Mutations of Self-Incompatibility Alleles in Trifolium Pratense and T. Repens.

HE existence of several allelomorphs at the S locus has been demonstrated in T Nicotiana spp. (EAST and YARNELL 1929), Veronica syriaca (LEHMANN 1926), Antirrhinum glutinosum ( GRUBER 1932), Trifolium pfatense (WILLIAMS 1939; and WILLIAMS and WILLIAMS 1947), Oenothera organensis (EMERSON 1939, 1940) and Trifolium repens (ATWOOD 1944). In Trifolium pratense, BATEMAN (1947) calculated that the number of S alleles operating in the material investigated was 212. The unusually high number of S alleles may have arisen in any of the following ways: (1) Direct mutation of one allele into another-any new mutation will have a selective advantage over the existing alleles and will easily spread into the population. It is expected that such new ‘constructive’ mutations, if they do occur in nature, could also be artificially induced. (2) An allele may give rise to a new allele in several steps, due to some balanced changes in its internal structure. The small initial selective advantage may ultimately lead to a new fully self-incompatible S allele. (3) All the S alleles may have been created independently from the original S gene complex (by the (2) method) during the course of evolution of the incompatibility system. In this case no mutation of one allele into another is involved. The technique of investigation may not be sensitive enough to pick up such minute changes as may be occurring in (2) and (3). A study of the spontaneous and X-ray induced mutations in the S gene in Prunus avium and Oenothera organensis by LEWIS (1948, 1949 and 1951) showed no indication of ‘constructive’ mutation. It was found that the S gene was composed of two parts, the part governing the reaction of pollen, and the part governing the reaction of style. Mutation could take place in one or both of these parts. But these changes always brought about self-compatibility and no evidence was found for the creation of a new S allele from another by direct mutation. The present study of spontaneous and X-ray induced mutations in S alleles of Trifolium pratense and Trifolium repens is directed towards further information regarding the nature of the S allele and its bearing upon the high number of these alleles encountered in the natural population.