EVIDENCE OF RELATIONSHIP BETWEEN EXTRA DIMINUTIVE CHROMOSOMES IN GEOGRAPHICALLY REMOTE RACES OF OENOTHERA

CLELAND, R. E. (Indiana U., Bloomington), and B. B. HYDE. Evidence of relationship between extra diminutive chromosomes in geographically remote races of Oenothera. Amer. Jour. Bot. 50(2): 179-185. Illus. 1963.-Two California races of 0. hookeri, Mono (from Mono Co.) and Mataguey (from San Diego Co.), have extra diminutive chromosomes. These diminutives do not associate with any of the normal chromosomes present. The extras from the 2 races, however, are able to synapse with each other and to form chiasmata, although the extras in Mono are distinctly larger than thcse in Mataguey. This suggests that they are at least partially homologous and that they have come from a common source. Two alternative hypotheses for their origin are suggested: (1) they have resulted from the loss of pairing ends through deletion, having descended from the same chromosome; (2) they represent normal chromosomes derived from another subgenus, through an inter-subgeneric cross followed by a backeross to hookeri. The facts tend to favor the latter hypothesis. MOST of the euoenotheras4 in California belong to the species 0. hookeri, as defined by Munz (1949). Among the many California races studied cytologically, 3 have shown extra diminutive chromosomes. These races are Hall 34 (from Bear Valley, in the San Bernardino Mountains), Mono (from Mono County), and Mataguey (from the Volcan Mountains in eastern San Diego County). A spread of approximately 500 miles separates the localities of Mono and Mataguey, with Hall 34 occupying an intermediate position. Hall 34 has been lost. In a detailed study of the structure and behavior of the extra diminutives (Cleland, 1951) it was found that the extras of Mono were about half the size of the normal chromosomes, whereas those of Mataquey were smaller (one-third to onequarter the size of the ordinary chromosomes). In neither race were the extras heteropyenotie, nor did their presence or absence have visible effects on the phenotype. Furthermore, in neither case did the extras show any tendency to pair with normal chromosomes, either within the races, where the chromosomes for the most part form pairs, or in outcrosses, where other configurations are usually found. Thus, their pairing I Received for publication July 31, 1962. 2 Supported by a grant from the Rockefeller Foundation. 3 Present address: Biological Laboratories, California Institute of TechnoIory, Pasadena, California. 4 We are informed that the subgenus of Oenothera, previously known as Onogra and later as Euoenothera, is now to be called Oenothera subgenus Oenothera, in accordance with the International Rules of Botanical Nomenclature. Since cytogeneticists have become accustomed to the Euoenothera, and since the use of the term Oenothera for both the genus and one of its subwenera may introduce ambiguities where traits are considered that characterize the subgenus but not the genus as a whole, we will continue to use the term "Euoenothera." ends were not homologous with any of the 14 ends found throughout the North American euoenotheras. Since the ends of the extra chromosomes in the 2 races were not homologous with the ends of the normal complement, it was of interest to know whether they were homologous with each other. If Mono is crossed with Mataquey, will the extras derived from the 2 races be able to pair with each other? If they are able to pair, does this suggest, in spite of the fact that Mono and Mataguey were found 500 miles apart, that the extras in the 2 races have been derived from a common source? Crosses in both directions were made in 1954 between the 2 races. The hybrids were grown in 1955; eight pedigrees were grown as follows: Mataguey X Mono 1954: