The evolutionary history of an invasive species: alligator weed, Alternanthera philoxeroides

The eco-evolutionary mechanisms of biological invasions are still not thoroughly understood. Alligator weed, Alternanthera philoxeroides (Martius) Gisebach (Amaranthaceae), is a plant native to South America and a weed in Australia and other countries. To better understand its success as an invader, we assessed the morphological and cytogenetic variability of 12 Argentine populations and the cytogenetic variability of seven Australian populations. We found differences in leaf shape (width-tolength ratio) and stem architecture in the Argentine populations, in reproduction (sexual with regular meiosis in two Argentine populations vs completely asexual with irregular meiosis and low pollen viability in all other populations) and ploidy level (tetraploids with sexual reproduction and seed production vs hexaploids with or without sexual reproduction). We propose a hypothesis about the mechanism that drove alligator weed to form highly invasive hybrid populations with vegetative reproduction from diploid ancestors, and we consider the implications for plant–herbivore interactions and biological control of this weed.