NONADAPTIVE EVOLUTION OF DISEASE RESISTANCE IN AN ANNUAL LEGUME

Local populations of the plant Amphicarpaea bracteata often contain genetically divergent lineages that differ strongly in disease resistance toward the specialist pathogen Synchytrium decipiens. In one population, lineages with disease resistance were observed to significantly decrease in frequency over a two-year period, despite the continued presence of pathogens. Extensive self-pollination in A. bracteata restricts the opportunity for recombination of alleles affecting separate traits, resulting in strong correlations between disease resistance and other ecologically important characters, including plant morphology, phenology, and patterns of reproductive allocation. Natural selection on these correlated characters may thus cause nonadaptive changes in disease resistance. These results imply that A. bracteata's mating system is a basic constraint interfering with its adaptation to pathogen attack.     

Individual variation in pathogen attack and differential reproductive success in the annual legume,Amphicarpaea bracteata

Summary Demographic analyses in two natural populations of the annual legume Amphicarpaea bracteate examined whether variation in attack by the host-specific fungal pathogen Synchytrium decipiens was associated with variation in the reproductive success of individual plants. In both populations, fungal infection early in life was significantly associated with reduced seedling growth rates. Laboratory inoculation experiments confirmed that S. decipiens infection had a negative impact on plant growth. The laboratory experiments further indicated that there was significant variation among the progenies of different plant genotypes in the degree of growth reduction caused by pathogen attack. Prereproductive mortality rates in natural environments were significantly higher for plants with infection intensities above the median; for the two populations studied, heavily infected plants had 3.8 and 12 gimes higher death rates compared with low infection plants. Among surviving plants, fungal infection intensity was significantly negatively correlated with total seed biomass in both populations. As a result of these associations between plant survivorship, fecundity, and fungal infection, lifetime relative fitness within both plant populations was strongly negatively correlated with the intensity of S. decipiens infection. These results demonstrate the existence of consistent natural selection for increased resistance to pathogen attack in this plant species.     

DISEQUILIBRIUM BETWEEN DISEASE-RESISTANCE VARIANTS AND ALLOZYME LOCI IN AN ANNUAL LEGUME

Polymorphism existed at 58% of the enzyme loci examined (11/19) in one population of the highly self-pollinated annual legume Amphicarpaea bracteata. Due to extreme gametic disequilibrium among loci, genetic variation in this population was structured into a small number of multilocus genotypes. Over 97% of the plants sampled could be grouped into two classes (biotypes “A” and “B”), each consisting of a few highly similar genotypes. The two classes had mutually exclusive sets of alleles at nine loci. These classes differed sharply in their disease resistance toward one isolate of the specialist fungal pathogen Synchytrium decipiens from their native habitat. All biotype A plants were strongly susceptible, and all biotype B plants were resistant. When plants of both biotypes were exposed to this pathogen in a greenhouse, the resistant biotype (B) exhibited a significantly higher growth rate. The strong association between plant disease-resistance phenotypes and allozyme variants implies that pathogen attack could be a major selective agent influencing the evolution of neutral or near-neutral alleles at enzyme loci in this plant.     

Geographic structure of lineage associations in a plant-bacterial mutualism

Two species linked by a mutualistic relationship may evolve correlated population differentiation if there is long-term continuity of interactions between specific partners. This phenomenon was analyzed by multilocus enzyme electrophoresis on the annual legume Amphicarpaea bracteata and its nitrogen-fixing bacterial symbionts (Bradyrhizobium sp.) sampled from >20 sites over a 1000 km area. Three analyses indicated that genetic differentiation was correlated in the two organisms. First, the genetic distance between bacterial populations at each pair of sites was significantly positively related to the genetic distance between their host plant populations, as evaluated by the Mantel test. Second, a cluster analysis revealed that several divergent lineages were present both among plants and among bacteria. Bacterial lineages showed a highly nonrandom distribution across plant lineages that was consistent in each of two regions sampled. Finally, there were numerous cases where populations of the same plant lineage 1000 km apart harbored bacterial isolates with an identical multilocus genotype. Thus, despite recurrent opportunities for partner switching, particular genotypes of these two organisms associate consistently across multiple habitats throughout their geographic range.     

DIVERGENCE IN SYMBIOTIC COMPATIBILITY IN A LEGUME-BRADYRHIZOBIUM MUTUALISM

Geographic variation in the mutualism between the legume Amphicarpaea bracteata and its nitrogen-fixing root nodule bacteria (Bradyrhizobium sp.) was analyzed by sampling genotypes from 11 sites separated by distances ranging from 0.6 km to more than 1000 km. Cross inoculation experiments revealed that plants were genetically differentiated in traits determining compatibility with mutualist partners from different sites. Combinations of plant and bacterial genotypes native to the same local habitat yielded 26% higher plant growth relative to non-native combinations (range across 4 experiments; 9% to 48%). Among non-native symbioses, plant growth was unrelated to the geographic distance between sites of plant and bacterial origin. However, compatibility varied significantly with the genetic distance among host populations (inferred by multilocus enzyme electrophoresis): genetically similar plants from separate sites showed superior growth with each other's mutualist partners. Nevertheless, the tree structure of population genetic similarity was not congruent in plants versus bacteria. This implies that adaptive variation in symbiotic compatibility has evolved without strictly parallel divergence in the two species.     

Effects of disease resistance genes on Rhizobium symbiosis in an annual legume

Summary The evolution of disease resistance in plants may be constrained if genes conferring resistance to pathogens interfere with plant responses toward other, nonpathogenic organisms. To test for such effects, we compared symbiotic nitrogen fixation in Amphicarpaea bracteata plants that differed at a major locus controlling resistance to the pathogen Synchytrium decipiens. Both resistant and susceptible plant genotypes nodulated successfully and grew significantly better in the presence of Rhizobium, although growth enhancement by Rhizobium was altered by different levels of nitrate fertilization. Plants homozygous for disease resistance achieved 2% higher growth than susceptible homozygotes across all treatments, but this difference was not significant. Resistant and susceptible plant genotypes did not differ in the mean number of nodules formed per plant or in nodule diameter. However, there was highly significant variation among replicate families within each disease resistance category for both nodulation characteristics. These results imply that genetic variation exists among A. bracteata plants both for diease resistance and for traits affecting symbiotic nitrogen fixation. However, there were no evident pleiotropic effects of disease resistance genes on the plant-Rhizobium symbiosis.     

LOCAL MALADAPTATION IN THE ANTHER-SMUT FUNGUS MICROBOTRYUM VIOLACEUM TO ITS HOST PLANT SILENE LATIFOLIA: EVIDENCE FROM A CROSS-INOCULATION EXPERIMENT

Conventional wisdom holds that parasites evolve more rapidly than their hosts and are therefore locally adapted, that is, better at exploiting sympatric than allopatric hosts. We studied local adaptation in the insect-transmitted fungal pathogen Microbotryum violaceum and its host plant Silene latifolia. Infection success was tested in sympatric (local) and allopatric (foreign) combinations of pathogen and host from 14 natural populations from a metapopulation. Seedlings from up to 10 seed families from each population were exposed to sporidial suspensions from each of four fungal strains derived from the same population, from a near-by population (< 10 km distance), and from two populations at an intermediate (< 30 km) and remote (< 170 km) distance, respectively. We obtained significant pathogen X plant interactions in infection success (proportion of diseased plants) at both fungal population and strain level. There was an overall pattern of local maladaptation of this pathogen: average fungal infection success was significantly lower on sympatric hosts (mean proportion of diseased plants = 0.32 ± 0.03 SE) than on allopatric hosts (0.40 ± 0.02). Five of the 14 fungal populations showed no strong reduction in infection success on sympatric hosts, and three even tended to perform better on sympatric hosts. This pattern is consistent with models of time-lagged cycles predicting patterns of local adaptation in host-parasite systems to emerge only on average. Several factors may restrict the evolutionary potential of this pathogen relative to that of its host. First, a predominantly selfing breeding system may limit its ability to generate new virulence types by sexual recombination, whereas the obligately outcrossing host 5. latifolia may profit from rearrangement of resistance alleles by random mating. Second, populations often harbor only a few infected individuals, so virulence variation may be further reduced by drift. Third, migration rates among host plant populations are much higher than among pathogen populations, possibly because pollinators prefer healthy over diseased plants. Migration among partly isolated populations may therefore introduce novel host plant resistance variants more often than novel parasite virulence variants. That migration contributes to the coevolutionary dynamics in this system is supported by the geographic pattern of infectivity. Infection success increased over the first 10–km range of host-pathogen population distances, which is likely the natural range of gene exchange.     

Genetic discontinuities and disequilibria in recently established populations of the plant pathogenic fungus Mycosphaerella fijiensis

Dispersal processes of fungal plant pathogens can be inferred from analysis of spatial genetic structures resulting from recent range expansion. The relative importance of long‐distance dispersal (LDD) events vs. gradual dispersal in shaping population structures depends on the geographical scale considered. The fungus Mycosphaerella fijiensis, pathogenic on banana, is an example of a recent worldwide epidemic. Founder effects in this species were detected at both global and continental scale, suggesting stochastic spread of the disease through LDD events. In this study, we analysed the structure of M. fijiensis populations in two recently (∼1979–1980) colonized areas in Costa Rica and Cameroon. Isolates collected in 10–15 sites distributed along a ∼250‐ to 300‐ km‐long transect in each country were analysed using 19 microsatellite markers. We detected low‐to‐moderate genetic differentiation among populations in both countries and isolation by distance in Cameroon. Combined with historical data, these observations suggest continuous range expansion at the scale of banana‐production area through gradual dispersal of spores. However, both countries displayed specific additional signatures of colonization: a sharp discontinuity in gene frequencies was observed along the Cameroon transect, while the Costa Rican populations seemed not yet to have reached genetic equilibrium. These differences in the genetic characteristics of M. fijiensis populations in two recently colonized areas are discussed in the light of historical data on disease spread and ecological data on landscape features.     

Clinal differentiation during invasion: <Emphasis Type="Italic">Senecio inaequidens</Emphasis> (Asteraceae) along altitudinal gradients in Europe

The dynamics of plant population differentiation may be integral in predicting aspects of introduced species invasion. In the present study, we tested the hypothesis that European populations of Senecio inaequidens (Asteraceae), an invasive species with South African origins, differentiated during migration from two independent introduction sites into divergent altitudinal and climatic zones. We carried out 2 years of common garden experiments with eight populations sampled from Belgian and ten populations from French altitudinal transects. The Belgian transect followed a temperature and precipitation gradient. A temperature and summer drought gradient characterized the French transect. We evaluated differentiation and clinal variation in plants germinated from field-collected seed using the following traits: days to germination, days to flowering, height at maturity, final plant height and aboveground biomass. Results showed that S. inaequidens populations differentiated in growth traits during invasion. During the 1st year of sampling, the results indicated clinal variation for growth traits along both the Belgium and French altitudinal transects. Data from the 2nd year of study demonstrated that with increasing altitude, a reduction in three growth traits, including plant height at maturity, final plant height and aboveground biomass, was detected along the French transect, but no longer along the Belgian one. Phenological traits did not exhibit a clear clinal variation along altitudinal transects. The possible evolutionary causes for the observed differentiation are discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Soil moisture mediated interaction between <Emphasis Type="Italic">Polygonatum biflorum</Emphasis> and leaf spot disease

Fungal pathogens can regulate the abundance and distribution of natural plant populations by inhibiting the growth, survival, and reproduction of their hosts. The abiotic environment is a crucial component in host–pathogen interactions in natural plant populations as favorable conditions drive pathogen development, reproduction, and persistence. Foliar plant pathogens, such as fungal lesions referred to generically as “leaf spot disease,” are particularly responsive to increased moisture levels, but the manner in which the abiotic environment drives disease dynamics, and how these diseases regulate natural plant populations, is not fully understood. We investigate (1) the impact of ambient soil moisture and diffuse light on the prevalence of a leaf spot pathogen (Phyllosticta sp.) in a natural population of Polygonatum biflorum, an understory herb native to deciduous forest understories in the eastern US, and (2) the effects of the fungal pathogen on the survival, growth, and abundance of the plants. We tracked six P. biflorum populations and disease incidence, as well as soil moisture and diffuse light, between 2003 and 2005 in the understory deciduous forest of the southern Appalachian Mountains, North Carolina, USA. Results show that both the occurrence of P. biflorum and the prevalence of P. biflorum leaf spot disease are highest where soil moisture is intermediate and diffuse light is lowest. Disease occurrence depends upon plant presence, but it also adversely impacts plant survival, abundance, and growth. These results suggest that leaf spot disease is likely to impact population dynamics, which in turn vary as a function of environmental drivers.     

Host plant trichomes and the advantage of being big: progeny size variation of the pipevine swallowtail

1. Plants possess numerous traits that confer resistance against insect herbivores, and herbivores, in turn, can evolve traits to ameliorate the effectiveness of these traits. The pipevine swallowtail, Battus philenor, is an extreme specialist on plants in the genus Aristolochia. The only host plant available to the California population of B. philenor is A. californica. Aristolochia californica is distinct from most other B. philenor host plants in that it is pubescent. 2. The progeny of B. philenor are larger in California compared with populations examined in Texas. Size differences persist throughout larval development. 3. Regardless of maternal host plant, population differences in progeny size persist, and crosses between California (large progeny) and Texas (small progeny) B. philenor populations resulted in offspring producing intermediate sized progeny, indicating a heritable component to progeny size variation. 4. California neonate caterpillars more easily overcame the trichomes of A. californica compared with Texas neonates. When trichomes were removed from A. californica, time to feeding establishment was reduced for caterpillars from both populations. Texas caterpillars established feeding sites on A. californica with trichomes removed, in the same time required to establish feeding on their non‐pubescent host plant, A. erecta. 5. This study shows that plant trichomes might impose selection pressure on progeny size.     

Differentiation and adaptation in <Emphasis Type="Italic">Brassica nigra</Emphasis> populations: interactions with related herbivores

Local adaptation and population differentiation of plants are well documented, but studies on interactions with natural enemies are rare. In particular, evidence for plant adaptation to the local biotic environment, such as herbivores remains poor. We used the black mustard Brassica nigra, an annual species of river valley and coastal habitats to (1) analyse population differentiation in plant traits and herbivory in a common garden experiment, (2) examine home versus away differences in a reciprocal transplant experiment and (3) test whether plants are adapted to local herbivores or vice versa under standard greenhouse conditions. In the common garden experiment, we found significant differentiation in plant traits, leaf damage and herbivore number among seven populations of B. nigra from France and Germany (distance 15–1,000 km). Differences were particularly strong among coastal and river valley populations and did not necessarily increase with geographical distance. A herbivore removal treatment did not change population differentiation when compared with the control allowing natural colonisation. The reciprocal transplant experiment at a scale of 15–30 km did not reveal local plant adaptation, whilst one dominant herbivore species (Meligethes aeneus) occurred in significantly higher numbers on local plants. A greenhouse experiment combining three aphid (Brevicoryne brassicae) and plant populations of the same provenance indicated herbivore adaptation to their local plants rather than plant adaptation, but overall contrasts between local and non-local combinations were not significant. The results suggest that herbivores may counteract local plant adaptation to other environmental factors. Our study has important implications for plant translocations in ecological restoration projects.     

Seed predation,pathogen infection and life-history traits in Brassica rapa

Herbivory and disease can shape the evolution of plant populations, but their joint effects are rarely investigated. Families of plants of Brassica rapa (Brassicaceae) were grown from seeds collected in two naturalized populations in an experimental garden. We examined leaf infection by the fungus Alternaria, seed predation by a gall midge (Cecidomyiidae) and plant life-history traits. Plants from one population had heavier seeds, were more likely to flower, had less fungal infection, had more seed predation and were more fecund. Fungal infection score and seed predation rate increased with plant size, but large plants still had the greatest number of undamaged fruits. Spatial heterogeneity in the experimental garden was significant; seed predation rate and fecundity varied among blocks. An apparent tradeoff existed between susceptibility to disease and seed predation: plants with the highest fungal infection score had the lowest seed predation rate. Alternaria infection varied between populations, but the disease had no effect on fecundity. Seed predation did reduce fecundity. Damaged fruits had 31.4% fewer intact seeds. However, evidence for additive genetic variation in resistance to seed predation was weak. Therefore, neither disease nor seed predation was likely to be a strong agent of genetically based fecundity selection.     

Consequences of a mixed reproductive system in the hog peanut,Amphicarpaea bracteata, (Fabaceae)

Summary The forest annual, Amphicarpaea bracteata L. can reproduce via aerial chasmogamous, aerial cleistogamous, and subterranean cleistogamous flowers. Both plant size and light intensity influenced the utilization of the three modes of reproduction. chasmogamous and aerial cleistogamous flower number and the ratio of chasmogamous flowers to the total number of aerial flowers increased with plant size. The latter demonstrated a shift to xenogamy and outbreeding in larger plants. Light intensity indirectly influenced reproductive modes through its infuence on plant size. Seed set by both types of aerial flowers was low and unrelated to plant size. Subterranean seed number and the total dry weight of subterranean seeds per plant increased with size. The subterranean seeds of Amphicarpaea bracteata are thirty-four times larger than the aerial seeds (fresh weight). Under field conditions, subterranean seeds had greater germination after one year than acrial seeds. The plants arising from subterranean seeds were significantly larger and more fecund than those from aerial seeds. Seeds produced by aerial cleistogamous, hand selfpollinated chasmogamous, and naturally pollinated chasmogamous flowers had equivalent germination rates and produced plants of equal size and fecundity. This suggests that the outbred progeny from chasmogamous flowers have no advantage over the inbred progeny from aerial cleistogamous flowers.     

HYBRID BREAKDOWN IN DEVELOPMENTAL TIME IN THE COPEPOD TIGRIOPUS CALIFORNICUS

Laboratory crosses were carried out among three genetically differentiated Los Angeles populations (all located within approximately 15 km) and one San Diego population (approximately 150 km away) of the intertidal copepod Tigriopus californicus. Despite high levels of allozyme differentiation, all crosses produced viable F₁ progeny. Most F₁ progeny had shorter developmental times and reduced variance in developmental times compared to the parental populations. Only one pair of populations failed to produce viable F₂ progeny; when the central Los Angeles population (AB) was crossed to the San Diego (SD) population, most larvae died during the late naupliar stages. Developmental times in the F₂ generation of the other Los Angeles × San Diego crosses were typically 40% longer than developmental times of the parental populations. Among the Los Angeles populations, only one cross (and not its reciprocal) showed a similarly large increase in developmental time. Variance in F₂ developmental times was greater than the parental variance in 5 of 10 crosses. These results are discussed with regard to the evolution of coadapted gene complexes and population differentiation in T. californicus.     

Local adaptation across a fertility gradient is influenced by soil biota in the invasive grass, <Emphasis Type="Italic">Bromus inermis</Emphasis>

Biotic soil factors, such as fungi, bacteria and herbivores affect resource acquisition and fitness in plants, yet little is known of their role as agents of selection. Evolutionary responses to these selective agents could be an important mechanism that explains the success of invasive species. In this study, we tested whether populations of the invasive grass Bromus inermis are adapted to their home soil environment, and whether biotic factors influence the magnitude of this adaptation. We selected three populations growing at sites that differed in soil fertility and grew individuals from each population in each soil. To assess whether biotic factors influence the magnitude of adaptation, we also grew the same populations in sterilized field soil. To further examine the role of one element of the soil biota (fungi) in local adaptation, we measured colonization by arbuscular mycorrhizal (AM) and septate fungi, and tested whether the extent of colonization differed between local and foreign plants. In non-sterilized (living) soil, there was evidence of a home site advantage because local plants produced significantly more biomass than at least one of the two populations of foreign plants in all three soil origins. By contrast, there was no evidence of a home site advantage in sterilized soil because local plants never produced significantly more biomass than either population of foreign plants. Fungal colonization differed between local and foreign plants in the living soil and this variation corresponded with biomass differences. When local plants produced more biomass than foreign plants, they were also less intensively colonized by AM fungi. Colonization by septate fungi did not vary between local and foreign plants. Our results suggest that biotic soil factors are important causes of plant adaptation, and that selection for reduced interactions with mycorrhizae could be one mechanism through which adaptation to a novel environment occurs.     

PATHOGEN IMPACT ON SEXUAL VS. ASEXUAL REPRODUCTIVE SUCCESS IN ARISAEMA TRIPHYLLUM

Populations of Arisaema triphyllum commonly harbor a systemic fungal pathogen, Uromyces ari-triphylli, which reduces leaf area and leaf longevity. Infected plants produced 42% fewer asexual progeny (cormlets). Furthermore, all asexual progeny from infected parents were themselves infected with Uromyces. Mean seed production of infected plants was only 21% of that among healthy plants. A hand pollination experiment demonstrated that 1) infected male plants can produce viable pollen, 2) the low seed production of naturally pollinated infected females was not due to inadequate pollination, and 3) seed production among healthy female plants was strongly pollinator limited. Seeds from infected and healthy parents had similar viability, and all seeds germinating from both types of parents were uninfected. The contrast in disease transmission to sexual vs. asexual progeny suggests that pathogen attack may be one selective factor favoring sexual reproduction in A. triphyllum populations.     

Evidence for genomic changes in transgenic rice (Oryza sativa L.) recovered from protoplasts

The occurrence of genomic modifications in transgenic rice plants recovered from protoplasts and their transmission to the self-pollination progeny has been verfied with the random amplified polymorphic DNA (RAPD) approach. The plant was the Indica-type rice (Oryza sativa L.) cultivar Chinsurah Boro II. The analysed material was: (1) microspore-derived embryogenic rice cells grown in suspension culture, (2) transgenic plants recovered from protoplasts produced from the cultured cells and (3) the self-pollination progeny (two successive generations) of the transgenic plants. DNA purified from samples of these materials was PCR-amplified with different random oligonucleotide primers and the amplification products were analysed by agarose gel electrophoresis. Band polymorphism was scored and used in band-sharing analyses to produce a similarity matrix. Relationships among the analysed genomes were expressed in a dendrogram.The extensive DNA changes evidenced in cultured cells demonstrate the occurrence of somaclonal variation in the material used to produce protoplasts for gene transfer. Quantitatively reduced DNA changes were also found in the resulting transgenic plants and i their self-pollination progenies.While confirming the stability of the foreign gene in transgenic plants, this work gives molecular evidence for the occurrence of stable genomic changes in transgenic plants and points toin vitro cell culture as the causative agent. RAPDs are shown to be a convenient tool to detect and estimate the phenomenon at the molecular level. The methodology is also proposed as a fast tool to select those transgenic individuals that retain the most balanced genomic structure and to control the result of back-crosses planned to restore the original genome.     

Potential for higher invasiveness of the alien Oxalis pes-caprae on islands than on the mainland

The higher vulnerability of islands to invasions compared to mainland areas has been partially attributed to a simplification of island communities, with lower levels of natural enemies and competitors on islands conferring vacant niches for invaders to establish and proliferate. However, differences in invader life-history traits between populations have received less attention. We conducted a broad geographical analysis (i.e. 1050 km wide transect) of plant traits comparing insular and mainland populations to test the hypothesis that alien plants from insular populations have the potential for higher invasiveness than their alien mainland counterparts. For this purpose plants of the annual geophyte Oxalis pes-caprae were grown from bulbs collected in the Balearic islands and the Spanish mainland under common greenhouse conditions. There were no significant differences in bulb emergence and plant survival between descendants from insular and mainland populations. However, Oxalis descendants from insular populations produced 20% more bulbs without reducing allocation to bulb size, above-ground biomass or flowering than descendants from mainland populations. Based on the lack of sexual reproduction in Oxalis and the dependence of invasion on bulb production, our study suggests that the higher occurrence of Oxalis in the Balearic islands than in the Spanish mainland can partially be explained by genetically based higher propagation potential of insular populations compared to mainland populations.     

The roles of geography and founder effects in promoting host‐associated differentiation in the generalist bogus yucca moth Prodoxus decipiens

There is ample evidence that host shifts in plant‐feeding insects have been instrumental in generating the enormous diversity of insects. Changes in host use can cause host‐associated differentiation (HAD) among populations that may lead to reproductive isolation and eventual speciation. The importance of geography in facilitating this process remains controversial. We examined the geographic context of HAD in the wide‐ranging generalist yucca moth Prodoxus decipiens. Previous work demonstrated HAD among sympatric moth populations feeding on two different Yucca species occurring on the barrier islands of North Carolina, USA. We assessed the genetic structure of P. decipiens across its entire geographic and host range to determine whether HAD is widespread in this generalist herbivore. Population genetic analyses of microsatellite and mtDNA sequence data across the entire range showed genetic structuring with respect to host use and geography. In particular, genetic differentiation was relatively strong between mainland populations and those on the barrier islands of North Carolina. Finer scale analyses, however, among sympatric populations using different host plant species only showed significant clustering based on host use for populations on the barrier islands. Mainland populations did not form population clusters based on host plant use. Reduced genetic diversity in the barrier island populations, especially on the derived host, suggests that founder effects may have been instrumental in facilitating HAD. In general, results suggest that the interplay of local adaptation, geography and demography can determine the tempo of HAD. We argue that future studies should include comprehensive surveys across a wide range of environmental and geographic conditions to elucidate the contribution of various processes to HAD.