The Linaria canadensis complex (Linaria section Leptoplectron or genus Nuttallanthus) comprises a group of morphologically variable taxa which are recognized at specific or varietal rank and are native to North America. Linaria canadensis and L. texana are locally abundant in sandy soils and appear to be naturalized in other areas of the western hemisphere. Linaria floridana is narrowly distributed, and is confined primarily to Atlantic and Gulf coastal areas where all three taxa occur occasionally in mixed populations. Although species boundaries are unresolved among New World toadflaxes, previous morphological studies support hypotheses that the complex forms a monophyletic group distinct from the 150 Eurasian species of Linaria. We examined variation within and among 54 populations of the Linaria canadensis complex at 15 putative isozyme loci encoding 12 enzyme systems. Chasmogamous flowers have showy blue corollas and are visited by butterflies and bees, but cleistogamy, self-pollination, and self-compatibility characterize plants from investigated populations. High levels of selfing are indicated by field and greenhouse mating studies and by low levels of infrapopulation genetic polymorphism and observed heterozygosity. Analyses of morphological variation differentiate three primary morphotypes, but isozyme data identify two primary groups of populations with many qualitative allelic differences and with genetic identity coefficients less than 0.62: 1) L. canadensis and 2) L. floridana + L. texana. Isozyme data also indicate that these two groups are reproductively isolated in areas of sympatry where mixed populations occur. Population clusters exhibit a pronounced geographical pattern which may reflect historical biogeographic isolation and recent migrational pathways.

Key words: Isozymes, Linaria, Scrophulariaceae, species delimitation