Latitudinal and environmental patterns of ploidy
In plants, abiotic factors such as temperature, light regime (especially photoperiod), drought, or salinity, as well as isolation, may influence the incidence of different ploidy states (e.g., Hagerup, 1932; Comai et al., 2005; te Beest et al., 2012; Husband et al., 2013; Šmarda et al., 2013; Ramsey & Ramsey, 2014; Lobato-de Magalhães et al., 2019). Polyploids often have broader ecological tolerances than their diploid antecedents (Soltis et al., 2016). Notwithstanding the strong spatial latitudinal patterns observed here, we found that individual environmental and biotic variables (though often following latitudinal gradients) are strongly predictive of, or associated with ploidy patterns in aquatic macrophytes. These primarily include temperature (as annual average, maximum of the warmest month, or annual temperature range), potential evapotranspiration, annual precipitation, total macrophyte species richness, and ecozone-endemic macrophyte species richness. Polyploid macrophyte species were massively more prevalent in harsh environments, particularly those with low temperatures and reduced growing season length, typical of sub-Arctic to Arctic latitudes.
Overall, our results provide evidence from macrophytes to support the general assertion that increased chromosome number may be associated with a wide range of environmental conditions, but especially those typifying the environmentally-stressed high latitudes (e.g., drastic changes in temperature and photoperiod between seasons). It should be noted that Cyperaceae (the most diverse family in floras of the tundra and taiga zone), Poaceae, and (in part) Juncaceae contribute strongly to the regional floras of high latitudes, and all are well represented in the macrophyte floras of such regions. In Cyperaceae it has been stated that “chromosome instability is almost the rule” (Tena-Flores et al., 2014). Agmatoploidy, dysploidy and symploidy have been reported in several aquatic and wetland genera of these three families, including Carex, Eleocharis, Luzula, and Rhynchospora (Tena-Flores et al., 2013, 2014; Luceño et al., 1998; Guerra, 2016).
Changes in ploidy perhaps cause changes in plant reproduction (e.g., apomixis or clonal recruitment), which can affect the evolution of species’ geographic distribution range (Eckert, 2002; Ulum et al. 2020). For example, aquatic mosses, as with their terrestrial cousins, developed various adaptations, such as the alternation of gametophyte (haploid) and sporophyte (diploid) generations, to aid survival in the often environmentally-stressed and highly-disturbed conditions (for example, fast-flowing or torrential upland streams), in which they typically occur (Grime, 1979; Lang & Murphy, 2011; Goga et al. 2018). Polyploids often show a tendency towards clonality, apomixis, and self-compatibility, all of which are adaptations that generally enhance reproductive assurance (Herben et al., 2017).
Recently, Ulum et al. (2020) observed that different cytotypes of Ranunculus auricomus L. (Ranunculaceae) (diploid, tetraploid, and hexaploid races, typically occurring in moist woodland conditions) vary their reproduction strategy (e.g., apomixis frequency) in response to extended photoperiod. These authors mention that the polyploid cytotypes of this species may better buffer environmental stress, thereby facilitating the establishment of the species, which is congruent with our findings. A wider range of environmental conditions (e.g., temperature and photoperiod) related to high latitudes could be also be playing a role as a driver of polyploidy in aquatic plants. Middle latitude occurrence of mixed ploidy species could reflect an adaptation by the ancestral diploid races of these species to expand the species range into higher latitudes, via an increase in chromosome number to form new races better suited to the higher-latitude conditions, while also retaining the original diploid cytotypes in their lower-latitude home areas. This could well contribute to an explanation of the observed higher occurrence of mixed ploidy macrophyte species in latitudes intermediate between the peaks of haploidy/diploidy occurrence at low latitudes, and polyploidy at very high latitudes (see schematic summary diagram, Fig. 1b).