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Stability of S Alleles in a Doublecross Hybrid of Red Clover¹

The S-allele system used for controlling crossing in doublecross hybrid red clover (Trlfoliura pratense L.) is expected in theory to produce four heterozygous allelic types arbitrarily designated S₁S₅, S₁S₇, S₃S₅, and S₃S₇. This study was undertaken to ascertain if this theoretical segregation is achieved in a doublecross produced under field cages where crosses are made with honey bees (Aphis mellifera L.). S-alleles of doublecross hybrid plants were identified by presence or absence of seed produced from crosses to four homozygous I₁ parents, and from chain crosses. Among 77 plants examined, 16 had S₁S₅, 11 had S₁S₇, 14 had S₃S₅, 11 had S₃S₇, 12 had one unknown allele, and 10 had two unknown alleles. Three exceptional plants were found that appeared to be S₅S₇ but set seed when intra-crossed suggesting an unusual genetic control. No S₁S₃ or S₁S₇ plants other than the three plants described above, were found. About 68% of the population conformed to theoretical expectations. The remainder apparently resulted from generation of new S-specificities, contamination, or both. Either possibility would have serious consequences for control of crossing by the S-allele system.

Key Words: Gametophytic system • Singlecrosses • Sallele segregatio • Self compatibilit • Pseudo-self-compatibili • Trifolium pratense L.

² Professor of agronomy, Univ. of Kentucky, Lexington, KY 40546-009

Received for publication March 26, 1982.