Introduction
Chromosome number change (e.g., polyploidy or whole-genome duplication; dysploidy or change in single chromosome numbers) is a common genetic mutation in plant species with complex effects on plant ecological responses and several evolutionary implications (Segraves, 2017). Ploidy state (“cytotype”) has been determined, initially by chromosome counts and more recently by techniques such as flow cytometry (Kron et al., 2007; Rice et al., 2015), though variation in DNA content among cytotypes is still unknown for many aquatic plant species, especially in tropical regions (Ramsey & Ramsey, 2014).
The existence of different distribution patterns both between and within species across geographical gradients such as latitude and altitude, in vascular plant populations showing different cytotypes has long been known (e.g., Tischler, 1935; Löve & Löve, 1949; Blackburn & Morton, 1957; Johnson & Packer, 1965; Hardy et al., 2000; Johnson et al., 2003; Kubátová et al., 2008; Martin & Husband, 2013). Recent work offers strong evidence for the existence of a latitude-related gradient of occurrence of polyploidy in angiosperms (Rice et al., 2019), which the authors suggested may be related to increasing climatic or other environmental stress and indirect effects of environment on life form and species richness occurring at high latitudes. In addition, there is previous evidence that ploidy state is a factor linked to range-size in angiosperms (Petit & Thompson, 1999; Lowry & Lester, 2006; Martin & Husband, 2009). Whether this applies to those vascular plant species that live in freshwater and brackish inland waterbodies (“aquatic macrophytes”), has, however, not been specifically investigated at a global-scale, and was the subject of our study.
As a group, aquatic macrophytes are generally considered to occupy a “stressful environment”, in terms of factors that may reduce their productivity, in all or most of the habitats where they are found (Grime, 1979; Santamaría, 2002). These factors include light, nutrient and carbon availability, temperature regime, sediment conditions, and alkalinity (e.g., Bornette & Puijalon, 2011; Iversen et al., 2019; Sun et al, 2019). However, the intensity of environmental stress and disturbance tolerated by individual species, and sometimes among individual populations of a given species across its range varies considerably. Aquatic macrophytes consequently show a wide range of resulting life-strategies for survival and growth (e.g., Garbey et al., 2004; Lui et al., 2005; Beck & Alahuhta, 2017) and changes in number of chromosomes could be one factor that contributes to the success of such survival strategies.
Given the assumption that the latitudinal distribution pattern seen in angiosperm ploidy may be particularly related to environmental stress conditions (e.g., Stebbins, 1984; Rice et al., 2019), it is clearly of interest to explore further how ploidy state varies in plants, such as aquatic macrophytes, which both occur widely across the planet and are inherently adapted to cope with potentially-high environmental stress conditions. These stressors may be associated with latitude and other spatial, environmental, and landscape-level drivers, both natural and human-related, known to influence the macroecology of these plants (e.g., Murphy et al., 2019, 2020; Alahuhta et al., 2020). If, as has been asserted, polyploidy can be a response to habitat loss and isolation, then high levels of ploidy may be a strategy of plant species which enhances their survival in human-impacted landscapes (Plue et al., 2018), as well as in natural habitats experiencing strong environmental stress or disturbance pressures (e.g., Chambers et al., 1999; Ulum et al. 2020). Despite the increasing evidence that ploidy is a factor of importance for environmental adaptation, relatively few studies have to date addressed the ecological drivers of change in ploidy state for aquatic plants (Šmarda et al., 2013; Soltis et al., 2016; Segraves, 2017).
Our objectives were to address hypotheses related to four principal questions: (1) Is latitude correlated with aquatic macrophyte ploidy, and in particular is there an opposing latitudinal pattern between haploid/diploid and polyploid species? (2) Can any such latitudinal ploidy patterns in aquatic macrophytes be alternatively explained by environmental and landscape variables? (3) Are there any signs of species interactions with ploidy patterns (e.g., effects associated with species richness)? (4) Do polyploids occupy larger geographical ranges than haploid/diploid aquatic macrophyte species?