APOMIXIS AND POLYEMBRYONY IN PACHIRA OLEAGINEA (BOMBACACEAE)

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. IN PLANTS, polymorphic chromosomal systems are usually hased on reciprocal translocations. Oenothera is, of course, the classic example. But the complex-heterozygotes of this genus are exceptional and extreme, since homozygous classes never appear. Members of other genera, such as Paeonia californica (Walters, 1942), Datura meteloides (Snow, 1959), and several species of the most closely related genus to Oenothera, Clarkia (including Godetia), exhibit a less extreme situation, heterozygotes 1 Received for publication August 24, 1959. In the years 1956 and 1957 this research was carried out during the tenure of National Science Foundation Predoctoral Fellowships. It is a distinct pleasure to acknowledge the interest and guidaniee of Dr. Harlan Lewis, under whom this study was conducted. Both he and Dr. G. Ledyard Stebbins have made many helpful suggestions concerning the manuscript. often being common, sometimes exceedingly so, but never to the exclusion of at least some classes of homozygotes. Translocation heterozygotes have been found in 14 of the 34 known species of Clarkia (Lewis and Lewis, 1955; Lewis, unpubl.; Snow, unpubl.), and it is likely that study of some species less well known cytologically would increase this number. These heterozygotes are very common in C. unguiculata (Lewis, 1951; Mooring, 1958), C. williamsonii (Lewis and Venkatesh, unpubl.), and C. amoena (Ha'kansson, 1942; Snow, unpubl.). This paper reports upon studies of translocation heterozygosity in Clarkia dudleyana, where, as will be shown below, heterozygotes are also common in nature at some localities. The results of interpopulation crosses have shown in addition that the various This content downloaded from 157.55.39.78 on Tue, 21 Jun 2016 07:08:01 UTC All use subject to http://about.jstor.org/terms