Genome-wide association analysis (GWAS)
A diversity panel, consisting of 220 B. napus inbred lines was
used for the GWAS in this study (Xu et al., 2021). More specifically,
the population included winter, semi-winter and spring oilseed rape. The
accessions originated from Asia, Europe, and North America. The B.
napus population was grown at the Farm Station in Yangling, Shaanxi
province, China (107°59′–108°08′E; 34°14′–34°20′N), where the local
temperature ranged from −15 to 10 °C in winter
(https://lishi.tianqi.com/xianyang/202101.html). Low-temperature
resistance was measured in the field in January of 2017 and 2018. The
grade of low-temperature resistance was evaluated with criteria
including grade 1 (the plants are normal, and the leaves are green),
grade 2 (a small part of leaves are necrotic), grade 3 (a half of leaves
are necrotic), grade 4 (most of leaves are necrotic), and grade 5 (all
leaves are necrotic). The association analysis for low-temperature
resistance was carried out using a MLM with the software package TASSEL
5 (https://www.softpedia.com/get/Science-CAD/TASSEL.shtml). The
population structure was calculated using PCA. The software PopLDdecay
(https://github.com/BGI-shenzhen/PopLDdecay) was used to calculate
LD. The parameter r2 =0.2 was read as LD length.
The threshold of the GWAS was set to P -value <
10–6 .