Incompatibility in Autotetraploid Trifolium Repens L. I. Competition and Self-Compatibility.

ELF-INCOMPATIBILITY is the inability of a plant producing functional male and female gametes to set seed upon self-pollination. The best known genetic system effecting incompatibility in plants is that in which a single series of oppositional alleles governs pollen-tube growth, with pollen behavior under autonomous, or gametophytic, genetic control (EAST and MANCELSDORF 1926). Self-incompatibility in diploid white clover, Trifolium repens L., was shown by ATWOOD (1940) to be conditioned by multiple oppositional (S) alleles of this type. The gametophytic S allele system is similarly operative in T. pratense (WILLIAMS and SILOW 1933), T. hybridum (WILLIAMS 1951 ; BREWBAKER 1951), and T. nigrescens (BREWBAKER, unpublished data). Superimposed on the self-incompatibility system in most species are one or more mechanisms which reduce or inactivate pollen-tube inhibition, thereby permitting some self-fertilization to occur. One such mechanism occurring in polyploids is that of competition (LEWIS 1947). When certain oppositional alleles occur together in the heterogenic diploid pollen produced by autotetraploids, they interact (competition interaction) in such a way that the inhibition of these grains in self-pollinations is suppressed or inactivated. In Trifolium repens (ATWOOD and BREWBAKER 1953) and T. hybridum (BREWBAKER 1953) heterogenic grains that exhibited competition were never inhibited in selfor cross-pollinations, and functioned with a frequency equal to that of other uninhibited pollen classes in cross-pollinations. This type of self-fertility is intrinsically attributable to certain S allele combinations, and has been termed self-compatibility. Self-compatible autotetraploids segregated in the F, family derived from two self-incompatible, colchicine-doubled, parental clones of white clover studied by ATWOOD (1944). The data from this and advanced generation families were interpreted on the hypothesis that competition occurred in one of the six heterogenic pollen classes (ATWOOD and BREWBAKER 1953). The present investigations were made in order to obtain more comprehensive information on the frequency and behavior of competition pollen classes and the resultant self-compatibility in autotetraploid white clover.