CHROMOSOMAL REPATTERNING INCLARKIA RHOMBOIDEAAS EVIDENCE FOR POST-PLEISTOCENE CHANGES IN DISTRIBUTION

Abstract

Clarkia rhomboidea Douglas (Onagraceae) is a widely distributed western North American annual of polyploid origin (Lewis and Lewis, 1955; Mosquin, 1961). Studies of meiotic associations of chromosomes in artificial interpopulational hybrids of this species indicate that since its origin a considerable amount of chromosomal reorganization has occurred among natural populations. The chromosomes have been altered both by reciprocal translocations and paracentric inversions. The purpose of this paper is to examine the evidence for the origin of C. rhomboidea and to determine the subsequent course of chromosomal differentiation in this species. When considered together with the recent climatic history of the western United States, the cytogenetic information provides unusual clues to the recent changes in distribution pattern and paths of migration followed by C. rhomboidea since its origin. Clarkia rhomboidea ranges from the mountains of southern California northward in a broad belt following the Sierra Nevada, the Coast Ranges, and the Cascade Range to Washington and extends eastward into Idaho. Widely disjunct populations occur in Utah and Arizona. The species is absent from the southwestern deserts and the arid interior of Oregon and Washington (fig. 1). The populations are adapted to a wide range of habitats. In Arizona, Utah, Nevada, and most of California they occur in the broad transition zone between the Arcto-Tertiary and the Madro-Tertiary Geofloras associated usually with yellow pine (Pinus ponderosa) and various oaks. However, from the higher elevations of the Sierra Nevada in California to the northern limits of the species range, the colonies are generally found in coniferous or montane forest. The wide range of ecological tolerance is especially evident on the western slopes of the Sierra Nevada of California where populations occur from the mesic margins of montane meadows at 10,000 feet elevation to the arid oak woodland and chaparral associations at about 1,800 feet. Plants of this species tend to be colonial with populations varying in size from a few individuals to many hundreds. However, single plants and small colonies often occur sporadically over many miles of favorable terrain. Herbarium specimens as well as field and garden observations indicate that interpopulational variation is especially pronounced in this species (Lewis and Lewis, 1955; Mosquin, 1961). The most conspicuous variables are flowering time, habit, leaf and petal shape, and flower size. With respect to breeding habit, populations with small flowers are automatic selfers whereas those with larger flowers tend to be outcrossed. The chief pollinators are various Hymenoptera. The plants are self-compatible and capsules of most plants set a full complement of seed when isolated from insects. As in other species of Clarkia, the seeds have no mechanism for longrange dispersal and are as a rule dropped near the parent plant. The chromosome number in C. rhomboidea is n = 12 and is based upon an examination of 74 plants from 47 populations 1 This investigation was carried out while the writer was a graduate student at the Department of Botany, University of California, Los Angeles, and is a portion of a dissertation submitted as part of the requirements for the degree of Doctor of PhilosoDhv.