THE GEOGRAPHICAL MOSAIC OF COEVOLUTION IN A PLANT–POLLINATOR MUTUALISM

Although coevolution is widely accepted as a concept, its importance as a driving factor in biological diversification is still being debated. Because coevolution operates mainly at the population level, reciprocal coadaptations should result in trait covariation among populations of strongly interacting species. A long-tongued fly ( Prosoeca ganglbaueri ) and its primary floral food plant ( Zaluzianskya microsiphon ) were studied across both of their geographical ranges. The dimensions of the fly's proboscis and the flower's corolla tube length varied significantly among sites and were strongly correlated with each other. In addition, the match between tube length of flowers and tongue length of flies was found to affect plant fitness. The relationship between flower tube length and fly proboscis length remained significant in models that included various alternative environmental (altitude, longitude, latitude) and allometric (fly body size, flower diameter) predictor variables. We conclude that coevolution is a compelling explanation for the geographical covariation in flower depth and fly proboscis length.     

Natural selection along an environmental gradient: a classic cline in mouse pigmentation

We revisited a classic study of morphological variation in the oldfield mouse ( Peromyscus polionotus ) to estimate the strength of selection acting on pigmentation patterns and to identify the underlying genes. We measured 215 specimens collected by Francis Sumner in the 1920s from eight populations across a 155-km, environmentally variable transect from the white sands of Florida's Gulf coast to the dark, loamy soil of southeastern Alabama. Like Sumner, we found significant variation among populations: mice inhabiting coastal sand dunes had larger feet, longer tails, and lighter pigmentation than inland populations. Most striking, all seven pigmentation traits examined showed a sharp decrease in reflectance about 55 km from the coast, with most of the phenotypic change occurring over less than 10 km. The largest change in soil reflectance occurred just south of this break in pigmentation. Geographic analysis of microsatellite markers shows little interpopulation differentiation, so the abrupt change in pigmentation is not associated with recent secondary contact or reduced gene flow between adjacent populations. Using these genetic data, we estimated that the strength of selection needed to maintain the observed distribution of pigment traits ranged from 0.0004 to 21%, depending on the trait and model used. We also examined changes in allele frequency of SNPs in two pigmentation genes, Mc1r and Agouti , and show that mutations in the cis -regulatory region of Agouti may contribute to this cline in pigmentation. The concordance between environmental variation and pigmentation in the face of high levels of interpopulation gene flow strongly implies that natural selection is maintaining a steep cline in pigmentation and the genes underlying it.     

Environmental heterogeneity enhances clonal interference

Clonal interference (CI) is a phenomenon that may be important in several asexual microbes. It occurs when population sizes are large and mutation rates to new beneficial alleles are of significant magnitude. Here we explore the role of gene flow and spatial heterogeneity in selection strength in the adaptation of asexuals. We consider a subdivided population of individuals that are adapting, through new beneficial mutations, and that migrate between different patches. The fitness effect of each mutation depends on the patch and all mutations considered are assumed to be unconditionally beneficial. We find that spatial variation in selection pressure affects the rate of adaptive evolution and its qualitative effects depend on the level of gene flow. In particular, we find that both low migration and high levels of heterogeneity lead to enhanced CI. In contrast, for high levels of migration the rate of fixation of adaptive mutations is higher when environmental heterogeneity is present. In addition, we observe that the level of fitness variation is higher and simultaneous fixation of multiple mutations tends to occur in the regime of low migration rates and high heterogeneity.     

Possible role of pectin-containing mucilage and dew in repairing embryo DNA of seeds adapted to desert conditions

BACKGROUND AND AIMS: Repair of damage to DNA of seed embryos sustained during long periods of quiescence under dry desert conditions is important for subsequent germination. The possibility that repair of embryo DNA can be facilitated by small amounts of water derived from dew temporarily captured at night by pectinaceous surface pellicles was tested. These pellicles are secreted during early seed development and form mucilage when hydrated. METHODS: Seeds of Artemisia sphaerocephala and Artemisia ordosica were collected from a sandy desert. Their embryos were damaged by gamma radiation to induce a standard level of DNA damage. The treated seeds were then exposed to nocturnal dew deposition on the surface of soil in the Negev desert highlands. The pellicles were removed from some seeds and left intact on others to test the ability of mucilage to support repair of the damaged DNA when night-time humidity and temperature favoured dew formation. Repair was assessed from fragmentation patterns of extracted DNA on agarose gels. KEY RESULTS: For A. sphaerocephala, which has thick seed pellicles, DNA repair occurred in seeds with intact pellicles after 50 min of cumulative night dew formation, but not in seeds from which the pellicles had been removed. For A. ordosica, which has thin seed pellicles, DNA repair took at least 510 min of cumulative night dewing to achieve partial recovery of DNA integrity. The mucilage has the ability to rehydrate after daytime dehydration. CONCLUSIONS: The ability of seeds to develop a mucilaginous layer when wetted by night-time dew, and to repair their DNA under these conditions, appear to be mechanisms that help maintain seed viability under harsh desert conditions.     

The evolution of multicellularity in animals as a shift in biological autonomy

The hypothesis is advanced that major evolutionary innovations are characterized by an increase of organismal autonomy in the sense of an emancipation from the environment. After a brief overview of the literature on this concept, increasing autonomy is defined as the evolutionary shift in the individual system-environment relationship, so that the direct influences of the environment are gradually reduced and a stabilization of self-referential, intrinsic functions within the system is generated. This is described as relative autonomy because numerous interconnections with the environment and dependencies upon it are retained. Elements of an increasing autonomy are spatial separations, an increase in homeostatic functions, internalizations and an increase in physiological and behavioral flexibility. These elements are described by taking the transition from single cells to metazoans as a case study. The principle of increasing autonomy is of central relevance for understanding this transition. The hypothesis does not contradict the principle of adaptation, but rather contributes to a further understanding of its elements as it supplies aspects for a reconsideration of the relationship between the outside and the inside, between organism and environment.     

Rapid evolution and the convergence of ecological and evolutionary time

Recent studies have documented rates of evolution of ecologically important phenotypes sufficiently fast that they have the potential to impact the outcome of ecological interactions while they are underway. Observations of this type go against accepted wisdom that ecological and evolutionary dynamics occur at very different time scales. While some authors have evaluated the rapidity of a measured evolutionary rate by comparing it to the overall distribution of measured evolutionary rates, we believe that ecologists are mainly interested in rapid evolution because of its potential to impinge on ecological processes. We therefore propose that rapid evolution be defined as a genetic change occurring rapidly enough to have a measurable impact on simultaneous ecological change. Using this definition we propose a framework for decomposing rates of ecological change into components driven by simultaneous evolutionary change and by change in a non‐evolutionary factor (e.g. density dependent population dynamics, abiotic environmental change). Evolution is judged to be rapid in this ecological context if its contribution to ecological change is large relative to the contribution of other factors. We provide a worked example of this approach based on a theoretical predator–prey interaction [ Abrams, P. & Matsuda, H. (1997) . Evolution, 51, 1740], and find that in this system the impact of prey evolution on predator per capita growth rate is 63% that of internal ecological dynamics. We then propose analytical methods for measuring these contributions in field situations, and apply them to two long‐term data sets for which suitable ecological and evolutionary data exist. For both data sets relatively high rates of evolutionary change have been found when measured as character change in standard deviations per generation (haldanes). For Darwin's finches evolving in response to fluctuating rainfall [ Grant, P.R. & Grant, B.R. (2002) . Science, 296, 707], we estimate that evolutionary change has been more rapid than ecological change by a factor of 2.2. For a population of freshwater copepods whose life history evolves in response to fluctuating fish predation [ Hairston, N.G. Jr & Dillon, T.A. (1990) . Evolution, 44, 1796], we find that evolutionary change has been about one quarter the rate of ecological change – less than in the finch example, but nevertheless substantial. These analyses support the view that in order to understand temporal dynamics in ecological processes it is critical to consider the extent to which the attributes of the system under investigation are simultaneously changing as a result of rapid evolution.     

Development of tolerance against toxic cyanobacteria in Daphnia

We tested whether previous exposure to a toxic strain of cyanobacteria (Microcystis) affects survival, growth, and reproduction of a common herbivore, Daphnia magna. Samples from three natural populations of D. magna were each divided into two parts; one part was fed a mixture of toxic Microcystis and the non-toxic green alga Scenedesmus whereas the other part was fed only Scenedesmus. After four weeks, we compared the ability of these two populations to withstand the toxic Microcystis by assessing survivorship, growth, and reproduction. We found that the ability of D. magna to cope successfully with toxic Microcystis is improved if the animals have experienced previous exposure to toxic Microcystis. This suggests that the toxin may less affect the D. magna populations that are repeatedly exposed to toxic cyanobacteria in their natural habitat than populations lacking prior exposure. Since the ability to tolerate toxins is manifested in both improved survival and larger size of the animals, it may have considerable impact on zooplankton community composition in fresh-waters.     

Effect of chlorophenols on the membrane lipids of bacterial cells

Chlorophenols, widespread soil and water contaminants and often degradation products of some pesticides, are a potential stress factor for survival of environmental bacteria. The effect of pentachlorophenol (PCP) and 2,4-chlorophenol (2,4-CP) on the growth, amount of lipid, and fatty acid composition in the membrane lipids was examined in a strain of the bacterium Kocuria varians, able to degrade chlorophenols. The index of fatty acid unsaturation in two main membrane lipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) decreased in the presence of chlorophenols. Transformation of stearic acid into oleic acid was significantly increased by PCP addition only in PE, but conversion of oleic acid into linoleic acid was blocked by PCP and 2,4-CP in both PC and PE. This observation may indicate that while Δ⁹ desaturase was sensitive mainly to 2,4-CP, activity of Δ¹² desaturase was inhibited by both PCP and 2,4-CP.     

Latitudinal differentiation in the effects of the toxic dinoflagellate Alexandrium spp. on the feeding and reproduction of populations of the copepod Acartia hudsonica

Blooms of the dinoflagellate Alexandrium spp. increase in their frequency, toxicity and historical presence with increasing latitude from New Jersey (USA) to the Gaspé peninsula (Canada). Biogeographic variation in these blooms results in differential exposure of geographically separate copepod populations to toxic Alexandrium. We hypothesize that the ability of copepods to feed and reproduce on toxic Alexandrium should be higher in copepods from regions that are frequently exposed to toxic Alexandrium blooms. We tested this hypothesis with factorial common environment experiments in which female adults of the copepod Acartia hudsonica from five separate populations ranging from New Jersey to New Brunswick were fed toxic and non-toxic strains of Alexandrium, and the non-toxic flagellate Tetraselmis sp. Consistent with the hypothesis, when fed toxic Alexandrium we observed significantly higher ingestion and egg production rates in the copepods historically exposed to toxic Alexandrium blooms relative to copepods from regions in which Alexandrium is rare or absent. Such differences among copepod populations were not observed when copepods were fed non-toxic Alexandrium or Tetraselmis sp. These results were also supported by assays in which copepods from populations both historically exposed and naïve to toxic Alexandrium blooms were fed mixtures of toxic Alexandrium and Tetraselmis sp. Two-week long experiments demonstrated that when copepods from populations naïve to toxic Alexandrium were fed a toxic strain of Alexandrium they failed to acclimate, such that their ingestion rates remained low throughout the entire two-week period. The differences observed among populations suggest that local adaptation of populations of A. hudsonica from Massachusetts (USA) to New Brunswick (Canada) has occurred, such that some populations are resistant to toxic Alexandrium.