Geographical distribution: ploidy state ranges
Confirming the known evidence that plant range distribution is
associated with ploidy state (Petit & Thompson, 1999; Lowry & Lester,
2006; Martin & Husband, 2009), our results demonstrated that polyploid
species generally occupy larger geographical ranges than diploid
macrophyte species. Additionally, we observed higher geographical and
latitudinal range size of mixed ploidy species compared with diploid and
polyploid species. Plant species that have both types of ploidy (D&P)
are likely to be able to occupy a wider range of environmental
conditions than species with more limited ploidy. This may occur because
their component polyploid populations are likely to be better suited to
more stressed high latitudes (or possibly high mountain environments)
while their haploid/diploid populations can occupy low-latitude habitats
experiencing more benign conditions, such as higher temperatures,
thereby leading to broader overall distribution. An example of a
macrophyte species showing geographically-demarcated populations of
mixed ploidy is Rotala ramosior (L.) Koehne (Lythraceae). In this
New World species Neotropical populations, from Mexico southwards, are
diploid, whilst Nearctic populations, occurring as far north as Canada,
are polyploid (Les, 2017). In other cases, different chromosome races of
a species may occur in the same area, as, for example, in Phragmites australis (Cav.) Trin. ex Steud. (Poaceae), which has
more than a dozen chromosome races varying from 2n=28 to 2n=120, most
frequent being 2n=48, 72, 96. Other examples are Ranunculus
gmelinii DC. (Ranunculaceae) with three chromosome races (2n=16 (2x);
2n=32 (4x); 2n=64 (8x)) occurring (with different frequency) throughout
its distribution area, and Comarum palustre L. (Rosaceae) which
has two main races (2n=28 (4x); 2n=42 (6x)) not correlated with either
morphology or geography (e.g. Rice et al., 2015). Although there is some
information in the literature, such as that for Rotala ramosior, on distributions of the different cytotypes of some macrophyte species,
very little work has been done hitherto on the detail of geographical
cytotype occurrence in macrophyte species, let alone the landscape-scale
environmental and other factors which may drive ploidy state
distribution in these plants.