CHROMOSOMAL DIFFERENTIATION IN CLARKIA DUDLEYANA

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Abstract
Snow, Richard. (U. California, Davis.) Chromosomal differentiation in Clarkia dudleyana. Amer. Jour. Bot. 47 (4) : 302—309. Illus. 1960.—Clarkia dudleyana (n=9) is a common, colonial annual of the early‐summer California flora. Of 275 individuals, derived from 9 natural populations and their garden‐grown representatives, 17.1% were heterozygous for reciprocal translocations. Supernumerary chromosomes were also found in about 2% of the plants examined. The translocation heterozygotes are not distributed regularly over the species range but are concentrated near the geographical center of distribution. Most of the populations contained none or only a few heterozygotes, but in one colony 69% of 42 plants sampled were heterozygous. Judging from the meiotic metaphase associations observed, at least 5 different chromosome arrangements are present at this locality. Hybrids between colonies have invariably been translocation heterozygotes, the largest association found in such hybrids being a chain of all 18 chromosomes (a potential ring of 18). No correlation is evident between geographical separation and degree of cytological differentiation. Heterozygotes with smaller rings of 4 or 6 chromosomes, whether from natural populations or resulting from interpopulation hybridization, are highly fertile owing to the regular alternate disjunction of the chromosomes of the rings. In the larger rings of 12 to 18 chromosomes, derived from interpopulation crosses, segregation is much more irregular and leads to high sterility. It is possible that at least in some localities the heterozygotes enjoy a selective advantage over their homozygous sibs. It is also postulated that homozygosity for a particular chromosome arrangement may be selectively favored in a certain habitat, as a result of a position effect attendant upon placing formerly non‐linked genes in the same linkage group through reciprocal translocation. The high degree of chromosomal differentiation between some populations of this species suggests that the complex heterozygotes of Oenothera have arisen as a result of hybridization of cytologically differentiated races.