BREEDING SYSTEMS AND LEVELS OF INBREEDING DEPRESSION IN GEOGRAPHICALLY RESTRICTED AND WIDESPREAD SPECIES OF ASTRAGALUS (FABACEAE)

Several workers have suggested that species with restricted ranges and few individuals are more likely to be self-compatible and to exhibit low levels of inbreeding depression than are geographically widespread congeners. To investigate these predicted patterns, controlled pollinations were performed in the field on populations of two restricted and two widespread species of Astragalus. All four species are xerophytic perennials which have similar floral size and morphology. Both restricted A. linifolius and restricted A. osterhouti are self-compatible, and A. linifolius is moderately autogamous. Widespread A. lonchocarpus is self-compatible, but widespread A. pectinatus is essentially self-incompatible. Neither the restricted nor the widespread species exhibited evidence of fecundity components of inbreeding depression (as measured by percent seed set and percent embryo abortion). Seedlings of A. linifolius and A. lonchocarpus that had been produced by selfand cross-fertilization were grown in a growth chamber to investigate progeny viability components of inbreeding depression. In contrast to the predicted patterns, inbreeding depression was detected in progeny of restricted A. linifolius, but not in progeny of widespread A. lonchocarpus. These patterns emphasize the importance of studying inbreeding depression in additional restricted and widespread species at several stages of the life cycle. THE REPRODUCTIVE BIOLOGY of plant species with limited ranges and few individuals has long been of interest to evolutionary biologists (Darwin, 1859; Meagher, Antonovics, and Primack, 1978; Harper, 1979; Drury, 1980; Frankel and Soule, 1981; Rabinowitz, 1981; Groves, 1982; Griggs and Jain, 1983; Hamrick, 1983; Kruckeberg and Rabinowitz, 1985; Fiedler, 1985,1986; Hodgson, 1986a, b; Menges, Waller, and Gawler, 1986; Karron, 1987a, b). Very little is known, however, about breeding systems and levels of inbreeding depression in geographically restricted taxa (Mehrhoff, 1983; ' Received for publication 12 February 1988; revision accepted 15 July 1988. This work is dedicated to the memory of Dr. Deborah Rabinowitz. I thank Y. Linhart for his advice and encouragement. R. Dodd, T. Moats, and C. Strand were most helpful with growth chamber and field studies. I am also grateful for suggestions from the following individuals: D. W. Crumpacker, B. Dale, J. Gehring, M. Grant, D. Hazlett, S. Jain, M. Lane, D. Marshall, J. Mitton, R. Monson, A. Moore, D. Rabinowitz, D. Schemske, W. Schuster, T. Veblen, and D. Waller. Field work on A. pectinatus was conducted on the Central Plains Experimental Range operated by the USDA and made available to the CPER LTER program administered by Colorado State University. Funds were provided by NSF Dissertation Improvement Grant BSR-85 14441 and a grant-in-aid of research from Sigma xi. 2 Current address: Department of Biology, University of New Mexico, Albuqueique, NM 87131. Kruckeberg and Rabinowitz, 1985; Clampitt, 1987; Karron, 1987a). Stebbins (1957) discussed two reasons why geographically restricted species might be likely to exhibit self-compatibility. The first, well known as Baker's rule (Baker, 1955,1967; Jain, 1976), states that following long distance dispersal, a solitary propagule is much more likely to reproduce and generate a sexually reproducing population if it is capable of self-fertilization. Second, Stebbins suggested that in species whose population sizes fluctuated greatly (e.g., Vandermeer, 1982; Griggs and Jain, 1983; Menges, Gawler, and Waller, 1985), self-pollinating morphs would be favored by natural selection when population size was small. Allard, Jain, and Workman (1968) and, more recently, Lande and Schemske (1985; Schemske and Lande, 1985; but note also the critique by Waller, 1986) provided a detailed mechanism for the latter hypothesis. They argued that species that have experienced repeated population bottlenecks or pollinator failures are likely to exhibit reduced levels of inbreeding depression (due to a reduction in genetic load) and may, therefore, have undergone selection favoring self-compatible morphs. Studies of geographically restricted and widespread species of Astragalus provide an