1.2 DNA extraction and sequencing
The plant genomic DNA extraction kit (TIANGEN) was used to extract the total DNA of the samples. After the quality of the total DNA samples was confirmed by Shanghai Hanyu Biotechnology Co., Ltd., the samples were subjected to bidirectional sequencing using an Illumina HiSeq 2500, and the raw data obtained were converted into raw reads by CASAVA base-calling analysis. The clean data obtained after removing the adaptor-containing, low-quality sequences were taken for subsequent analysis. Data processing was performed by Trimmomatic v0.32 (Bolger et al., 2014) with the following steps: (1) removal of sequences containing N bases; (2) removal of adaptor sequences in the reads; (3) removal of low-quality bases (Q value < 20) from the reads in the 3′ to 5′ direction; (4) removal of low-quality bases (Q value < 20) from the reads in the 5′ to 3′ direction; (5) removal of four bases with an average base quality of less than 20; and (6) removal of the reads and their pairs with a length less than 50 nt. Velvet v1.2.03 (Zerbino & Birney, 2008) was used to assemble the clean data.