The contribution of evening primrose (Oenothera biennis) to a modern synthesis of evolutionary ecology

Transcription

1 Popul Ecol (2011) 53:9 21 DOI /s SPECIAL FEATURE: REVIEW Linking Genome to Ecosystem The contribution of evening primrose (Oenothera biennis) to a modern synthesis of evolutionary ecology Marc T. J. Johnson Received: 24 May 2010 / Accepted: 13 October 2010 / Published online: 16 November 2010 Ó The Society of Population Ecology and Springer 2010 Abstract In this review, I consider the contribution that common evening primrose (Oenothera biennis) has made towards integrating the ecology, evolution and genetics of species interactions. Oenothera biennis was among the earliest plant models in genetics and cytogenetics and it played an important role in the modern synthesis of evolutionary biology. More recently, population and ecological genetics approaches have provided insight into the patterns of genetic variation within and between populations, and how a combination of abiotic and biotic factors maintain and select on heritable variation within O. biennis populations. From an ecological perspective, field experiments show that genetic variation and evolution within populations can have cascading effects throughout communities. Plant genotype affects the preference and performance of individual arthropod populations, as well as the composition, biomass, total abundance and diversity of arthropod species on plants. A combination of experiments and simulation models show that natural selection on specific plant traits can drive rapid ecological changes in these same community variables. At the patch level, increasing genotypic diversity leads to a greater abundance and diversity of omnivorous and predaceous arthropods, which is also associated with increased biomass and fecundity of plants in genetically diverse patches. Finally, in questioning whether a community genetics perspective is needed in biology, I review several multifactorial This manuscript was submitted for the special feature based on the symposium in Kyoto, held on 18 October M. T. J. Johnson (&) Department of Plant Biology, North Carolina State University, Box 7612, Raleigh, NC 27695, USA marc_johnson@ncsu.edu experiments which show that plant genotype often explains as much variation in community variables as other ecological factors typically identified as most important in ecology. As a whole, research in the O. biennis system has contributed to a more complete understanding of the dynamic interplay between ecology, evolution and genetics. Keywords Coevolution Community genetics Extended phenotype Genecology Indirect genetic effect Tritrophic Introduction A modern synthesis of evolutionary ecology is currently underway that merges the traditionally disparate disciplines of ecology, evolution, genetics and genomics into a single synthetic framework of study (Johnson and Stinchcombe 2007; Wade 2007; Whitham et al. 2008). This synthesis seeks to understand the dynamic interplay between ecology and evolution by examining the evolutionary consequences of ecological interactions (Ford 1964; Hutchinson 1965), as well as the ecological consequences of evolutionary change within populations (Lack 1965; Antonovics 1992; Whitham et al. 2003; Johnson and Stinchcombe 2007). This effort is important because of its interdisciplinary focus, and also because of the potential benefits that such a unified approach offers for both basic and applied problems in biology. For example, problems relating to the conservation of rare habitats and species, the threat of biological invasions to native biodiversity, and the effects of biotic and abiotic stressors in agriculture and silviculture, all stand to benefit from the tools and perspectives that come through the successful unification of these disciplines.

2 10 Popul Ecol (2011) 53:9 21 Evolutionary ecologists have long sought to bridge the questions and techniques from ecology and evolution (Lack 1965; Collins 1986). With a few notable exceptions (Pimentel 1961), evolutionary ecology has typically focused on how biotic (e.g., competition) and abiotic (e.g., temperature) ecological factors influence the evolution of one or more populations within a community (Ehrlich and Raven 1964; Ford 1964; Hutchinson 1965; Ohgushi 1997; Schluter 2000; Thompson 2005). Although it has long been recognized that evolution over long periods of time plays a fundamental role in affecting the assembly and structure of contemporary communities (Darwin 1859; Rosenzweig and MacArthur 1963; Macarthur and Levins 1967; Schluter 2000), ecologists traditionally ignored genetic variation and evolution within species because it was believed that its effects were likely negligible in comparison to those factors typically identified as most important in affecting the abundance, distribution and diversity of species. The factors traditionally thought to be most important include abiotic environmental variation, disturbance, predation, resource competition, mutualistic interactions and primary productivity, among others (Menge and Sutherland 1987; Hunter and Price 1992). In recent years, ecologists have increasingly embraced the idea that genetic variation and evolution might represent important ecological factors in their own right. This tide shift has come with the recognition that most populations harbor substantial genetic variation for numerous phenotypic traits (Mousseau and Roff 1987; Falconer and Mackay 1996; Geber and Griffen 2003), and that populations have the capacity to rapidly evolve in response to natural selection (Linhart and Grant 1996; Thompson 1998; Grant and Grant 2006), which might affect temporal and spatial dynamics of populations, communities and ecosystems over relatively short timescales (Price 1983; Ohgushi 1991; Antonovics 1992; Whitham et al. 2003). Such ecological effects of evolution could also feedback to alter the evolution of interacting populations. The study of these types of interactions and feedbacks have become the domain of several subdisciplines, which have been variously named eco-evolutionary dynamics (Fussmann et al. 2007; Pelletier et al. 2009), community and ecosystem genetics (Antonovics 1992; Collins 2003; Whitham et al. 2003, 2006), diffuse selection (Janzen 1980; Iwao and Rausher 1997; Stinchcombe and Rausher 2001; Strauss et al. 2005) and the geographic mosaic of coevolution (Thompson 2005). Regardless of what we call it, all the questions addressed by these areas investigate different sides of the same evolutionary ecology coin. Several systems have played a prominent role in the integration of ecology, genetics and evolution. This review focuses on the contribution of a single plant system, common evening primrose (Oenothera biennis L., Onagraceae), which has and continues to be a model for testing general predictions in ecology and evolution. Moreover, this review represents the first comprehensive synthesis of more than a century of experimental evolutionary ecology research focusing on O. biennis. I first provide a concise overview of the natural history and early contributions this system made to genetics and the modern evolutionary synthesis. I then review the results of experiments that answer the questions: what is the pattern of genetic variation within and between O. biennis populations, and what are the evolutionary consequences of biotic and abiotic selection on these populations? In recent years, O. biennis has made its most novel contributions to the emerging subdiscipline of community genetics, and I provide a review of these findings by answering the question: what are the ecological consequences of genetic variation and evolution in O. biennis for communities of plants and arthropods? Finally, I ask: what is the relative importance of genetic variation in O. biennis versus other ecological factors in affecting the abundance, distribution and diversity of species commonly associated with O. biennis? The answer to this question comes in the form of a synthesis of several multifactor experiments that together test the merit of recent claims that a community genetics perspective is not needed in community and ecosystem ecology (Morin 2003; Ricklefs 2003; Hersch-Green et al. 2010). As a whole, this review highlights the role that O. biennis has played in addressing general problems in evolutionary ecology, as well as how the results from this system compare to the findings of other systems. Common evening primrose as a model system Taxonomy, ecology and life-history Oenothera biennis L. is a member of the evening primrose plant family (Onagraceae), which is nested within the Myrtales Order and the Rosids subclass. This taxonomic position is notable because many of the current genetic models (e.g., Arabidopsis, Populus, Eucalyptus) are members of these clades, which facilitates the development of molecular tools in Oenothera. Oenothera biennis is an herbaceous plant native to eastern North America that has become naturalized on all continents except Antarctica (Dietrich et al. 1997). In its native range, O. biennis is common in a wide diversity of open habitats where populations vary greatly in size and density (Gross and Werner 1982). Small isolated patches of several plants are most common, but dense local stands consisting of thousands of individuals are frequent along beaches, ditches, and other recently disturbed open habitats.

3 Popul Ecol (2011) 53: There is considerable life-history variation in O. biennis. All plants initially form a base of leaves called a rosette, which bolt to form a flowering stalk during the first (annual), second (biennial), or third year (triennial) of growth (Reekie et al. 1997; Johnson 2007a; Johnson et al. 2009a). Reproduction is typically fatal, but some plants have the capacity to reproduce in each of their first 2 years (Johnson 2007a). This variation in the timing and number of reproductive events is influenced by the plant s environment and genotype (Johnson 2007a). Cytology, genetics and genomics Evening primrose is best known for the role it played in the rediscovery of Mendel s laws and for its peculiar genetic system, which helped resolve the chromosomal theory of genetic inheritance (Darlington 1980). It was using O. glazioviana Micheli in Martius, a hybrid of O. biennis and O. elata subsp. hookeri, that Hugo de Vries rediscovered Mendel s original findings of genetic inheritance by the independent assortment of alleles (de Vries 1900). Despite his findings, O. biennis and many other Oenothera spp. (Holsinger and Ellstrand 1984; Johnson et al. 2009b) do not exhibit independent assortment. A peculiar genetic system called permanent translocation heterozygosity (PTH) prevents recombination and segregation and results in the production of seeds that are genetically identical to their parent plant and to one another. In this way, PTH reproduction renders individuals functionally asexual, much like apomixis. This genetic system occurs in as many as eight plant families, including 43 species of Oenothera where it has been independently derived at least 20 times (Holsinger and Ellstrand 1984; Johnson et al. 2009b). A thorough review of the cytological mechanisms of the PTH genetic system can be found in Cleland (1972) and in recent summaries (Holsinger and Ellstrand 1984; Rauwolf et al. 2008; Johnson et al. 2009b, 2010). Despite the prominence that Oenothera played in the earliest development of plant genetics, cytogenetics and evolutionary biology, the development of modern genetic and genomic tools have been slow. Tools that exist include five completed chloroplast genome sequences of Oenothera spp. (including O. biennis) (Greiner et al. 2008), a modest EST library (Mracek et al. 2006), various types of polymorphic molecular markers (Levin 1975; Larson et al. 2008; Rauwolf et al. 2008), and a rich multilocus phylogenetic dataset (Hoggard et al. 2004; Levin et al. 2004; Evans et al. 2005; Johnson et al. 2009b, 2010). These resources will greatly increase in the near future because we are now deep read sequencing the nuclear genome of O. biennis (A. Agrawal, unpublished data). The pros and cons of Oenothera biennis as a model system The biology of O. biennis has numerous features that make it an attractive model system. Historically, O. biennis and various members of the Oenothera genus contributed to important breakthroughs in plant genetics and cytogenetics (Darlington 1980), mating system evolution (Emerson 1938; Steiner and Levin 1977; Raven 1979), systematics (Wagner et al. 2007) and ecology (Gross and Werner 1982; Kinsman 1982; Gross 1984). Personally, I started to work on O. biennis for three main reasons. First, it is a shortlived species in which it is easy to measure total lifetime fitness within large experiments. Second, the PTH genetic system makes it possible to replicate single genotypes from seed hundreds of times across multiple environments and treatments. And third, O. biennis plays host to a wide diversity of generalist and specialist arthropod species, making it possible to examine the bottom-up effects of genetic variation and evolution on relatively complex species assemblages, as well as the effects of natural enemies on the evolution of O. biennis (Kinsman 1982; Johnson 2007b). An additional attractive feature of this system is that it has a small number of chromosomes (x = n = 7) and a relatively small genome size (Mracek et al. 2006). Thus, the development of genomic resources is more feasible in this system than other species with larger and more complex genomes (Mracek et al. 2006; Greiner et al. 2008). Many other systems have also played important roles in addressing problems in evolutionary ecology, and it is important to consider when and where O. biennis might be the preferred model. Some of the more commonly used species used to study how ecological and demographic factors drive evolution within populations include Arabidopsis thaliana and its wild relatives, Mimulus guttatus, Trifolium repens, Anthoxanthum odoratum, and Ipomoea purpurea, to name a few. These species generally have short generation times, are easy to grow, and have an abundance of genetic tools, especially in the case of Arabidopsis and Mimulus. By contrast, relatively few systems have contributed to questions relating to the ecological and ecosystem-level consequences of evolution. Chief among these systems are three tree and shrub species: Eucalytpus (E. amygdalina E. risdonii, E. globulus) (e.g., Dungey et al. 2000; Barbour et al. 2009), Populus (P. fremontii, P. angustifolia) (e.g., Whitham et al. 2008), Salix (S. lasiolepis, S. sericea, S. eriocephala) (Fritz and Price 1988; Hochwender and Fritz 2004), and the herbaceous Solidago altissima (Maddox and Root 1987; Abrahamson and Weis 1997; Crutsinger et al. 2006). These species often form large dominant stands that can act as

4 12 Popul Ecol (2011) 53:9 21 key drivers of ecosystem processes over large areas. Unlike models such as Arabidopsis, these species also interact with a large and diverse array of other plants, arthropod and microbial species, making them ideal candidates for understanding community and ecosystem-level effects of genetic variation. Additionally, the first two systems (Eucalyptus and Populus) contain congeners with completely sequenced and well-annotated genomes. However, the long-lived perennial nature of these species, and the relatively slow growth in the case of the woody species, make it difficult to conduct multiple well-replicated experiments in various biotic and abiotic environments. The ability to experimentally study natural selection and evolutionary change is also limited in these systems. Oenothera biennis allows researchers to combine many of the best attributes of the long-lived (e.g., Populus) and short-lived (Arabidopsis) model systems. For example, O. biennis harbors and interacts with a diverse community of plants, arthropods and fungi, while having a short lifecycle, making it tractable for large-scale experiments that jointly measure genetic variation and selection, as well as the ecological effects of these factors. Additionally, the functionally asexual production of seeds makes it possible to study evolutionary dynamics in situ, by tracking the frequency of individual genotypes through time using molecular markers (Meyer et al. 2006). The main limitation of the O. biennis system is that it rarely dominates a community and is therefore unlikely to have large effects on large-scale ecosystem processes, an important area of investigation. Thus, O. biennis is best suited for testing general hypotheses and specific predictions about the interplay between ecology and evolution, which can provide insight into whether these types of interactions could be important in other systems. Evolution of Oenothera biennis The size and structure of O. biennis populations has a large impact on its evolution, as does its PTH genetic system. As mentioned earlier, local stands of O. biennis are often small, and large continuous populations are uncommon. Although it is difficult to delimit populations of functionally asexual species, I view O. biennis populations as being comprised of multiple small subpopulations, separated by 100 m to several kilometers across a landscape, connected by dispersal and gene flow. Evolution within local subpopulations is likely to be influenced by a combination of natural selection and relatively strong stochastic forces, such as founder effects, random dispersal, and genetic drift. Therefore, the evolution of subpopulations is likely nonequilibrial, but how this influences the evolution and adaptation within populations as a whole is unclear. The use of allozymes provided early insight into the evolution and population genetics of O. biennis. There is relatively high heterozygosity within individual subpopulations (0 20% of loci are heterozygous, mean = 4.5%, n = 44 populations) (Levin 1975), comparable to that found in outcrossing Oenothera species (Ellstrand and Levin 1980). Subpopulations typically contain few unique genotypes, with just one or two allozyme phenotypes (mean = 1.7) (Levin 1975), although as many as 14 genotypes have been recorded (Steiner and Levin 1977). By contrast, there is strong genetic differentiation among subpopulations for polymorphic loci (mean F ST = 0.61) (Levin 1975). These patterns arise because of O. biennis PTH genetic system, where functional asexuality maintains heterozygosity in a fixed state. The low variation within populations and high differentiation between populations occurs because relatively few genotypes initiate new subpopulations, and drift and selection quickly erode any variation that exists. Ecological genetics experiments conducted in field and greenhouse conditions show that there is substantial quantitative genetic variation for phenotypic traits within populations (Johnson 2008; Johnson et al. 2008, 2009a, c). Studies that have measured traits from genotypes collected in multiple subpopulations have found significant broad-sense heritability values for life-history traits (e.g., flowering time), morphological traits (e.g., biomass allocation), physical leaf traits (e.g., trichome density), primary chemistry and physiology (e.g., % leaf N), secondary chemistry (e.g., total phenolics), and resistance to herbivores (Table 1) (Johnson 2007a, 2008; Johnson et al. 2008, 2009a, c; Parker et al. 2010). These results suggest that O. biennis populations contain substantial quantitative genetic variation for most phenotypic traits (Table 1). However, a summary of the existing data show that heritability estimates are on average 2 39 higher for secondary metabolites than other traits. There is also considerable genetic variation for life-history traits, and slightly lower amounts of variation for other types of traits (Table 1). The high level of heritable variation within O. biennis populations is likely maintained by multiple mechanisms. Theory suggests that genotype-by-environment interactions (GEI) can maintain genetic variation within structured populations, like those of O. biennis (Christiansen 1974, 1975; Gillespie and Turelli 1989). A single field experiment revealed that variation among habitats can result in dramatic GEI on lifetime fecundity of O. biennis and that this interaction is in part explained by crossing reaction norms (Johnson 2007a). This form of GEI is sufficient to maintain genetic variation within populations (Stanton and Thiede 2005), especially when gene flow between subpopulations is low (Christiansen 1975).

5 Popul Ecol (2011) 53: Table 1 Broad-sense heritability estimates for different types of plant traits in O. biennis Trait type H 2 SE % Sig. No. distinct n Life-history Morphology Physical leaf traits Primary chemistry/ physiology Secondary chemistry Herbivore resistance Overall Across five studies, there have been 67 estimates of broad-sense heritability values for various traits. We categorized each trait according to one of six categories and then estimated: the mean heritability across all 67 estimates (H 2 ); the standard error of this mean H 2 ; the % of estimates (relative to the total number of independent estimates) that were statistically significant at P \ 0.05; the number of distinct traits estimated per category (sometimes the different studies measured the same trait); and the total number of estimates across studies per category (n). Data taken from Johnson (2007a, 2008), Johnson et al. (2008, 2009a, c) and Parker et al. (2010) Herbivorous insects on O. biennis can be a potent agent of natural selection acting on heritable plant traits. Oenothera biennis plays host to over 50 generalist and specialist herbivore species (Dickerson and Weiss 1920; Kinsman 1982; Johnson et al. 2006; Johnson 2007b), which can impose substantial damage on plants. For example, in a field experiment in North Carolina (Johnson et al. 2009b), naturally occurring generalist herbivores removed on average 24% (SE = 3.3%, n = 40) of a plant s leaf area by late summer (mid-august). A survey of 84 plants from 11 populations in Alabama and Florida (USA) showed lower mean levels of herbivory (mean = 3.9%, SE = 0.4) yet substantial variation among subpopulations (M. Johnson, unpublished data). A separate experiment in SE Canada found that herbivores reduced allocation to aboveground biomass by 14% (M. Johnson, unpublished data) and decreased total lifetime fruit production by 11% (Johnson and Agrawal 2005), suggesting that insect herbivores have the ability to impose selection on plant populations. The clearest evidence for this comes from genetic variation in the fitness effects of several specialist moth species. Three moth species exclusively feed on either the flowers (Mompha stellella, Momphidae; Schinia florida, Noctuidae) and/or fruits (M. brevivitella, S. florida) of O. biennis, and therefore these insect species have direct impacts on plant fitness. The densities of these insects can be high, and they dramatically vary among plant genotypes, where some genotypes are completely resistant to a particular insect herbivore species whereas others are highly susceptible (Kinsman 1982; Johnson and Agrawal 2007). Evidence also indicates that pathogenic fungi can impose strong selection on O. biennis. Unpublished data collected in the experiment of Johnson et al. (2006) found that lifetime fruit production was negatively genetically correlated with the severity of infection by powdery mildew on leaves (genotypic correlation: r p = 0.67, 0.02, n = 12 genotypes). It is unclear whether this negative relationship occurred due to the direct effect of mildew infection on plants, or whether plant genotypes with inherently low fitness were also most susceptible to infection. The community-level effects of genetic variation in O. biennis In 1992, Janis Antonovics articulated the need for the formation of a new area of study community genetics. His vision was that community genetics would merge the questions, concepts and theories of population genetics and evolutionary biology with community ecology, to understand the interplay between the ecology and evolution of multiple interacting populations, and not just a single population, which had been the focus within ecological genetics and evolutionary ecology (Ford 1949; Lack 1965). Community genetics offered a less restrictive framework than coevolution, which had typically been defined as reciprocal natural selection and evolutionary change between two or more populations (Ehrlich and Raven 1964; Janzen 1970; Thompson 1994). Community genetics could involve such reciprocity but it was not exclusive to it. Many researchers have answered Antonovics rallying cry by testing the general hypothesis that: evolution and genetic variation within single species populations can have cascading ecological effects throughout communities and ecosystems. This research has involved work on a wide diversity of organisms, including microbial parasites and bacteria (Bohannan and Lenski 2000; de Roode et al. 2004), phyto- and zooplankton (Yoshida et al. 2003), vascular plants (Whitham et al. 2006), insects (Ferrari et al. 2004; Tetrad-Jones et al. 2007; Hazell and Fellowes 2009), and fish (Post et al. 2008; Harmon et al. 2009; Bassar et al. 2010). Within this section, I review the role that O. biennis has played in understanding the community-level effects of: (1) genetic variation among individual plants (genotype identity), (2) the amount of genotypic diversity within plant populations, and (3) the effects of evolution of plant traits on communities. I also compare the results from O. biennis to similar studies in other plant systems. Effects of plant genotype on arthropods Genetic variation in O. biennis populations has clear bottom-up effects on arthropod communities. At the simplest level, plant genotype affects the preference and performance of individual arthropod species on plants by

6 14 Popul Ecol (2011) 53:9 21 influencing the amount of herbivory by specialist and generalist herbivores (McGuire and Johnson 2006; Johnson et al. 2009a), the number of eggs laid by specialist weevils on plants (Johnson and Agrawal 2007), and the abundance of more than half the common arthropods that naturally colonize O. biennis plants (Kinsman 1982; Johnson and Agrawal 2007). These effects of plant genotype also influence the population dynamics of herbivores, where genotype identity of O. biennis caused per capita population growth rate of a specialist aphid herbivore (Aphis oestlundi) to vary between 0 and 0.11 aphids day -1 mother -1 among plant genotypes (Johnson 2008). This same experiment revealed that plant genotype had direct effects on the abundance of predators and mutualistic ants that tended aphids, as well as indirect effects on the ants which were mediated by variation in aphid density (Johnson 2008). The observed tritrophic effects of plant genotype appear to be general across many plant systems. Although it has long been recognized that genetic variation in plant traits affects the preference and performance of herbivorous insects (Maddox and Root 1987; Karban 1992; Carmona et al. 2010), the effects of plant genotype on long-term herbivore population dynamics (Underwood and Rausher 2000, 2002; McIntyre and Whitham 2003), plant and herbivore mutualists (Wimp and Whitham 2001; Rudgers and Strauss 2004; Mooney and Agrawal 2008), and the third trophic-level (Fritz 1995; Stiling and Rossi 1996; Bailey et al. 2006) have been recognized more recently. The responses of multiple arthropod populations to genetic variation in O. biennis are often not independent of one another, which is associated with effects of plant genotype on the composition of arthropod assemblages. In a single field experiment, we found that the most common arthropod species positively covaried in their abundance among plant genotypes (Johnson and Agrawal 2007). The strength of this covariation was stronger among some arthropod species than others, such that we detected three to four distinct groups of species which responded more similarly to genetic variation within groups, and less similarly between groups. Membership to these covarying groups of species was not consistently related to the taxonomy or functional classification (e.g., feeding guild) of the different species, which is consistent with previous findings in goldenrods (Maddox and Root 1990) and willows (Roche and Fritz 1997), and is predicted to lead to diffuse selection by herbivores on plant populations (Stinchcombe and Rausher 2001; Strauss et al. 2005). When we examined the effects of plant genotype on the composition (i.e., relative abundance and presence/ absence) of these common arthropods using ordination techniques, we found significant differences among plant genotypes (Johnson and Agrawal 2007), consistent with similar studies conducted on dominant tree species (Dungey et al. 2000; Wimp et al. 2005; Tovar-Sánchez and Oyama 2006). However, these effects of plant genotype varied among habitats (i.e., statistically significant GEI) indicating that plant genetic variation is important in affecting arthropod community composition, but the nature of these effects depends on the environment in which the plants occur. These bottom-up effects of plant genotype on arthropod communities lead to clear impacts on higher order properties of communities. In two large field experiments, O. biennis genotype influenced the overall biomass, abundance, species richness, evenness in abundance and Simpson s diversity of a diverse arthropod community comprised of over 120 herbivorous, omnivorous and predaceous arthropod species (Johnson and Agrawal 2005, 2007; Johnson et al. 2006). The effects of plant genotype were strongest on the abundance and richness of herbivorous arthropods and attenuated at higher trophic levels, where plant genotype had significant but weaker effects on omnivorous and predaceous arthropod species (Johnson and Agrawal 2005). This finding of weaker effects of plant genotype on predators than herbivores is consistent with recent meta-analyses that show the ecological effects of plant genetic variation are expected to be strongest on components of the community that interact most closely with the focal plant, and weaker on components of the community (e.g., predators) whose interactions are indirect (Bailey et al. 2009; but see Crutsinger et al. 2009). Several of the studies reviewed above also identified heritable plant traits that predict variation in arthropod communities. Morphological (e.g., plant size, number of branches) and life-history (annual/biennial reproduction, flowering time) traits were most strongly correlated with variation in complex community variables, such as the total abundance, species richness, and Simpson s diversity of arthropods on plants (Johnson and Agrawal 2005). An idiosyncratic suite of traits predicted variation in the growth rate and abundance of individual herbivore and predator species. For example, genetic variation in leaf water content, the density of trichomes and the amount of leaf nitrogen, together explained 49% of the variation in aphid growth rate among plant genotypes (Johnson 2008). Ants that tended these aphids were positively related to aphid density and trichome density, while none of the measured traits predicted variation in the number of predators (Johnson 2008). By contrast, damage by the generalist Japanese beetle (Popillia japonica) was most strongly related to genetic variation in the concentrations of secondary compounds in leaves, such as ellagitannins and flavonoid glycosides (Johnson et al. 2009a). Recent findings in other systems suggest that idiosyncratic effects of specific plant traits on individual arthropod populations

7 Popul Ecol (2011) 53: might be the rule rather than the exception (Agrawal 2004, 2005; Bailey et al. 2006), and the genetic variation in secondary chemistry has less of an effect on herbivores than variation in life-history and morphological plant traits (Carmona et al. 2010). Effects of plant genotype on competing plants The effects of O. biennis genotype on the performance and diversity of competing plant species is relatively weak compared to the bottom-up effects of plant genetic variation. In greenhouse experiments, genetic variation in shoot:root ratio of O. biennis explained as much as 41% of the variance in the performance of a competing grass species (Johnson et al. 2008). This effect of plant genotype was swamped out by the effects of soil nutrients in the greenhouse, and plant genotype had no effect on the performance or diversity of competing plant species that naturally colonized a freshly ploughed field (Johnson et al. 2008). These findings suggest that the ecological effects of genotype identity on competitors are biologically unimportant in O. biennis. The limited literature examining this problem in other systems suggests that strong effects of plant genetic variation on the performance and coexistence of competing plant species are most likely when plant species exhibit heritable variation in allelopathic chemicals (Iason et al. 2005; Lankau and Strauss 2007) or strong variation in growth form and the propensity for vegetative clonal growth (Proffitt et al. 2005; Crutsinger et al. 2010). Effects of plant genotypic diversity on arthropods and the focal plant population To fully understand the community-level consequences of genetic variation requires studies to scale-up from the individual plant-level to the population level (Urban et al. 2008; Tack et al. 2010). The first generation of studies to do this investigated how genetic diversity (i.e., the magnitude of genetic variation) within plant patches or populations affects community and ecosystem properties (Hughes et al. 2008; Bailey et al. 2009). Experiments using O. biennis were among the first to show that the ecological effects of genotypic diversity could have qualitatively and quantitatively similar effects on arthropod communities and plant fecundity compared to the effects of plant species diversity (Johnson et al. 2006). Increasing genotypic diversity in small patches of O. biennis from one to eight genotypes led to an 18% increase in the number of arthropod species on plants, and explained 16% of the total variation, a result that appears to be remarkably consistent across other plant species (Bailey et al. 2009). Surprisingly, the species richness and abundance of the third trophic level was more strongly influenced by increased genotypic diversity in O. biennis than were the richness or abundance of herbivores. This result is in stark contrast to the effects of plant genotype identity, where effects were stronger on herbivores than on predators in O. biennis. A possible explanation for this discrepancy is that herbivores and predators often search at different scales, where herbivores cue into trait variation at the individual plant level, whereas predators search at a larger scale, searching for entire patches with an abundance of prey (Crutsinger et al. 2009). These bottom-up effects of genotypic diversity were also associated with increased plant performance, where four independent experiments found that genotypically diverse patches of O. biennis experience higher survival, lifetime fecundity or increased net primary productivity compared to monocultures (Johnson et al. 2006; Parker et al. 2010; S. McArt and S. Cook, personal communications). A recent experiment by Parker et al. (2010) suggests that the observed increase in plant performance is related to positive effects of genotypic diversity on resistance to biotic stressors. In a large field experiment, Parker et al. (2010) found that diverse O. biennis patches lost 76% fewer seeds to specialist seed-feeding insect herbivores, experienced 11% lower herbivory by voles (a major source of plant mortality), and experienced smaller impacts of deer herbivory on plant fitness. These findings parallel similar results from aquatic environments where increased genotypic diversity in seagrass (Zostera marina) provided resistance against grazing by birds and stress due to extreme temperatures (Hughes and Stachowicz 2004; Reusch et al. 2005). Can evolution drive ecological changes within communities? One of the greatest gaps in evolutionary ecology is our lack of understanding about whether evolution in one population can drive corresponding ecological changes in a second population or even within entire communities and ecosystems (Fussmann et al. 2007; Hersch-Green et al. 2010). Theoretical models show that evolution within prey populations can alter predator prey population dynamics (Abrams and Matsuda 1997), and microcosm experiments focusing on rotifer algae and bacteria virus interactions have empirically validated these predictions (Bohannan and Lenski 2000; Yoshida et al. 2003). It is still unclear whether evolutionary changes within populations cause corresponding changes in natural communities over time (Johnson and Stinchcombe 2007; Post and Palkovacs 2009). To help fill this gap, Johnson et al. (2009c) proposed three necessary conditions, that if empirically validated provide strong evidence that evolution by natural selection in one population can cause ecological changes in communities.

8 16 Popul Ecol (2011) 53:9 21 These conditions include: (1) heritable genetic variation in phenotypic traits of the focal population; (2) natural selection on heritable variation; and (3) the genetic variation in plant traits subject to natural selection must cause variation in some aspect of the community. Using a combination of experimental data, theory and simulations, we showed that natural selection measured in a field experiment acted on life-history variation in O. biennis and was expected to result in evolutionary changes within plant populations that then cause rapid changes in arthropod communities. The exact biotic and abiotic agents of selection are unknown, but the predicted community-level changes due to selection included both decreased and increased relative abundances of dominant insect species, a rapid rise in the total abundance of arthropods, and a steady decline in the number of arthropod species found in plant populations. However, we also showed that these predictions are sensitive to the size of the focal population, as the strength of genetic drift can swamp out the effects of selection in small populations (Johnson et al. 2009c). How important is genetic variation versus other ecological factors? The need for a community genetics perspective in ecology was questioned shortly after the first studies on this topic appeared (Morin 2003; Ricklefs 2003). In response to this criticism, many biologists have conducted experiments which show that genetic variation in one population can explain a large amount of the variation in community and ecosystem properties (Whitham et al. 2006; Bailey et al. 2009). These studies provide an important and necessary first step towards understanding whether there can be community-level effects of genetic variation. Nevertheless, it has recently been argued (Hersch-Green et al. 2010; Tack et al. 2010) that this first generation of community genetics studies provide little information about how important genotype identity and genotypic diversity are relative to other ecological factors, which community ecologists have traditionally identified as most important in community ecology (Menge and Sutherland 1987; Hunter and Price 1992). One might even argue that the amount of variation in community and ecosystem properties attributed to plant genotype (e.g., Johnson and Agrawal 2005; Shuster et al. 2006; Schweitzer et al. 2008) has been exaggerated by conventional experimental methods; most experiments collect genotypes over large geographic areas and heterogeneous environments, which are then randomized in common gardens that minimize external variation (Tack et al. 2010). This important problem can only be addressed with multifactorial field experiments, where plant genotype and at least one additional ecological factor are manipulated. With these experiments, one can quantify the amount of variation explained by plant genotype and other ecological factors, and the interactions between them (Hersch-Green et al. 2010). I have used this multifactorial approach to understand whether a community genetics perspective is needed in the O. biennis system. To understand the factors that most strongly affect the growth rate of a specialist aphid herbivore (Aphis oestlundi), I compared the relative roles of plant genotype, density-dependent population growth and aphid ant mutualistic interactions (Johnson 2008). After five aphid generations, plant genotype explained 29% of the total variation in aphid growth rate, which was greater than the variation explained by negative density-dependence (0.2% of the total variation), the presence of ants (0.9% of the total variation), or interactions between these factors (\1% of variation) (see Johnson 2008: Table 1). In another experiment, we compared the effects of plant genotype with interspecific competition among three herbivore species, and assessed how these factors affected the preference and performance of insects on plants (McGuire and Johnson 2006). Across all species, plant genotype explained on average 20% of the variation in herbivore performance, which was much greater than the \2% variation explained by either the effect of competition or the interaction between genotype and competition (McGuire and Johnson 2006). To compare the community-level effects of plant genotype to variation in the environment, we replicated plant genotype among five habitats that varied along a productivity gradient, as well as among random spatial blocks within habitats (microhabitat variation) (Johnson and Agrawal 2005). Plant genotype explained approximately 29 more variation than microhabitat for Simpson s diversity (% variation explained by plant genotype, % variation explained by microhabitat: 14, 6%), species richness (11, 5%), and the total abundance of arthropods (20, 10%). At larger spatial scales, habitat variation was the most important factor, explaining 39% of the total variation in Simpson s Diversity and 59% of the variation in both arthropod richness and total abundance, compared to 2 11% of variation explained by plant genotype or genotype 9 habitat interactions. However, it should be noted that habitats were selected to maximize environmental variation, so the large effect of habitat likely represents the upper limit to the effects of the environmental variation at a landscape scale. Nevertheless, these results highlight that the importance of plant genotype relative to the environment depends on spatial scale, as has been seen in other systems (see Johnson and Agrawal 2005: Appendix E; Bangert et al. 2006). A series of greenhouse and field experiments also assessed the factors that affect the performance and

9 Popul Ecol (2011) 53: Predator (abundance, diversity) Mutualistic ants (abundance) Habitat and abiotic environment Herbivore (preference, growth rate, abundance, covariation, composition, diversity) Interspecific competition Intraspecific competition Plant competitor (growth rate, biomass, diversity) Plant genotype Fig. 1 A graphical summary of the relative importance of Oenothera biennis genotype versus other ecological factors affecting plant and arthropod community members. The weight of the line provides a qualitative depiction of the strength of a particular factor on a community variable. Solid lines depict main effects of one factor on a variable and dashed lines indicate factors which interact with plant genotype. Although the relative importance of plant genotype versus other ecological factors (e.g., habitat) have been manipulated in multifactor experiments, we have not yet examined interactions between all possible non-genetic factors using multifactor experiments, nor have we independently manipulated the presence/absence of the third trophic level, other than ants which act as mutualists and predators in this system diversity of plant species (Johnson et al. 2008). As described above, these experiments showed that plant genotype can affect the performance of competitors, but multifactorial manipulations of soil nutrients and habitat reveal that these latter factors are far more important than any effect of plant genotype (Johnson et al. 2008). Based on this synthesis of results, I conclude that a community genetics perspective is needed to understand the ecology of multitrophic interactions in the O. biennis system (Fig. 1). Genetic variation in O. biennis has strong bottom-up effects on arthropod communities that explain as much and sometimes more variation than ecological factors traditionally identified as most important in community ecology (Fig. 1). A community genetics perspective is not needed to understand the ecological consequences of competition within the O. biennis system. Future directions The next 10 years represent an exciting time for evolutionary ecologists. Technological advancements (e.g., deep read sequencing) and new statistical tools (e.g., Bayesian methods, community phylogenetic tools, etc.) are making it possible to answer previously intractable questions. Moreover, the growing uncertainty associated with the impacts of global climate change, the sustainability of farming practices, and environmental degradation due to habitat destruction and invasive species, requires the combined input of ecology, evolution and genetics. Some of the most pressing questions facing evolutionary ecologists include: at what rate do natural populations adapt to changes in their biotic (e.g., invasive species) and abiotic (e.g., increased drought) environment; does evolutionary change within populations alter the structure, function and stability of communities and ecosystems; how does demography (i.e., population size) influence adaptive evolution within populations; and what types of genes mediate adaptation to the abiotic and biotic environment? Many researchers are now looking to Oenothera to answer these questions, and I encourage additional groups to explore the benefits and exciting tools this system has to offer. For example, field experiments are already underway that will reveal whether natural selection by insect herbivores drives rapid evolution within O. biennis populations, and whether this evolution causes changes in the abundance and diversity of arthropod populations through time. Researchers continue to investigate the ecological consequences of genotypic diversity (e.g., Parker et al. 2010) and their effects relative to plant species diversity (S. McCart and S. Cook, personal communication). We are also now beginning to implement ultra high throughput sequencing technologies to identify candidate genes responsible for large ecological effects on communities, which will ultimately enable us to build a

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