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).