Distribution of self-incompatibility alleles and breeding structure of open-pollinated cultivars of Brussels sprouts

Summary

Nineteen different S alleles were found in 488 plants representing 16 cultivars of Brussels sprouts. Twelve of these S alleles have previously been found in kale, and 7 are known only from Brussels sprouts. The number of S alleles per cultivar varied from 4 to 13, but for most cultivars the number fell in the range 7–10. Relatively unselected cultivars tend to have more S alleles than highly selected ones. In every cultivar the S allele frequencies are very unequal. The alleles with the greatest overall frequency are S₂ and S₅ and in almost all the cultivars tested one of these two alleles is the commonest. The rare S alleles not only occur at very low frequencies, but also occur in few cultivars. The commonest S alleles are recessive and the rare alleles are dominant. Dominant S alleles normally confer a higher degree of self-incompatibility than recessive S alleles. In Brussels sprouts cultivars a dynamic balance between outbreeding and inbreeding is achieved through the S allele system. The recessive S alleles, when present at high frequencies, allow some inbreeding, while the dominant S alleles promote outbreeding even when they are present at low frequencies. Because dominant S alleles confer high levels of self-incompatibility it is desirable to incorporate them into inbred lines used for the production of F₁ hybrid cultivars of Brussels sprouts. Such dominant alleles do occur in Brussels sprouts, but are rare and at least 20, and preferably 50, plants of a cultivar should be tested if rare S alleles are being sought.