color, a* <0, greener color; b* >0, yellowish color, and b* <0, bluer color[41]. The anterior, middle, and posterior margins of the right shell of 140 mussels were measured, and the difference in the color parameter was calculated as follows: dE=(L2+a2+b2)½, L=Lx-L0, a=ax-a0, b=bx-b0. Lx, ax, and bx are the color parameter values of different shells. L0, a0, and b0 are the color parameters of standard white inner shell mussels and ML, Ma, Mb, and MdE represent the average value of L, a, b, and dE.
Screening of SNP loci in the HcPKC gene of H. cumingii
The HcPKC gene was compared with the PKC gene in the genome of the Trina clam to determine the exon and intron regions. Primers specific to exonic regions were designed (Table 6). The DNA samples of 10 white mussels and 10 purple mussels were selected randomly for sequence amplification, and the amplified products were sent to MAP BIOTECH (Shanghai) for sequencing. Sequence 5.4.6 was used to obtain the SNP site from the compared sequencing results.
Table 6 The primers of SNP in the HcPKC gene of H. cumingii
primers
|
(5′-3′)sequence of primers
|
F1
|
CTTTATTGACAATGGCAGAGCA
|
R1
|
AGTTCTGCTAAACCCCTCCAT
|
F2
|
TAACCATGATGATTTGTCTTCCTCT
|
R2
|
TTCCAGCAAACAGGACTGATTAT
|
Data analysis
Genetic parameters such as observed heterozygosity, expected heterozygosity, and polymorphism content were analyzed using Popgene software[42]. The chi-square test was performed using SPSS software to analyze the correlation between the genotypes of different SNPs in the HcPKC gene fragment and the inner shell color of the mussels [43]. Analysis of linkage disequilibrium and haplotype construction with SHEsis online software (http://analysis.bio-x.cn/)[44, 45].
Statistical analysis
Data are shown as the mean ± SD and was analysed using SPSS 17.0 software. Differences were recognized as signifificant when p < 0.05 and highly signifificant when p < 0.01.