The genomic DNA of each sample was purified by Wizard Genomic DNA Purification Kit (Promega). 1 μg of the genomic DNA was firstly fragmented into 300 bp by the Covaris LE220R. The methylated adapters were ligated to the fragmented DNA. Bisulfite treatment was performed with EZ DNA Methylation-Gold Kit (Zymo Research) according to the instruction manual. The resulting single-stranded DNA were amplificated using KAPA HiFi HotStart Uracil+ ReadyMix (2×) (Roche). The concentration of purified libraries was measured with the Qubit fluorometer dsDNA HS Assay (Thermo Fisher Scientific). Size distribution of the sequencing libraries was analyzed using Agilent 2100 Bioanalyzer. WGBS was performed using an Illumina NovaSeq 6000 platform for 150 bp paired-end sequencing