Oppositional Alleles Causing Cross-Incompatibility in Trifolium Repens.

N SEVERAL species in which cross-incompatibility is determined by I multiple oppositional alleles, different types of tests have shown that these series of alleles are very extensive. The test yielding the most decisive results and involving the fewest crosses consists of making all possible reciprocal combinations (that is, diallel crosses) between plants which have one S factor in common. This procedure was applied first by EAST and YARNELL (1929); they found 15 different alleles among the 32 tested in Nicotiana. A similar test has since been made by WILLIAMS (1937) using red clover, in which 34 out of 40 alleles proved to be different. More recently, WILLIAMS (1941) reported on two other series of red clover plants in which 41 out of 48 and 37 out of 40 alleles, respectively, were different. Likewise, EMERSON (1940) has identified 45 different alleles in Oenothera organensis . A series of oppositional alleles conditioning cross-incompatibility has been found in plants of white clover, irrespective of whether they were selfincompatible, pseudo-self-compatible, or self-compatible (ATWOOD 1940; in press; 1941a). Matings between unrelated plants have very rarely been incompatible, and when selected plants were studied intensively (ATWOOD 1940; in press), seven of the eight alleles that were identified proved to be different. This evidence has suggested that the series of alleles in white clover may be just as extensive as that found in other species.