Project overview
Biologists have recently recognized that contemporary evolution may play an important role in successful species invasions. For plant species, theory and the preponderance of empirical data suggest that latitudinal clines should evolve with regard to palatability and defense traits (palatability should increase and defenses decrease with increasing latitude). However, this theory has never before been applied to species that are involved in continent-wide invasions. Moreover, no one has ever examined how interactions between invading plants and closely related native plants, via their shared herbivores, might vary across broad latitudinal gradients. My collaborator, Laura Meyerson (Natural Resources Science, University of Rhode Island), and I have recently begun to address these issues with the cosmopolitan invasive grass, Phragmites australis (common reed; Fig. 1). This plant species is native and widespread in Europe and North America, but a European genotype has been spreading rapidly in North America over the past 100 years. This study system affords us the unique opportunity to examine parallel cline formation between native and invasive genotypes along the same latitudinal gradient, and assess how differences along this gradient could affect invasion success.

An informational (tri-fold) pamphlet on this research project is available as a PDF file in English, French, Spanish and Norwegian.

Fig. 1. Richard Stevens, LSU, setting rodent traps in a P. australis patch

Background
P. australis represents an ideal system for studying geographic and clinal variation in palatability, defense levels, and herbivore pressure; and how the invasion process is affected by this type of variation. Key features that make this an ideal system include, the co-occurrence of native and invasive genotypes across a wide (continental) range, a well documented history of invasion in North America, and a well known herbivore fauna associated with this plant in both native and introduced ranges. We provide a brief description of this system below.

Fig 2. Distribution of P. australis haplotypes in North America. M (red circles) = invasive, I (blue squares) = Gulf Coast, all others = endemic native haplotypes (after Saltonstall 2003)

In the 1800s, botanical records described P. australis as being rare or not common, but by the early 1900s,it had become common and was spreading (Saltonstall 2002, Tewksbury et al. 2002). However, in the ensuing years, P. australis spread throughout the U.S. and Canada and is now recognized as a dominant plant species in disturbed wetlands. The recent increase in P. australis, particularly along the east coast of the U.S., has been attributed to the introduction of an aggressive Eurasian genotype (Saltonstall 2002).

Based on chloroplast DNA analysis of fresh tissues and herbarium records, Saltonstall (2002) identified 27 distinct haplotypes worldwide, 13 of which were endemic to North America. These native haplotypes are found in the Northeast, Midwest and west of the Rockies (see Fig. 2). Two additional haplotypes found in North America and elsewhere were designated haplotypes M and I. Haplotype M was the most common haplotype in present-day samples from North America, Europe and Asia. In North America, haplotype M comprised only 6% of the herbarium records examined prior to 1910, but made up over 60% of the examined specimens after 1960. Haplotype M  is considered invasive and first appeared in records from wetlands in the northeast, but has subsequently spread west (Saltonstall 2002, Lambertini et al. 2006, Meyerson et al. in review).

Haplotype I is found near the coastline of all the Gulf Coast states (Fig. 1) and is informally known as the Gulf Coast haplotype (Pellegrin & Hauber 1999, Saltonstall 2002, Howard et al. 2008). It also occurs in a few isolated pockets in the southwest U.S. (Saltonstall 2002) but appears to be rapidly spreading (Meyerson et al. in review). In addition, this haplotype is the dominant type in South America and has been found in Australia, Asia and several islands in the south pacific.

Project Objectives
The main objectives of this proposal are two-fold. First, the PIs will test whether P. australis has undergone rapid evolution to form a cline with regard to traits associated with its interactions with herbivores (palatability, defense levels). Second, the PIs will quantify differences in palatability, defense levels, and herbivore pressure between sympatrically occurring native and invasive genotypes of P. australis, and evaluate how these differences may affect invader success. Using field surveys and common-garden experiments, the following specific questions will be addressed. (1) Does invasive P. australis exhibit a latitudinal cline in palatability, defense levels, and herbivore pressure that is parallel to the clines found for the same genotype in its native range (Europe) and with sympatrically occurring native genotypes (North America)? (2) Is invasion success associated with the invading genotype being less palatable, better defended, or having lower herbivore pressure than the sympatrically occurring native genotypes? (3) Is the geographical, latitudinal or within site (native vs. invasive genotype) variation in P. australis palatability and defense levels genetically based? (4) In areas of sympatry, does hybridization occur between native and invasive genotypes? If so, how does it influence palatability, defense levels, and herbivore pressure and what impact would it have on invader success?

Fig. 3. Common herbivore of P. australis, the generalist aphid Hyalopterus pruni (A) and damage from a stem borer (B)

Research Program
Our research plan involves the following three phases:

1) Conduct surveys to examine whether there is latitudinal variation in traits affecting P. australis defense and herbivore pressure. We will establish two transects, one in eastern North America (NA; native and invasive haplotypes) and the other in Europe/North Africa (EUR; native range of the North American invasive haplotype). Surveys will be conducted to assess plant fitness and defense levels, herbivore pressure, environmental conditions, and landscape composition. These data will be used to address the following questions: (1) Does native P. australis in NA and EUR exhibit parallel clinal variation in plant fitness and defense traits and parallel variation in herbivore pressure as predicted by theory (defense levels and herbivore pressures decrease with increasing latitude)? (2) Is there parallel cline formation between haplotype M in its invasive (NA) and native (EUR) range and between sympatrically occurring native and invasive haplotypes in NA? (3) Are differences between co-occurring native and invasive haplotypes in NA related to differences in plant fitness, clonal growth and present-day rates of spread? (4) What is the relative contribution of local, regional and global factors to the variation observed in plant fitness, defense and herbivore pressure? And (5) In areas of sympatry, does hybridization occurs between native and invasive haplotypes? If so, do hybrids differ from parental stocks in defense levels and herbivore pressure? Although theory and existing empirical data strongly favor the occurrence of either a cline or geographic variation in defenses and herbivore pressure, any pattern, including the absence of a cline, would be extremely informative regarding the invasion process at large spatial scales.

2) Conduct common-garden experiments to determine if latitudinal patterns are genetically based.The above survey is informative regarding the large-scale variation in P. australis-herbivore interactions, but controlled experiments will be required to ascertain whether the observed variation is attributable to genetic differences among sites, or a consequence of phenotypic plasticity in response to an environmental gradient. At both LSU and URI, we will establish replicate common gardens that are populated with P. australis ramets from each of our sites in NA and EUR. We will then test the hypothesis that traits associated with P. australis fitness, defense, and palatability have a genetic basis. For native and invasive haplotypes in sympatry, we will also conduct a series of herbivore preference experiments to assess whether herbivores are likely to impede or facilitate the invasion of non-native P. australis haplotypes in areas where native haplotypes are present.

Newly constructed greenhouse at LSU South Campus

3) Assess effects of hybridization on P. australis fitness, defense levels and palatability. Using P. australis haplotypes from the six locations where they occur in sympatry, we will perform hybrid crosses and compare hybrid fitness, defense levels, palatability and herbivore preference for them relative to parental stock. Our goal is to test whether hybridization could facilitate invasion success by making the hybrids better fit or better defended from their herbivores.

References

Howard, R. J., S. E. Travis, and B. A. Sikes. 2008. Rapid growth of a Eurasian haplotype of Phragmites australis in a restored brackish marsh in Louisiana, USA. Biological Invasions 10:369-379.

Lambertini, C., M. H. G. Gustafsson, J. Frydenberg, J. Lissner, M. Speranza, and H. Brix. 2006. A phylogeographic study of the cosmopolitan genus Phragmites (Poaceae) based on AFLPs. Plant Systematics and Evolution 258:161-182.

Meyerson, L.A., A. Lambert, K. Saltonstall. In review. Three invasion fronts of Phragmites australis in North America: research and management needs in the face of common reed expansion in the west and Gulf regions. Invasive Plant Science and Management.

Pellegrin, D. and D. P. Hauber. 1999. Isozyme variation among populations of the clonal species, Phragmites australis (Cav.) Trin. ex Steudel. Aquatic Botany 63:241-259.

Saltonstall, K. 2002. Cryptic invasion by a non-native genotype of the common reed, Phragmites australis, into North America. Proceedings of the National Academy of Sciences of the United States of America 99:2445-2449.

Tewksbury, L., R. Casagrande, B. Blossey, P. Hafliger, and M. Schwarzlander. 2002. Potential for biological control of Phragmites australis in North America. Biological Control 23:191-212.