The native North American toadflaxes (Linaria section Leptoplectron) comprise a morphologically-variable complex of taxa that have been collectively recognized as a distinct genus (Nuttallanthus D. Sutton) and individually assigned to specific and varietal rank. Linaria canadensis and L. texana are locally abundant in sandy soils throughout much of temperate North America; L. floridana is confined primarily to Atlantic and Gulf coastal areas where all three taxa occur occasionally in dense, mixed populations. These plants produce cleistogamous flowers early in the growing season; showy, fragrant chasmogamous flowers appear later and attract numerous Lepidopteran and Hemipteran visitors. Chromosome counts reveal no variation in ploidy level within the complex. Field and greenhouse breeding-system studies indicate high levels of self-fertilization: all individuals studied were entirely self-compatible and many “chasmogamous” flowers selfed prior to anthesis; pollen is required for seed production. While hand-performed cross pollinations between plants within each taxonomic group resulted in reduced seed set (possibly due to limited pollen transfer), no intertaxon hand-pollinations produced viable seeds. Field tests conducted in several populations of L. texana involved five treatments: open pollination, pollinator exclusion, floral emasculation and pollinator exclusion, emasculation and open-pollination, and hand/self-pollination of emasculated flowers. These tests indicate L. texana is facultatively xenogamous; while many potential pollinators were observed visiting successive flowers, successful pollen transfer and seed set was rare. These results are consistent with an analysis of isozyme variation, which revealed low levels of observed heterozygosity and infrapopulation genetic polymorphism throughout the complex. Analyses of morphological and genetic variation within and among 50 populations of Linaria canadensis, L. texana and L. floridana yield broadly consistent results and suggest that these taxa should be recognized as distinct species. Phenetic analyses identify numerous quantitative and qualitative morphological differences among these taxa; isozyme data indicate that these taxa are reproductively isolated in areas of sympatry.

Key words: breeding system, isozymes, Linaria, morphology, Nuttallanthus