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?