Introduction
Separating environmental from genetic contributions to phenotypic
variation is central to evolutionary ecology since it illuminates how
species respond to their local environment and produce phenotypes
capable of maintaining positive fitness and thus population persistence.
When a population experiences new environmental conditions, either by
environmental change or range expansion, existing genotypes may shift
their phenotypic expression via physiological change (aka. adaptive
plasticity), or evolution may shift genotype frequencies leading to
local adaptation via genetic change (Sultan 2000, Kingsolver et al.
2002). Natural selection, acting on novel mutation and standing genetic
variation, should reach beyond the limits of plasticity and maximize
population mean fitness (Auld et al. 2010). The rate of evolutionary
change is however dependent on the amount of genetic variation in a
population, namely genetic variation in fitness (Burt 1995). Like
phenotype, fitness itself is comprised of environmental and genetic
components (Schoen et al. 1994). Genetic variation in fitness is the raw
material natural selection acts on, and determines a population’s rate
of adaptive evolution in the face of environmental change and genetic
degradation caused by deleterious mutation, maladaptive gene flow,
genetic drift and inbreeding depression (Burt 1995, Hendry et al. 2018).
Despite the critical role of intraspecific genetic variation to species
adaptation and survival (Booy et al. 2000), the amount of genetic
diversity in natural populations remains largely unknown. Whereas much
recent work has focused on the genetic structure of endangered
populations, it is equally important to understand how genetic variation
may contribute to the successes of populations that thrive.
In this study we quantified environmental and genetic variation in
morphological traits and in fitness for the plant Lemna minor(lesser duckweed), a tiny floating aquatic plant in the familyLemnaceae found in eutrophic ponds and wetlands. Among the
smallest of all angiosperms, L. minor consists of only a single
leaf-like frond, a few mm across, to which a single unbranched root is
attached. Its reproduction is almost exclusively asexual and vegetative
with daughter fronds budding out the mother frond’s lower surface
(Landolt 1986, Lemon and Posluszny 2000). Daughter fronds remain
attached to the mother for a certain period of time before splitting
apart after abscission (Landolt 1986, Lemon et al. 2001). Their
generation time may be as short as just a few days, and their small size
results in populations of hundreds of thousands to millions of
individuals in a single pond. Because they are widespread and abundant,
are easily maintained and manipulated in the laboratory, and possess
highly reduced morphology and simplified physiology, they are being
increasingly used as a tractable model system in ecology and evolution
(Laird and Barks 2018, Hart et al. 2019, Vu et al. 2019).
We quantified phenotypic variation in two morphological traits: frond
area and root length. The extremely simplified morphology of L.
minor means that these two traits essentially capture the totality of
biomass allocation between shoot and root tissue, responsible for the
capture of light and nutrients. The frond is essentially a
photosynthetic sheet whose area may fluctuate to balance light capture
and photosynthesis (growth) with the production of daughter fronds
(reproduction) (Vasseur et al. 1995). Root length on the other hand has
been shown to vary depending on nutrient levels, since uptake rates are
proportional to root surface area (Cedergreen and Madsen 2002). Optimal
phenotype in L. minor ’s root-shoot ratio should then vary in the
field as a function of local availability of light and nutrients, with
the plant investing more biomass into the tissue responsible for the
uptake of the limiting resource. Such phenotypic variation could arise
via plasticity or local adaptation, or both, with consequences for
within and among site genetic diversity. By measuring phenotypic
expression in a common garden assay (Kawecki and Ebert 2004) we can
quantify environmental and genetic components of variation in frond area
and root length to determine if these traits have a genetic basis and
result from local adaptation.
Our study had three main objectives. First, we ask if phenotypic
variation in L. minor is correlated with natural gradients of
resource levels. We hypothesize that biomass allocation and phenotype
should match the environment to increase uptake rates of limiting
resources, such that low light environments produce plants with larger
fronds, and low nutrient environments produce plants with longer roots.
Secondly, we ask if such phenotypic variation is due primarily to
plasticity or local adaptations. We do this by quantifying the
environmental and genetic contributions to phenotypic variation. Thirdly
we aim estimate adaptive potential in natural populations of L.
minor by quantifying the standing genetic variation in fitness.