CHARACTERIZATION OF INVASIVE CONYZA SPECIES (ASTERACEAE) IN EUROPE: QUANTITATIVE TRAIT AND ISOZYME ANALYSIS

The genus Conyza (Asteraceae) represents one of the foremost examples of intercontinental plant invasions from the New World to the Old World that has resulted in a number of taxonomic problems owing to the dearth of knowledge concerning the biosystematics of the genus. In this study, quantitative trait and enzyme electrophoresis analysis was used to assess the pattern of variation within and among five short‐lived taxa of Conyza (C. blakei, C. bonariensis, C. canadenis, C. floribunda, C. sumatrensis), introduced into Europe, and to test the hypothesis of a recent hybrid origin of C. floribunda. The five taxa exhibit marked differences in morphological and life‐history characteristics that are concordant with divergence in genes encoding isozymes. In addition, a recent hybrid origin of C. floribunda is clearly not supported by either quantitative or electrophoretic data. These findings suggest that the five taxa differentiated genetically in their native range prior to their introduction in Europe. As formerly suggested by Arthur Cronquist (Bulletin of the Torrey Botanical Club 70: 629–632, 1943), C. canadensis appears to be more closely related to the genus Erigeron than the other taxa. The electrophoretic results also confirm that this particular species is diploid, while the presence of multiple bands and fixed heterozygosity demonstrates that the other Conyza taxa are all genetic allopolyploids. Both quantitative and electrophoretic data suggest the presence of two groups among the polyploid taxa: one comprising C. bonariensis and C. sumatrensis, the other C. blakei and C. floribunda. The latter grouping is supported by the observation that C. blakei and C. floribunda, both native to Argentina, exhibit a striking ability to shift from semelparous to iteroparous reproduction, while the other taxa are strictly semelparous.