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Genetic Variation and Aquatic Plant Management: Key Concepts and Practical Implications

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Genetic Variation and Aquatic Plant Management: Key Concepts and Practical Implications J. Aquat. Plant Manage. 56s: 101–106 Genetic variation and aquatic plant management: Key concepts and practical implications RYAN A. THUM* INTRODUCTION different subdisciplines, and I refer readers to any of the numerous college-level genetics textbooks available for Evolution can occur rapidly, and over contemporary further introduction to these subfields (e.g., Pierce 2014, timescales (Hairston et al. 2005). Adaptation can occur Brooker 2015, Snustad and Simmons 2016). This review during time frames that are relevant to natural resource focuses on two specific subfields that are most immediately management. Terrestrial weed scientists have learned this accessible to genetic studies of invasive aquatic plants: first hand by watching in real time the rapid evolution of population genetics and ecological genetics. resistance by many species to many different herbicides (see Heap 2017). The evolutionary potential of weeds is determined by genetic variation, which can be much greater POPULATION GENETICS than is recognized by some traditional weed scientists that Population genetics is the subfield of genetics concerned do not have background or interest in genetics. with the patterns, causes, and consequences of genetic Genetic variation has not historically been a major focus variation in populations, and how genetic variation varies of aquatic plant management research or practice. Yet, over space and time. Population genetic variation is most genetic studies reveal that managed aquatic plant taxa can commonly quantified by measuring allele frequencies for exhibit genetic variation that can be relevant to their one to many molecular markers, as opposed to quantifying potential for growth, spread, impact, and control efficacy. phenotypic variation (see Ecological Genetics section For example, the genetic basis of fluridone resistance in below). The molecular markers used for population genetic hydrilla [Hydrilla verticillata (L.f) Royle] has been uncovered analyses are commonly considered to be ‘‘neutral,’’ and not (Michel et al. 2004), and genetic screening of hydrilla directly associated with any phenotypic traits. However, this populations can be used to predict fluridone efficacy is not always the case, and many current population genetics (Benoit and Les 2013). Similarly, evidence for manage- studies are specifically designed to look for genetic loci that ment-relevant genetic variation has been identified in are under selection and/or related to particular phenotypic watermilfoil (Myriophyllum spp.; e.g., Berger et al. 2012, traits of interest (see ‘‘Detecting selection and adaptation’’ Thum et al. 2012, LaRue et al. 2013), fanwort (Cabomba below). caroliniana Gray; Bultemeier et al. 2009), and flowering rush Basic population genetic descriptions can be informative (Butomus umbellatus L.; Lui et al. 2005). Understanding where, for aquatic plant managers. Understanding how much when, and how genetic variation will be important for genetic diversity occurs within a population can provide management decisions and outcomes is still unknown, and insight into the potential for variation in ecological traits. the study of genetic variation and its impacts on manage- For example, many aquatic plant species are capable of ment therefore provides exciting prospects for the invasive asexual propagation, and it is therefore possible for a aquatic plant management research agenda. population to contain anywhere from a single genetic clone In this paper, I provide a conceptual overview of the to many genetic clones that might differ in traits that are of types of genetic studies that could be areas of research focus interest to managers. Molecular markers can be used to test for invasive aquatic plant management. It is not intended to these alternatives. Similarly, molecular markers can be used be an exhaustive review of genetics, nor is it intended to to determine whether two populations are genetically review specific methodology and protocols, because those similar or different, which can provide insight into the are ever-changing with rapid technological and computa- potential for different populations to exhibit different tional advances in genetics. Rather, it is intended to ecological traits of interest to managers. Identifying introduce some of the salient features of the broad field genetically distinct populations can be helpful for designing of genetics that aquatic plant researchers could consider experiments that test for significant variation in phenotypic when developing and designing projects. traits of interest. Finally, genetic surveys of individual The term ‘‘genetics’’ broadly encompasses many different populations over time can determine whether populations concepts and methods. Therefore, before undertaking a are genetically stagnant versus dynamic, which might project about genetic variation, it is helpful to identify provide insight into the potential for ongoing and contem- which specific aspects of genetics are of interest. The larger porary adaptation (e.g., herbicide-resistance evolution). field of genetics can be broadly divided into numerous In addition to basic population genetic descriptions, I *Assistant Professor, Plant Sciences and Plant Pathology Department, highlight below three prominent emphases of current Montana State University, Bozeman, MT, USA. Corresponding author’s population genetics for which there are very few, if any, E-mail: [email protected]. published studies on invasive aquatic plants, but that could J. Aquat. Plant Manage. 56s: 2018 101 be pursued by interested students and researchers. The collect data on 500 individuals, it would be better to sample molecular techniques and statistical analyses used will 10 individuals from each of 50 locations scattered across the depend on the specific questions being asked, and logistical geographic range of the focal species than it would be to constraints in any given study system. An exhaustive review sample 50 individuals from each of 10 locations across the of methods is beyond the scope of this paper, but I briefly species’ range. In addition, when attempting to infer highlight considerations for molecular markers and sam- geographic origins of invasive species, the geographic pling schemes in each area. coverage and number of locations sampled throughout the native range should be maximized. PHYLOGEOGRAPHY LANDSCAPE GENETICS The term ‘‘phylogeography’’ refers to the study of historical processes that have influenced the contemporary A related subfield of population genetics is landscape geographic distribution of genetic variation across the genetics. This subfield is similar to phylogeography in its landscape. For example, numerous studies have considered emphasis on spatial patterns of genetic variation, but is how Quaternary geology (e.g., glacial–interglacial cycles distinct from phylogeography in its emphasis on present- during the past 2 million yr) has influenced contemporary day factors influencing genetic variation. At its core, genetic variation (e.g., see Hewitt 1999 and Soltis et al. 2006 landscape genetics combines genetic data on populations for extensive reviews of examples). Many of these studies with landscape ecology methods to relate contemporary have found genetic signatures that indicate the isolation of patterns of gene flow to the landscape. This is a rapidly lineages into distinct geographic refugia during glacial developing field, and I refer readers to a recent text by maxima, followed by expansion of these lineages following Balkenhol et al. (2015) for further details. glacial retreat. The signatures of these events are still Landscape genetics could be immediately relevant to identifiable in contemporary populations using certain aquatic plant managers in at least two ways. First, landscape molecular markers. genetic approaches could be used to identify features of the Phylogeographic studies of aquatic plants can be relevant landscape that facilitate versus impede the movement of to aquatic plant management. For example, biogeographic invasive aquatic plant taxa among water bodies, which could patterns of genetic variation in invasive aquatic plant taxa help inform management efforts to limit and prevent their can be used to infer their geographic origins and spread. For example, landscape genetic studies of intro- introduction pathways and vectors. Information on geo- duced red fox (Vulpes vulpes) in California have helped graphic origins is of specific interest to researchers identify areas of introduction and routes of geographic searching for biocontrol agents, which are often specific spread, and have provided insight into the importance of to geographic regions. Furthermore, inferences about localized reproduction versus immigration in introduced pathways and vectors for introduction can be used to populations (Sacks et al. 2016). Second, landscape genetics inform prevention efforts. In addition, because of its studies increasingly attempt to identify genetic signatures emphasis on history and species formation, phylogeo- that indicate adaptation to environmental conditions across graphic studies can lead to the identification of genetically the landscape. Such approaches could be used to inform distinct groups that can represent cryptic taxa (e.g., Thum et questions about the extent to which invasive aquatic plant al. 2011). Because genetically distinct cryptic taxa can species adapt to environmental conditions, including exhibit ecological differences that are relevant to managers, management (see also next section). subsequent
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