Ploidy of invasive aquatic macrophyte species
Change in ploidy is sometimes thought to be associated with the success of species invasion and invasiveness, as well as rapid environmental and climatic adaptation over varying spatial and time scales (te Beest et al., 2012; Wani et al., 2018; Levin, 2019). Regarding invasiveness, of the 3496 known macrophyte species 52 are seriously invasive into at least one other world ecozone outwith their native ecozone(s) (e.g., Pieterse & Murphy, 1993; Hussner et al., 2017; Hill et al., 2020), and we have ploidy information for 49 of those species (Appendix A5). The proportion of invasive species exhibiting each ploidy state is similar: haploid/diploid (17 species), polyploid (15), and mixed ploidy (17). There are differences in the incidence of invasive macrophytes with different ploidy between ecozones, but there is little or no consistent pattern to suggest that ploidy is important in determining invasiveness in aquatic macrophyte species. Our findings are supported by those of Kubátová et al. (2008), who looked at ploidy state in native (Old World) and invasive (North American) populations of Lythrum salicaria L. (Lythraceae), and found that invasive spread was not associated with differences in ploidy level (2x, 3x, 4x, 6x).
It is also interesting to examine in some detail the results of a recent study of invasive plant ploidy at a local scale, in this case the Kashmir Himalaya region, straddling the boundary between Orient and Palaearctic, and mostly located at high elevation (Dar et al., 2020). The authors provide evidence to support their assertion that polyploidy is strongly associated with invasiveness in the alien flora of this region. However, we doubt that polyploid species are actually predominant in the alien macrophyte flora of Kashmir: mixed ploidy species are more likely to play this role in the region. The reason for the difference between our and their conclusion is that in their study the authors cite only a single reference for ploidy state of each species, often quite old and in many cases geographically very limited, although some of the species are widely distributed across the world. In contrast we used multiple additional sources in allocating ploidy state, often more recent, and usually covering additional areas of the planet – the net consequence being a higher probability of finding races of the target species which show both types of ploidy, should these exist, resulting in more likely allocation of mixed ploidy status to the species. Of 390 alien plant species included in their study 55 (14.1%) are macrophytes, which were categorised by Dar et al. (2020) as 18.2% D, 72.7% P and 9.1% D&P species. However, the outcome using ploidy information for the same species from our dataset suggests a breakdown of 9.1% D, 32.7% P and 58.2% D&P for these species, which does not support the assertion that polyploidy predominates in this set of species invasive in Kashmir (though it remains possible that polyploid races of the species concerned are more invasive than their diploid cytotypes: no data are however available on this issue). This example demonstrates the relevance of global ploidy datasets on seeing the “whole picture”, though it will be interesting to see the outcomes of more studies along these lines, at local scale, from other parts of the world.