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.