COEVOLUTIONARY CLINES ACROSS SELECTION MOSAICS

Infectious Disease Dynamics in Heterogeneous Landscapes

Infectious diseases dynamics are affected by both spatial and temporal heterogeneity in their environments. Our ability to quantify and predict how this heterogeneity impacts risks of infection and disease emergence is the key to successful disease prevention efforts. Here, we review the literature on infectious diseases from human, agricultural, and wildlife ecosystems to describe the rapid ecological and evolutionary responses in pathogens to environmental heterogeneity, with expected impacts on their epidemiology. To date, the underlying network structures through which disease transmission proceeds have been notoriously difficult to quantify because of this variation. We show that with recent advances in statistical methods and genomic approaches, it is now more feasible than ever to trace disease transmission networks, the molecular underpinning of infection, and the environmental variation relevant to disease dynamics. We end by identifying major new opportunities and challenges in understanding disease dynamics in an ever-changing world.

Mutualists and antagonists drive among-population variation in selection and evolution of floral display in a perennial herb

Spatial variation in the direction of selection drives the evolution of adaptive differentiation. However, few experimental studies have examined the relative importance of different environmental factors for variation in selection and evolutionary trajectories in natural populations. Here, we combine 8 y of observational data and field experiments to assess the relative importance of mutualistic and antagonistic interactions for spatial variation in selection and short-term evolution of a genetically based floral display dimorphism in the short-lived perennial herb Primula farinosa. Natural populations of this species include two floral morphs: long-scaped plants that present their flowers well above the ground and short-scaped plants with flowers positioned close to the ground. The direction and magnitude of selection on scape morph varied among populations, and so did the frequency of the short morph (median 19%, range 0-100%; n = 69 populations). A field experiment replicated at four sites demonstrated that variation in the strength of interactions with grazers and pollinators were responsible for among-population differences in relative fitness of the two morphs. Selection exerted by grazers favored the short-scaped morph, whereas pollinator-mediated selection favored the long-scaped morph. Moreover, variation in selection among natural populations was associated with differences in morph frequency change, and the experimental removal of grazers at nine sites significantly reduced the frequency of the short-scaped morph over 8 y. The results demonstrate that spatial variation in intensity of grazing and pollination produces a selection mosaic, and that changes in biotic interactions can trigger rapid genetic changes in natural plant populations.

Geographic structure in a widespread plant?mycorrhizal interaction: pines and false truffles

Mutualistic interactions are likely to exhibit a strong geographic mosaic in their coevolutionary dynamics, but the structure of geographic variation in these interactions is much more poorly characterized than in host-parasite interactions. We used a cross-inoculation experiment to characterize the scales and patterns at which geographic structure has evolved in an interaction between three pine species and one ectomycorrhizal fungus species along the west coast of North America. We found substantial and contrasting patterns of geographic interaction structure for the plants and fungi. The fungi exhibited a clinal pattern of local adaptation to their host plants across the geographic range of three coastal pines. In contrast, plant growth parameters were unaffected by fungal variation, but varied among plant populations and species. Both plant and fungal performance measures varied strongly with latitude. This set of results indicates that in such widespread species interactions, interacting species may evolve asymmetrically in a geographic mosaic because of differing evolutionary responses to clinally varying biotic and abiotic factors.

EVOLUTION OF MIXED STRATEGIES OF PLANT DEFENSE ALLOCATION AGAINST NATURAL ENEMIES

In this study we present a simple optimization model for the evolution of defensive strategies (tolerance and resistance) of plants against their natural enemies. The model specifically evaluates the consequences of introducing variable costs and benefits of tolerance and resistance and nonlinear cost-and-benefit functions for tolerance and resistance. Incorporating these assumptions, the present model of plant defense predicts different evolutionary scenarios, not expected by previous work. Basically, the presence of an adaptive peak corresponding to intermediate levels of allocation to tolerance and resistance can arise when the shape parameter of the cost function is higher than the corresponding of the benefit function. The presence of two alternatives peaks of maximum tolerance and maximum resistance occurs only when benefits of tolerance and resistance interact less than additive. Finally, the presence of one peak of maximum resistance or maximum tolerance depends on the relative values of the magnitude of costs for tolerance and resistance. An important outcome of our model is that under a plausible set of conditions, variable costs of tolerance and resistance can represent an important aspect involved in the maintenance of intermediate levels of tolerance and resistance, and in favoring adaptive divergence in plant defensive strategies among populations. The model offers a framework for future theoretical and empirical work toward understanding spatial variation in levels of allocation to different defensive strategies.

Phenotype matching in wild parsnip and parsnip webworms: causes and consequences

According to the geographic mosaic theory of coevolution, selection intensity in interactions varies across a landscape, forming a selection mosaic; interaction traits match at coevolutionary hotspots where selection is reciprocal and mismatch at coldspots where reciprocity is not a factor. Chemical traits play an important role in the interaction between wild parsnip (Pastinaca sativa) and the parsnip webworm (Depressaria pastinacella). Furanocoumarins, produced as plant defenses, are detoxified by the webworms by cytochrome P450 monooxygenases; significant additive genetic variation exists for both furanocoumarin production in the plant and detoxification in the insect, making these traits available for selection. To test the hypothesis that differences in selection intensity affect the distribution of coevolutionary hotspots and coldspots in this interaction, we examined 20 populations of webworms and wild parsnips in Illinois and Wisconsin that varied in size, extent of infestation, proximity to woods (and potential vertebrate predators), and proximity to a chemically distinct alternate host plant, Heracleum lanatum (cow parsnip). Twelve of 20 populations displayed phenotype matching between plant defense and insect detoxification profiles. Of the eight mismatched populations, a logistic regression model related matching probability to two predictors: the presence of the alternate host and average content of xanthotoxin (one of the five furanocoumarins produced by P. sativa). The odds of mismatching were significantly increased by the presence of the alternate host (odds ratio = 15.4) and by increased xanthotoxin content (odds ratio = 6.053). Parsnips growing near cow parsnip displayed chemical phenotypes that were chemically intermediate between cow parsnip and parsnips growing in isolation. Rapid phenotype matching in this system is likely due in part to differential mortality every season; larvae transferred to a plant 30 m or more from the plant on which they developed tended to experience increased mortality over larvae transferred to another umbel on the same plant on which they had developed, and plant populations that mismatched in 2001 displayed a change in chemical phenotype distribution from the previous year. Trait mixing through gene flow is also a likely factor in determining mismatch frequency. Populations from which webworms were eradicated the previous year were all recolonized; in three of seven of these populations, infestation rates exceeded 90%. Our findings, consistent with the geographic mosaic theory, suggest that the presence of a chemically distinct alternate host plant can affect selection intensity in such a way as to reduce the likelihood of reciprocity in the coevolutionary interaction between wild parsnip and the parsnip webworm.

Gene flow and the coevolution of parasite range

The geographic range of many parasites is restricted relative to that of their hosts. We study possible evolutionary mechanisms for this observation using a simple model that couples coevolution and demography. The model assumes that the environment consists of two habitats connected by movement and that coevolution is governed by quantitative traits. Our results demonstrate that host gene flow is an important determinant of parasite geographic range. Fluctuations in the rate of host gene flow cause shifts in parasite population densities and associated range expansions or contractions. In extreme cases, changing the rate of host gene flow can lead to global extinction of the parasite. Through a process we term demographic compensation, these shifts in parasite density may occur with little or no change in parasite adaptation to the host. As a consequence, reciprocal adaptation between host and parasite can become uncoupled from the rate of host gene flow.

The evolutionary response of predators to dangerous prey: hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts

The "geographic mosaic" approach to understanding coevolution is predicated on the existence of variable selection across the landscape of an interaction between species. A range of ecological factors, from differences in resource availability to differences in community composition, can generate such a mosaic of selection among populations, and thereby differences in the strength of coevolution. The result is a mixture of hotspots, where reciprocal selection is strong, and coldspots, where reciprocal selection is weak or absent, throughout the ranges of species. Population subdivision further provides the opportunity for nonadaptive forces, including gene flow, drift, and metapopulation dynamics, to influence the coevolutionary interaction between species. Some predicted results of this geographic mosaic of coevolution include maladapted or mismatched phenotypes, maintenance of high levels of polymorphism, and prevention of stable equilibrium trait combinations. To evaluate the potential for the geographic mosaic to influence predator-prey coevolution, we investigated the geographic pattern of genetically determined TTX resistance in the garter snake Thamnophis sirtalis over much of the range of its ecological interaction with toxic newts of genus Taricha. We assayed TTX resistance in over 2900 garter snakes representing 333 families from 40 populations throughout western North America. Our results provide dramatic evidence that geographic structure is an important component in coevolutionary interactions between predators and prey. Resistance levels vary substantially (over three orders of magnitude) among populations and over short distances. The spatial array of variation is consistent with two areas of intense evolutionary response by predators ("hotspots") surrounded by clines of decreasing resistance. Some general predictions of the geographic mosaic process are supported, including clinal variation in phenotypes, polymorphism in some populations, and divergent outcomes of the interaction between predator and prey. Conversely, our data provide little support for one of the major predictions, mismatched values of interacting traits. Two lines of evidence suggest selection is paramount in determining population variation in resistance. First, phylogenetic information indicates that two hotspots of TTX resistance have evolved independently. Second, in the one region that TTX levels in prey have been quantified, resistance and toxicity levels match almost perfectly over a wide phenotypic and geographic range. However, these results do not preclude the role the nonadaptive forces in generating the overall geographic mosaic of TTX resistance. Much work remains to fill in the geographic pattern of variation among prey populations and, just as importantly, to explore the variation in the ecology of the interaction that occurs within populations.