Cryptic species within Amphicarpaea bracteata (Leguminosae): evidence from isozymes, morphology, and pathogen specificity

All North American plants of the genus Amphicarpaea are currently classified into a single variable species (Amphicarpaea bracteata). However, an isozyme analysis of 33 populations from a 1000-km region revealed the presence of two divergent lineages (I, II) that shared no alleles in common at 7 of 18 loci examined. These lineages coexisted on both a local and a regional scale with virtually no hybridization: among nearly 1000 plants analyzed, only a single hybrid individual was found that had alleles from both lineages. A multivariate analysis of seven leaf traits showed that these lineages were morphologically distinct and that populations from the same lineage resembled each other regardless of geographic origin. The two lineages also showed divergent responses to isolates of the pathogen Synchytrium decipiens: pathogens sampled from either lineage failed to reproduce successfully on hosts of the other lineage. The correlated pattern of variation for all traits examined implies that these lineages constitute two separate biological species. Consistent (though less extreme) trait differences were also detected among two clusters of populations within lineage I, suggesting that this lineage may be further subdivided into two reproductively isolated sets of populations. Keywords: Amphicarpaea, electrophoresis, geographic variation, hybridization, Leguminosae.