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.