Oppositional Allelism in Diploid and Autotetraploid Trifolium Hybridum L.

HE selfand cross-incompatibilities of a number of obligately cross-polliT nated species of flowering plants are genetically determined by series of multiple oppositional alleles. The oppositional or personate incompatibility system was first described, with substantiating data, by EAST and MANGELSDORF (1925) for Nicotiana species. The existence of oppositional alleles has been subsequently demonstrated in at least 17 other species, including four species of the genus Trifolium-T. pratense (WILLIAMS and SILOW 1933), T. repens (ATWOOD 1940), T. hybridum (WILLIAMS 1951; BREWBAKER 1951), and T. nigrescens ( BREWBAKER, unpublished data). The literature pertaining to incompatibilities of flowering plants has been summarized in reviews by SEARS (1937), STOUT (1938), LEWIS (1949) and ATWOOD and BREWBAKER (1953). Incompatibility relationships in colchicine-induced tetraploids have been studied most extensively in Oenothera orgunensis (LEWIS 1947) and in Trifoliuwa repens (ATWOOD 1944; ATWOOD and BREWBAKER 1953). In Trifolium repens colchicine-doubled 4N sectors were commonly self-incompatible like the diploids upon which they arose, although occasional self-incompatible 2N clones produced self-compatible tetraploid sectors following colchicine treatments ( BREWBAKER 1952a). In Oenothera the personate pollen-tube inhibition was reduced in some tetraploids, but not to a sufficient degree for self-fertilization to occur. The present investigations with alsike clover were started in 1948 in conjunction with incompatibility studies in related Trifolium species. The first objective was to determine the genetic basis for selfand cross-incompatibilities in diploid alsike. In addition, the investigations were extended to colchicineinduced tetraploids. Specifically, the latter studies were conducted to determine the genetic basis for the change from a self-incompatible diploid to a self-compatible tetraploid which was observed among the first series of colchicinetreated plants.