REPRODUCTIVE BIOLOGY OF ERYTHRONIUM GRANDIFLORUM VARIETIES GRANDIFLORUM AND CANDIDUM (LILIACEAE)

This four-year (1982-1986) study compared the pollination biology and cytology of a relatively rare taxon, Erythronium grandiflorum Pursh var. candidum (Piper) Abrams, with that of a geographically widespread sister taxon, E. grandiflorum var. grandiflorum, in the Pacific Northwest. Breeding studies were used to study the reproductive systems of the taxa, to test for interfertility between them, and to seek abnormalities in the breeding system of the rare taxon that might account for its relative rarity. Chromosomes were counted in both taxa and in hybrids from artificial crosses. Ranges of the two taxa were determined from field studies and herbarium specimens. Although the ranges of the taxa overlap in part, populations are usually allopatric. Phenological factors are probably minor in preventing cross-pollination between the taxa. Both taxa are protogynous obligate outcrossers pollinated by bees. Seed production apparently is not pollinator-limited. The two taxa are interfertile only when individuals from allopatric populations are artificially crossed, and thus should be considered two biological species. When the taxa overlap in floral phenology in sympatric populations, they exhibit reciprocal incompatibility. Such incompatibility between closely related perennial species is unusual. It may have arisen via natural selection in mixed populations. Alternatively, if populations of each taxon vary with regard to their incompatibility alleles, they may have acquired their fertility barriers by chance before populations of the two taxa came into contact with each other. THE PROTECTION and management of rare plant species is impossible without a clear understanding of their biology. There are different degrees and kinds of rarity, and certainly there is no one cause of it. Suggested causes have been youth, senescence, lack of success, genetic depletion, and narrow ecological tolerances of the taxon (Drury, 1980), but none of these explanations is all-encompassing, because of numerous and complex factors (Stebbins, 1980). For example, limited genetic variability and ecological amplitude may suggest that a species is newly derived, but other explanations are possible. The species may have gone through a genetic bottleneck (e.g., population restriction during glaciation). Steep ecological graI Received for publication 1 July 1986; revision accepted 8 April 1987. I would like to thank Jerrold I Davis, David E. Bilderback, James R. Habeck, Charles N. Miller, David Alt, Meredith A. Lane, Alexander F. Motten, and Peter Bernhardt for advice and review of the manuscript. Curators of several herbaria graciously loaned specimens for this study. Don 0. Loftsgaarden suggested appropriate statistical tests. Robert D. Pfister, Cliff Martinka, and Robert C. Haraden authorized my use of study sites. Martha Apple provided field assistance. Terry Griswold of the University of Utah identified the insects. I am especially grateful to Richard P. Fritz-Sheridan for field assistance and encouragement throughout this project. This research was supported by the National Audubon Society, Sigma Xi, a Bertha Morton Scholarship administered by the University of Montana Graduate School, and the University of Montana Botany Department. dients surrounding populations may reduce the environmental variation at population sites. The ability of the plants to disperse propagules to a suitable habitat may be limited. As a result, Stebbins (1980) concluded that the rarity of a species can not be correlated consistently with its amount of within-population genetic variation. Likewise, while most rare and localized species occur in ecological islands, a correlation is foiled because other species in these islands may be widespread. Nor are historical correlations consistent, because "rare and localized species can be either young, ancient, or of an intermediate age" (Stebbins, 1980). Thus, the cause of the rarity of a species must be sought by using a synthetic approach in which ecological, genetic, and historical factors are considered and their interactions elucidated (Drury, 1980; Stebbins, 1980; Endler, 1982). Studies that compare a rare plant taxon with a closely related nonrare species provide important contributions toward understanding rare species. Once compiled for many groups of plants, comparative data will better enable us to "judge whether the traits of rare plants are unique to them or are some random sample of plant traits in general and unrelated to the rare state" (Rabinowitz, 1981). This study compared the pollination biology and cytology of a rare taxon, Erythronium grandiflorum Pursh var. candidum (Piper) Abrams, with that of the closely related and widespread E. gran-