design: Sus scrofa ovarian mitochondrial fraction was enriched and total DNAs were extracted by QIAGEN DNeasy Blood & Tissue kit. Enriched mtDNAs were mixed with lambda DNA (dam-: dcm-) and pBR322 plasmid DNA, and BS-seq library was constructed by Zymo Pico Methyl-Seq Library Prep Kit, according to the manufacturers instructions.  Each NGS library was sequenced by Illumina NextSeq instrument using 150 bp paired-end sequencing v2.5 chemistry.