GENOTYPIC VARIATION IN AGAMOSPERMOUS ERIGERON COMPOSITUS (ASTERACEAE)

Few integrative analyses of the structure of agamospermous plant popul ations have been conducted. Erigeron compositus occurs in montane west ern North America and comprises both sexual and agamospermous populati ons. Sexual E. compositus has previously been characterized as outcros sing and predominantly diploid (2n = 18). Agamic E. compositus is usua lly hexaploid (2n = 54), though counts herein range from 2n = 36 to 2n = 80. Starch-gel electrophoresis, cytology, and analysis of pollen pr oduction were used to evaluate variation within and among agamospermou s populations. Fifteen enzyme loci were used to identify 24 unique mul tilocus genotypes in seven populations, an average of 3.4 genotypes pe r population. Proportion of distinct genotypes per population sample s ize (G/N) and measures of genetic diversity (D) and evenness (E) are 0 .10, 0.48, and 0.61, respectively, which indicate that E. compositus m aintains levels of diversity similar to other agamospermous taxa. Most agamospermous populations are mosaics comprising groups of geneticall y distinct individuals that are frequently distinguished by cytotype a nd capacity for pollen production. The geographical and ecological sep aration of sexual and agamospermous populations make it unlikely that gene flow from sexual populations is a direct source of genetic variat ion in agamospermous populations. Instead, crossing between geneticall y distinct facultative agamosperms probably accounts for most variatio n. Genetic and morphological evidence document one such putative cross ing event. Agamospermous E. compositus is very similar genetically to sexual E. compositus. Allozyme analysis further shows that genetic var iation in agamospermous population is partitioned among a few highly h eterozygous genotypes, whereas sexual populations maintain numerous ge notypes of relatively low heterozygosity.