Isozyme Systematics of Astragalus Sect. Leptocarpi Subsect. Californici (Fabaceae)

Crossing studies and enzyme electrophoresis were utilized to examine the genetic structure and relationships of populations of seven taxa of Astragalus sect. Leptocarpi subsect. Californici and five related species. The examined species were all self-compatible and no interspecific crosses were successful, demonstrating strong reproductive barriers. Forty populations were surveyed electrophoretically for 11 enzymes encoded by 17 putative loci. All conspecific populations exhibited high genetic identity values above 0.961. The mean genetic identity among all species was 0.69. Cluster analysis of genetic distance data indicates that the annuals (including A. gambelianus) are well-differentiated from the three desert species and the perennial outgroup A. nothoxys. Intraspecific genetic divergence and variation were low for both morphologically uniform and variable species, as well as for species with disjunct populations. Recent speciation is considered to be the most likely explanation for this pattern. Perhaps the ease with which reproductive isolation originates in the group has facilitated speciation and the ability of recently diverged species to become secondarily sympatric without genetic homogenization. Astragalus L. is the largest genus of flowering plants in the world with 2500 to 2750 species (Podlech 1986; Townsend 1974) and is especially well represented in western North America with approximately 400 species (Barneby 1964; Welsh 1978). Despite the large size of the genus, little is known about the genetic constitution of Astragalus species, the degree of divergence between them, or the evolutionary processes that have led to the unparalleled diversity of this genus. The taxonomy of the North American species of Astragalus has been expertly summarized by Barneby (1964). Two notable aspects of this monograph facilitate evolutionary study in the genus. First, Barneby maintains a broad generic concept, using sections and subsections to circumscribe groups of presumably closely related species (Spellenberg 1976). The genetic segregations of some earlier classifications of North American Astragalus (e.g., Rydberg 1929) were clearly artificial and resulted in the separation of even conspecific varieties (Barneby 1964). The sections and subsections recognized by Barneby, on the other hand, are reasonable candidates for monophyletic groups suitable for phylogenetic analysis. The second advantage of Barneby's work is his explicit rationale of taxonomic decisions and the inclusion of statements concerning interrelationships of the taxa. Astragalus sect. Leptocarpi Jones as circumscribed by Barneby (1964) encompasses caulescent annuals or short-lived perennials with free stipules and generally elongate, trigonously compressed, bilocular pods. The 24 species of the section are placed in eight subsections. Subsection Californici (Gray) Barneby is the largest with nine species. The Californici are distinguished from other subsections by their predominantly annual habit, relatively shallow calyx tube, and legume that dehisces only after falling from the peduncle. The five cismontane species of the subsection, which are distributed in the mediterranean climate regions of California, were the primary focus of this study (table 1, fig. 1). Each species of subsect. Californici is distinguished by a characteristic legume and edaphic specializations (Barneby 1964; Liston 1990a, 1990b, 1990c). Astragalus breweri Gray (2n = 22) has a 7-12 mm long, fusiform pod with an elongate beak, and grows on serpentine outcrops of the North Coast Range. Astragalus clarianus Jepson (2n = 22) has a 13-18 mm long, crescentic pod borne on a gynophore, and is restricted to volcanic clay substrates in Sonoma and Napa counties. Astragalus pauperculus Greene (2n = 24) has a 12-17 mm long, laterally compressed, trigonous pod, and is found on volcanic soils on the eastern side of the Sacramento Valley. Astragalus rattanii Gray (2n = 22) has a similarlyshaped pod to A. clarianus, but it lacks a gynophore. The fruit is 24-43 mm long in var. rattanii vs. 12-20 mm long in var. jepsonianus Barneby.