Rare Production of Seeds by Invasive Alternanthera philoxeroides (Alligator Weed) in North America Observed in Terrestrial Populations

Abstract

Fruiting in angiosperms is a complex process that is partially influenced by the external environment. In the case of invasive Alternanthera philoxeroides, introduction of this species to multiple global regions may have caused the failure of its sexual reproductive system due to genetic bottleneck and a propensity for clonal propagation. Alternanthera philoxeroides fruits rarely in its invaded ranges which raises questions regarding environmental stimuli that affect fruiting; we propose here that A. philoxeroides fruiting is partly caused by terrestrial conditions. Two instances of fruiting A. philoxeroides were observed in disparate locations in northern Mississippi, USA during 2022. Despite prolific fruiting in western Mississippi, no seeds germinated under any dormancy treatments at increasing durations. In both populations, fruiting was only observed in terrestrial forms of A. philoxeroides, even when nearby aquatic forms exhibited flowering but no fruiting. The exclusivity of fruiting to terrestrial A. philoxeroides seems to suggest drier conditions are an environmental trigger for A. philoxeroides fruiting. However, this process does not account for non-viability of seeds. Previous research suggests low genetic diversity can result in formation of nonviable seeds whether due to inbreeding depression or another unknown mechanism. We postulate that aquatic conditions present a barrier to A. philoxeroides fruiting and that if habitats transition from wet to dry, fruiting may be induced. However, environments are shifting and multiple A. philoxeroides haplotypes have been observed in the invaded range thus future work should assess the effects of terrestrial conditions on fruiting and seed viability in different A. philoxeroides haplotypes.