Developing and characterising Ricinus communis SSR markers by data mining of whole-genome sequences

Ricinus communis is a versatile industrial oil crop that is cultivated worldwide. Genetic improvement and marker-assisted breeding of castor bean have been slowed owing to the lack of abundant and efficient molecular markers. As co-dominant markers, simple sequence repeats (SSRs) are useful for genetic evaluation and molecular breeding. The recently released whole-genome sequence of castor bean provides useful genomic resources for developing markers on a genome-wide scale. In the present study, the distribution and frequency of microsatellites in the castor bean genome were characterised and numerous SSR markers were developed using genomic data mining. In total, 18,647 SSR loci at a density of one SSR per 18.89 Kb in the castor bean genome sequence (representing approximately 352.27 Mb) were identified. Dinucleotide repeats were the most frequently observed microsatellites, although the AAT repeat motif was also prevalent. Using six cultivars as screening samples, 670 polymorphic SSR markers from 1,435 primer pairs (46.7 %) were developed. Trinucleotide motif loci contained a higher proportion of polymorphisms (48.5 %) than dinucleotide motif loci (39.2 %). The polymorphism level in the SSR loci was positively correlated with the increasing number of repeat units in the microsatellites. The phylogenetic relationship among 32 varieties was evaluated using the developed SSR markers. Cultivars developed at the same institute clustered together, suggesting that these cultivars have a narrow genetic background. The large number of SSR markers developed in this study will be useful for genetic mapping and for breeding improved castor-oil plants. These markers will also facilitate genetic and genomic studies of Euphorbiaceae.