Polyploidy determines the stage of invasion: clues from Kashmir Himalayan aquatic flora

Invasive species pose a major threat to native biodiversity and ecosystem integrity in many ecologically sensitive parts of the globe. Many research efforts have so far been made mainly with a focus on morphology, physiology and reproductive biology of invasive species to explain what determines the patterns of invasion. Recently, polyploidy has been reported to significantly influence plant invasiveness. Notwithstanding the profound management and conservation implications, determining the genetic basis of plant invasiveness is a challenging task for ecologists. Variation in ploidy levels of species promises to yield some useful insights in this direction and we, therefore, aimed to test the relation between polyploidy and species invasiveness. We documented chromosome number and ploidy level of 118 alien aquatic plant species including 50 alien non-invasive and 70 alien invasive species, belonging to 64 genera and 39 families distributed across various aquatic habitats of the Kashmir Himalaya at different stages of invasion. Results yielded 47 (39.83%) diploid species, 53 (44.91%) polyploids and the remaining 18 (15.25%) species were reported to have mixed ploidy, i.e., both diploids and polyploids. The invasive plant species exhibited about 40% of intraspecific ploidy polymorphism in contrast to about 28% found in non-invasive plant species. Alien invasive species (at stage IVa, IVb and V senso Colautti and MacIsaac 2004) were found to have more polyploidy (60%) than non-invasive species (at stage II and III) having only 45.23% polyploidy. Our results depict a clear pattern that non-invasive species are disproportionately more diploids with lower ploidy ratios, while invasive plant species on the contrary exhibit higher chromosome counts, thereby being predominantly polyploids. Invasion stage III with 17 diploid species represents by far the highest relative proportion of diploid species, while invasion stage V having 16 polyploid species is highest in terms of polyploid species. Regression analysis based on the stage of invasion and the ploidy status revealed that the relative proportion of polyploids on progressing stage of invasion increases significantly (p < 0.01). These results indicate that polyploidy may be one of the potential determinants of plant invasion. The implications of these results in timely prediction and better management of invasive species are discussed.