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.