INCOMPATIBILITY STUDIES IN OENOTHERA : THE DISTRIBUTION OF S I ALLELES IN BIENNIS 1 POPULATIONS

The studies of the population structure of the North American Euoenotheras which have been carried out largely by Cleland and his associates have analyzed the segmental arrangement and phenotypes of the genomes in plants collected from a wide range of geographical locations (see Cleland, 1958 for a summary of this work). Although some collections have included three or four different plants from the same population, other than the work of Winterheimer (unpublished), relatively little has been done to investigate in more detail a sizable sample from a single stand of plants. Because this type of study requires on the average 15 to 20 hybrids for each collection analyzed, the size of the population sample undertaken for investigation tends to be limited to modest numbers. The discovery of an incompatibility allele system in the complex heterozygotes of Oenothera (Steiner, 1956) suggested another approach to population analysis, which, although providing in part different data, allows an estimate of the diversity of single populations as well as enabling an appreciable number of plants to be included in the study. The group of oenotheras best suited for this type of analysis is known as the biennis group 1 (Cleland et al., 1950). Members of this group are typical complex heterozygotes, composed of two genomes or "complexes" involving seven chromosomes each. These differ completely in the arrangement of their chromosome segments, so that metaphase I of meiosis is characterized by a ring of 14 chromosomes. The adjacent chromosomes of the ring regularly go to opposite poles at anaphase so that only two kinds of spores, and subsequently, gametes, are formed. Although these plants are normally self-pollinated, the homozygous complex combinations do not occur because of a balanced lethal system. Thus, the offspring are, like the parent, complete translocation heterozygotes which continue to breed true in succeeding generations. When different strains of complex heterozygotes are outcrossed, it becomes apparent that of the two complexes making up a plant, one is usually transmitted more readily through the egg, and the other more frequently through the pollen. On this basis, the two complexes are identified as alpha and beta, the alpha characteristically coming through the egg, the beta through the pollen. In addition, the alpha and beta complexes of a particular strain differ not only in the segmental arrangement of their chromosomes and their transmission behavior, but also produce distinctive phenotypes. Races differ in the extent of transmission of their complexes through egg and pollen. Some pass on only the alpha through the egg and the beta through the pollen; others may transmit the alpha through the pollen and the egg but only the beta through the pollen. Still others show offspring receiving both alpha and beta through pollen and egg; races also occur which transmit the alpha alone through the egg, but both complexes through the pollen. By intercrossing appropriate biennis group 1 races, alpha alpha combinations can readily be obtained. These are invariably self-incompatible because of the failure of pollen tubes to develop; when these plants are outcrossed either as male or female parents, they are perfectly compatible. An analysis of the compatibility behavior of biennis 1 alpha alpha showed that the alpha complexes carry an incompatibility allele system of the gametophytic type 1 Supported by Grant G-23596 from the National Science Foundation.