Barbara E. Giles

KIN-STRUCTURED COLONIZATION AND SMALL-SCALE GENETIC DIFFERENTIATION IN SILENE DIOICA

We investigated the genetic structure of a single island population of the dioecious plant Silene dioica in the Skeppsvik Archipelago, Umeå, Sweden. The population is less than 10 years old and consists of approximately 700 individuals growing within an area of about 200 m2. Despite the small scale of the study, levels of genetic differentiation among contiguous patches are greater than or comparable to what is observed over larger scales in the archipelago. The results suggest that the small‐scale structuring occurs during population expansion, soon after island colonization, and that the observed patterns of genetic differentiation can be attributed to the population being substructured into family groups. This family structure results from kin‐structured dispersal processes (colonization and migration) as the population expands over the island. As plant densities increase over time, either spatial fusion or temporal fusion of patches reduce the among patch variation. These processes, however, do not completely eradicate the genetic differentiation established by the kin‐structured dispersal processes. We discuss some implications of kin structuring for evolution through either kin or interdemic selection.

A COMPARISON BETWEEN QUANTITATIVE AND BIOCHEMICAL VARIATION IN THE WILD BARLEY HORDEUM MURINUM

A small number of studies have been reported in which the levels of allozyme variation differ appreciably from the levels of variation in morphometric characters (Maxson and Wilson, 1974; Turner, 1974; Avise et al., 1975; King and Wilson, 1976; Moran and Marshall, 1978; Turner et al., 1979; Hauptli and Jain, 1980; Jain et al., 1980; Kahler et al., 1980). Evolutionary inferences drawn from biochemical data do not reflect those drawn from morphological data in these cases. As both sets of characters merely represent two pictures of the same organism, it is disturbing that they are discordant. In the present study, the levels of variation revealed by starch gel electrophoresis and biometrical analyses were compared in several natural populations of Hordeum murinum. Again, discordance between the two suites of characters was observed. The wild barley, H. murinum, is a colonizing annual grass and weed, allotetraploid and cleistogamous throughout its range (Rajhathy and Morrison, 1962; Davison, 1970, 1971, 1977). Although limited to disturbed habitats, it has attained nearly world-wide distribution. From its supposed center of origin in the Mediterranean and Western Asia, successful colonization of Europe, Britain, Africa, New Zealand, and both Americas has occurred (Davison, 1970, 1971, 1977; Cocks et al., 1976).

Restricted gene flow and subpopulation differentiation in Silene dioica

The size of breeding units and the hierarchical population structure of the dioecious perennial herb Silene dioica were investigated on four closely situated island populations in the Skeppsvik Archipelago in northern Sweden. F-statistics analyses of nine polymorphic allozyme loci revealed that plants on single islands are divided into many small breeding units, between 0.2 m2 and 6 m2. Hierarchical analyses showed that levels of differentiation among subpopulations within islands (FPL=0.080) were about twice as high as among islands (FLT=0.048). These results are discussed in the light of what is known about pollen and seed movement in the archipelago.

Past and current gene flow in the selfing, wind-dispersed species Mycelis muralis in western Europe

The distribution of genetic diversity in Mycelis muralis, or wall lettuce, was investigated at a European scale using 12 microsatellite markers to infer historical and contemporary forces from genetic patterns. Mycelis muralis has the potential for long‐distance seed dispersal by wind, is mainly self‐pollinated, and has patchily distributed populations, some of which may show metapopulation dynamics. A total of 359 individuals were sampled from 17 populations located in three regions, designated southern Europe (Spain and France), the Netherlands, and Sweden. At this within‐region scale, contemporary evolutionary forces (selfing and metapopulation dynamics) are responsible for high differentiation between populations (0.34 < FST < 0.60) but, contrary to expectation, levels of within‐population diversity, estimated by Neis unbiased expected heterozygosity (HE) (0.24 < HE < 0.68) or analyses of molecular variance (50% of the variation found within‐populations), were not low. We suggest that the latter results, which are unusual in selfing species, arise from efficient seed dispersal that counteracts population turnover and thus maintains genetic diversity within populations. At the European scale, northern regions showed lower allelic richness (A = 2.38) than populations from southern Europe (A = 3.34). In light of postglacial colonization hypotheses, these results suggest that rare alleles may have been lost during recolonization northwards. Our results further suggest that mutation has contributed to genetic differentiation between southern and northern Europe, and that Sweden may have been colonized by dispersers originating from at least two different refugia.

Variation in Breeding System Traits within and among Populations of Microbotryum violaceum on Silene dioica

Breeding systems exert profound effects on the amount and distribution of genetic diversity within and among populations. Knowledge of breeding systems is also important for understanding dynamics between coevolving organisms, e.g., pathogen‐host interactions. Here we study the breeding system of the obligate anther smut Microbotryum violaceum on Silene dioica. Microbotryum violaceum is capable of both inbreeding and outcrossing, but several recent studies on other host races have indicated that automixis via intrapromycelial mating is the predominant breeding system. Compared with conjugations between cells from different meioses, automixis results in slower loss of heterozygosity and faster production of infectious hypha. However, high rates of intrapromycelial matings have been suggested to invoke a fitness cost due to production of fewer infectious dikaryons. Working with single strains under standardized laboratory conditions, we studied traits that could influence the distribution of genetic variability and pathogen fitness. We found that intrapromycelial mating is the dominant conjugation form for M. violaceum var. dioica but that the breeding system varies, partly because of genetic differences, both within and among populations. Further, we did not find the predicted fitness reduction for intrapromycelial matings, suggesting that intrapromycelial mating is a highly favorable breeding system for M. violaceum.

Enzymatic responses of Drosophila melanogaster to long- and short-term exposures to ethanol

Abstract The effects of environmental ethanol on larva-to-pupa survival and on the activities of four enzymes were investigated in three Drosophila melanogaster strains. The strains had different allelic combinations at the Odh and Aldox loci on their third chromosomes, but they all carried the AdhS-GpdhF allelic combination on the second chromosome. Replicates of each of the strains were exposed to three different ethanol treatments: (i) no ethanol in the medium (control); (ii) 5% ethanol for a single generation (short-term exposure); (iii) 5% ethanol for 20 generations (long-term exposure). In all experiments, the activities of four enzymes (ADH, ODH, GPDH and AOX) were measured in larvae, pupae and adults. The results showed that (i) the larval and adult metabolic responses to environmental ethanol were different; (ii) enzyme activity changes under short-term exposure differed from those measured under long-term exposure; (iii) the activities of the allozymes common to all strains (ADH-S and GPDH-F), differed depending on the genetic background. Changes in larva-to-pupa survival were seen when the larvae of control and exposed lines of the three strains were confronted with various concentrations of ethanol. In all three strains, the exposed lines had significantly higher initial survival rate and ethanol tolerance than the control lines. Strain-specific differences were observed in the ethanol tolerance of both types of line.

Differences in environmental temperature, ethanol and sucrose associated with enzyme activity and weight changes in Drosophila melanogaster

Activity changes of three enzymes (ADH, ODH and AOX) of Drosophila melanogaster were followed under different environmental conditions. The influences of ethanol, starvation (no carbohydrates in the medium) and ethanol stress during starvation were studied at both 18 and 26 degrees C. Two strains that were monomorphic for different alleles at the Odh and Aldox loci but otherwise identical were used. The investigated environmental conditions affected ADH induction by exogenous ethanol differently in the two strains. The different allozymes of ODH and AOX also responded differently to the treatments. We observed that the sucrose content of the medium on which ethanol exposure took place and the temperature strongly affected the responses within any single strain. Correlations were estimated among the three enzymes in the larval and adult stages of each strain separately. At both temperatures, differences between strains were observed in the patterns of associations of the response variables, in the larval, but not in the adult stages.

Patterns of disease and host resistance in spatially structured systems

We use data from species of the anther-smut fungi and the host plants Lychnis alpina and Silene dioica to show that spatial structuring at different scales can influence patterns of disease and host resistance. Patterns of disease and host resistance were surveyed in an archipelago subject to land-uplift where populations of S. dioica constitute an age-structured metapopulation, and in three contrasting areas within the mainland range of L. alpina, where population distributions range from continuous, through patchy but spatially connected to highly isolated demes. In S. dioica, disease levels depend on the age, size and density of local patches and populations. Disease is most predictably found in larger dense host patches and populations of intermediate age, and more frequently goes extinct in small old populations. The rate of local disease spread is affected by the level of host resistance; S. dioica populations showing an increase in disease over time are more susceptible than populations where the disease has remained at low levels. Among-population variation in resistance is driven by founding events and populations remain differentiated due to limited gene flow between islands. As observed in the L. alpina system, when populations are more connected, a greater fraction of populations have disease present. Results from a simulation model argue that, while increased dispersal in connected systems can increase disease spread, it may also favour selection of host resistance which ultimately reduces disease levels within populations. This could explain the observed lower disease prevalence in L. alpina in regions where populations are more continuous.

Ecotype Differentiation in the Face of Gene Flow within the Diving Beetle Agabus bipustulatus (Linnaeus, 1767) in Northern Scandinavia

The repeated occurrence of habitat-specific polyphyletic evolved ecotypes throughout the ranges of widely distributed species implies that multiple, independent and parallel selection events have taken place. Ecological transitions across altitudinal gradients over short geographical distances are often associated with variation in habitat-related fitness, these patterns suggest the action of strong selective forces. Genetic markers will therefore contribute differently to differences between ecotypes in local hybrid zones. Here we have studied the adaptive divergence between ecotypes of the water beetle Agabus bipustulatus along several parallel altitudinal gradients in northern Scandinavia. This water beetle is well known for its remarkable morphological variation associated with mountain regions throughout the western Palaearctic. Two morphological ecotypes are recognised: a montane type with reduced flight muscles and a lowland type with fully developed muscles. Using a multilocus survey of allozyme variation and a morphological analysis with landmark-based morphometrics, across thirty-three populations and seven altitudinal gradients, we studied the local adaptive process of gene flow and selection in detail. Populations were sampled at three different elevations: below, at and above the tree line. The results indicate that the levels of divergence observed between ecotypes in morphology and allele frequencies at α-Glycerophosphate dehydrogenase relative to those shown by neutral molecular markers reflects local diversifying selection in situ. Four main lines of evidence are shown here: (1) A repeated morphological pattern of differentiation is observed across all altitudinal transects, with high reclassification probabilities. (2) Allele and genotype frequencies at the α-Gpdh locus are strongly correlated with altitude, in sharp contrast to the presumable neutral markers. (3) Genetic differentiation is two to three times higher among populations across the tree line than among populations at or below. (4) Genetic differentiation between ecotypes within independent mountain areas is reflected by different sets of allozymes.