Contrasting flower biology and reproductive patterns of two threatened Utricularia L. species

The reproductive biology of threatened plant species is a critical determinant of their persistence, yet remains poorly understood for many aquatic taxa. We investigated the sexual reproduction of two aquatic Utricularia species, U. gibba and the sterile hybrid U. × neglecta , in natural Portuguese freshwater populations. Field observations and experimental assays assessed floral traits, reproductive systems, flower-insect interactions, and sexual fitness across four U. gibba populations and seven U. × neglecta populations. Both species share bilabiate yellow flowers with nectariferous spurs, indicative of adaptation to insect pollination, yet they exhibit contrasting reproductive strategies. U. gibba shows a mixed mating system with delayed selfing and facultative outcrossing, pollination-induced floral senescence, and occasional cross-pollination by cryptic, unidentified insects, ensuring reproductive assurance while maintaining genetic diversity. Nectar production, flower size, and display traits were strongly associated with pollinator-mediated pollen transfer in U. gibba , suggesting potential selection on floral traits. In contrast, U. × neglecta exhibits larger flowers, higher nectar production, and a thigmonastic stigma, which mediates interactions with pollinators; however, experimental pollinations confirm functional male sterility, preventing sexual reproduction and forcing exclusive reliance on vegetative propagation. These divergent strategies translate into contrasting conservation vulnerabilities, highlighting the need to integrate floral morphology, reproductive functionality, and pollinator dynamics to develop effective, species-specific conservation strategies for threatened aquatic plants. • Floral traits and pollinator interactions shape reproduction in two Utricularia species • U. gibba shows a mixed mating system with delayed selfing and occasional outcrossing • U. × neglecta exhibits functional male sterility and relies on vegetative propagation • Nectar production and floral traits influence pollinator-mediated pollen transfer • Divergent strategies imply contrasting conservation needs for aquatic Utricularia species