Douglas C. Boyes, Mikhail E. Nasrallah, Julia Vrebalov, June B. Nasrallah, The Self-Incompatibility (S) Haplotypes of Brassica Contain Highly Divergent and Rearranged Sequences of Ancient Origin, The Plant Cell, Vol. 9, No. 2 (Feb., 1997), pp. 237-247
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Boyes et al. (1997) studied this question.