AN ANALYSIS OF OUTCROSSING IN CERTAIN COMPLEX‐HETEROZYGOUS EUOENOTHERAS. I. FREQUENCY OF OUTCROSSING

HOFF, VICTOR J. (U. Arkansas, Fayetteville.) An analysis of outcrossing in certain complexheterozygous Euoenotheras. I. Frequency of outcrossing. Amer. Jour. Bot. 49(7): 715-721. 1962.Eleven complex-heterozygous races of Euoenothera were used for study. The races were grown in the midst of the experimental garden in which plants from approximately 150 races and hybrid combinations were growing together in close proximity so as to afford the maximum opportunity for outcrossing. Many capsules (126) resulting from unguarded flowers were collected from the chosen races and the seed grown to determine the number of hybrid plants resulting from outcrossing within each fruit. Among the various capsules, the number of outcrosses ranged from 0% to over 50% of the plants grown, while the total amount of outcrossing ranged from 0.7 to 7.8% among 5 different collections of capsules. A relationship between the amount of pollen produced by a flower or normally deposited on its stigma and the degree of outcrossing was noted. It is concluded that outcrossing probably occurs with sufficient frequency to account for the large number of complex-combinations found in nature. FROM the Rocky Mountains eastward, the presence of circles of 14 chromosomes (014),2 balanced lethals, and the self-pollinating habit have given to the subgenus Euoenothera a peculiar population structure consisting of a multitude of genetically diverse true-breeding races which are reproductively isolated from one another. Proceeding across the continent, the Euoenotheras fall into 10 major groupings based upon their cytological and genetical characteristics. The process by which the present major groupings presumably arose has been outlined by Cleland et al. (1950) and Cleland (1956, 1958, 1960). At least 4 distinct populations are thought to have arisen at different periods and spread across the country from a center of origin in Mexico or Central America. Each population was characterized by large-flowered, open-pollinated plants with mostly paired chromosomes free from lethals. After the first population had spread across the continent, a second later came to occupy the same area as the first, and crosses occurred with the earlier population which resulted, in many cases, in hybrids with 0914 because of different histories of interchange in the 2 populations. Balanced lethals and later the self-pollinating habit developed to produce true-breeding races. The process was again repeated when a third population crossed with both the first and second, and a fourth population with the third. Thus, the present major groupings, including the Received for publication December 16, 1961. Based on part of a dissertation submitted to the Graduate School of Indiana University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This work was supported in part by a Rockefeller Grant to Dr. R. E. Cleland. 2 0 14 indicates a circle of 14 chromosomes. parvifloras, the biennis-I, -II and -III assemblages and the strigosas, arose and are now recognized as possible species. The crosses between the original populations probably occurred in many localities, resulting from the beginning in many different hybrid lines. Because of heterogeneity of segmental arrangement within each population, these lines may have differed somewhat in the segmental arrangement of their complexes. At present, the number of true-breeding lines within each of the major groupings is large, and the question has been raised regarding whether the number of lines has increased since the original hybridizations and if it is still increasing. The question of outcrossing thus becomes important because of its evolutionary implications. Reproductive barriers found within true-breeding, permanent, heterozygous lines impose obstacles to further evolution by impeding gene flow. Genetic variability could, however, be produced through increasing the number of true-breeding lines by hybridization between already existing lines. Therefore, it is important to know whether genetic diversity has occurred through outcrossing within the Euoenotheras and if it is still taking place to bring about a further evolutionary development of the group. It has been assumed by Cleland et al. (1950) that outcrossing does occasionally occur, and that it has been an important evolutionary factor. There are, however, no definite data to support such a supposition at present. MATERIALS AND METHODS-The work was conducted within the experimental garden in which plants from approximately 150 different races and hybrid combinations were growing together in close proximity so as to afford the maximum opportunity for outcrossing. Eleven races, listed