Within-species variation among populations of the Carex flava complex as a function of habitat conditions

Background: Variable habitat conditions contribute to morphological variability that plays a substantial part in evolution of plants. Understanding the extent of phenotypic plasticity has important implication for assigning individuals to taxa.

Aims: I analysed morphological variability among populations within species of the Carex flava group and tested to what extent morphological variability was associated with habitat conditions.

Methods: A total of 571 specimens from 20 populations of four species from the C. flava complex were sampled in Poland and tested by Discriminant Function Analysis (DFA). The relationship between morphological characters and habitat factors was explored by means of the redundancy analysis (RDA).

Results: Variability of the generative and vegetative traits was fairly similar in Carex lepidocarpa, C. flava and Carex demissa, while it was somewhat higher in Carex viridula. The morphological traits were mostly related to soil organic matter content, calcium and carbonate, as well as to habitat fertility (C:N ratio), elevation and habitat disturbance. The results obtained do not support the separation of C. viridula var. pulchella from C. viridula var. viridula.

Conclusions: Phenotypic variability in the species of the C. flava complex is related to habitat conditions and this can lead to the differentiation of morphotypes within species.     

Density-dependent natural selection does not increase efficiency

Summary Populations ofDrosophila melanogaster kept at high population density (K-selected) for 125 generations have higher larval viability than populations kept at low densities (r-selected) when both are raised under crowded conditions. In additionK-selected adults that emerge from crowded cultures are larger than theirr-selected counterparts. These differences cannot be explained by differences in efficiency of food use. The minimum food required for successful pupation is actually greater in theK-selected populations. I conjecture that there may be a trade-off between minimum food requirements and competitive ability, which has changed substantially in theK-selected populations. The possibility thatK-selected larvae can dig more more deeply and gain access to unused food is examined and rejected as a possible explanation of the viability differences. Evidence is provided supporting the hypothesis that the differences in viability may be due to an increased tendency of theK-selected larvae to pupate off the surface of the medium.     

Life-history variation in Crepis tectorum (Asteraceae)

Summary Populations of the monocarpic plant Crepis tectorum were grown in a series of uniform environments to test the hypothesis that weedy populations are more r-selected than populations from a more natural habitat. Weedy populations exhibited a combination of r- and K-selected traits. The relatively rapid growth, the potential for a summer annual habit, and the relatively high fecundity that characterized at least one of the two weed populations studied were considered as r-selected traits favored in habitats of unpredictable duration. However, high levels of competition from other weedy species or from the crop in arable fields may explain at least some presumably K-selected traits observed in the weedy populations, e.g. relatively large seeds and late flowering in the summer. Results indicated that stress due to abiotic factors (strong winds, desiccation and nutrient deficiency) has been a more important selective factor than r- or K-selection, in non-weedy populations from calcareous grasslands (alvars) on the Baltic islands.     

DENSITY-DEPENDENT EVOLUTION OF LIFE-HISTORY TRAITS IN DROSOPHILA MELANOGASTER

Populations of Drosophila melanogaster were maintained for 36 generations in r- and K-selected environments in order to test the life-history predictions of theories on density-dependent selection. In the r-selection environment, populations were reduced to low densities by density-independent adult mortality, whereas populations in the K-selection environment were maintained at their carrying capacity. Some of the experimental results support the predictions or r- and K-selection theory; relative to the r-selected populations, the K-selected populations evolved an increased larval-to-adult viability, larger body size, and longer development time at high larval densities. Mueller and Ayala (1981) found that K-selected populations also have a higher rate of population growth at high densities. Other predictions of the thoery are contradicted by the lack of differences between the r and K populations in adult longevity and fecundity and a slower rate of development for r-selected individuals at low densities. The differences between selected populations in larval survivorship, larval-to-adult development time, and adult body size are strongly dependent on larval density, and there is a significant interaction between populations and larval density for each trait. This manifests an inadequacy of the theory on r- and K-selection, which does not take into account such interactions between genotypes and environments. We describe mechanisms that may explain the evolution of preadult life-history traits in our experiment and discuss the need for changes in theories of density-dependent selection.     

Patterns of genetic variability within and among populations of wild radish,Raphanus raphanistrum (Brassicaceae)

The genetic structure of populations is an important determinant of the evolutionary potential of a species. Colonizing plants tend to be characterized by low within- and high among-population variability. Genetic differentiation of both floral traits and isozymes was studied in six populations of wild radish (Raphanus raphanistrum). Evidence for differentiation in both sets of traits was found, but patterns of differentiation of floral traits did not coincide with isozyme differentiation. Contrary to most colonizing species, wild radish showed high within- and only moderate among-population variability at isozyme loci. In addition, levels of differentiation did not correspond to geographic distance between the populations. These results are likely due at least in part to the self-incompatibility system of this species, long-distance movement of large numbers of wild radish seeds by humans, and introgression from cultivated radish (R. sativus).     

Reproductive strategy in Drosophila melanogaster: Significance of a genetic divergence between temperate and tropical populations

Summary Reproductive capacities of tropical and temperate populations of D. melanogaster were compared using three complementary techniques: (1) measure of egg production by females grown in the laboratory under uncrowded conditions and provided as adults with abundant food; (2) study of egg production of flies of unknown ages, collected in nature and then kept in similar conditions; and (3) analysis of ovarian activity of wild females dissected just after their capture.Tropical populations showed a lower fecundity in the laboratory and this was also observed in laboratory reared adults. On the average, flies also appeared to be older in the tropics than in temperate countries. These data, together with ecological observations showing that tropical populations live in a more predictable and stable environment, suggest that temperature populations are r-selected, while tropical ones are K-selected. The study of ovarian activity of wild females failed however to confirm this expectation. Tropical flies, which have a lower genetic fecundity, generally appeared to produce more propagules than did temperate flies. Such a contradiction shows how the ideas of r- and K-selection are difficult to apply to natural populations of Drosophila. Population density and interindividual competition are probably not the main selective forces in nature. Attention must also be paid to the necessity of exploring the environment to find resources, to the role of predation and parasitism, and to the occurrence in temperate countries of seasonal fluctuations with different selective pressures on successive generations.     

Local adaptations in bryophytes revisited: the genetic structure of the calcium‐tolerant peatmoss Sphagnum warnstorfii along geographic and pH gradients

Bryophytes dominate some ecosystems despite their extraordinary sensitivity to habitat quality. Nevertheless, some species behave differently across various regions. The existence of local adaptations is questioned by a high dispersal ability, which is thought to redistribute genetic variability among populations. Although Sphagnum warnstorfii is an important ecosystem engineer in fen peatlands, the causes of its rather wide niche along the pH/calcium gradient are poorly understood. Here, we studied the genetic variability of its global populations, with a detailed focus on the wide pH/calcium gradient in Central Europe. Principal coordinates analysis of 12 polymorphic microsatellite loci revealed a significant gradient coinciding with water pH, but independent of geography; even samples from the same fens were clearly separated along this gradient. However, most of the genetic variations remained unexplained, possibly because of the introgression from phylogenetically allied species. This explanation is supported by the small heterogeneous cluster of samples that appeared when populations morphologically transitional to S. subnites, S. rubellum, or S. russowii were included into the analysis. Alternatively, this unexplained variation might be attributed to a legacy of glacial refugia with recently dissolved ecological and biogeographic consequences. Isolation by distance appeared at the smallest scale only (up to 43 km). Negative spatial correlations occurred more frequently, mainly at long distances (up to 950 km), implying a genetic similarity among samples which are very distant geographically. Our results confirm the high dispersal ability of peatmosses, but simultaneously suggested that their ability to cope with a high pH/calcium level is at least partially determined genetically, perhaps via specific physiological mechanisms or a hummock‐forming ability.     

Population‐level traits that affect,and do not affect,invasion success

What allows some species to successfully colonize a novel environment while others fail? Numerous studies in invasion biology have sought to answer this question, but those studies have tended to focus on traits of species or individuals (e.g. body size, seed size, seed number), and these traits have largely been found to be weak predictors of invasion success. However, characteristics of colonizing populations (e.g. genetic diversity, density, age structure) might also be important for successful establishment, as the authors of a study published in this issue of Molecular Ecology show ( Crawford & Whitney 2010 ). By experimentally manipulating the density and genetic diversity of colonizing populations of Arabidopsis thaliana, the authors found that genetic diversity, but not population density, increased colonization success. Importantly, the effects of genetic diversity on colonization success were both additive and non‐additive, suggesting that traits associated with particular genotypes and complimentarity among genotypes contribute to colonization success. This research highlights the importance of considering within‐species variation and characteristics of entire populations in predicting colonization success.     

LOW GENIC DIFFERENTIATION AMONG ISOLATED POPULATIONS OF THE CALIFORNIA FAN PALM (WASHINGTONIA FILIFERA)

The purpose of this study was to assess the relative roles of population size and geographic isolation in determining population-genetic structure. Using electrophoretic techniques to quantify allozymic variation at 16 genetic loci, we measured genic variation within and among 16 natural populations of the California fan palm (Washingtonia filifera). Genotypes were determined for every individual in each population so that parametric values rather than sample estimates for measures of genic variability were obtained. Palm populations displayed low levels of within-population variability. The proportion of polymorphic loci and observed heterozygosity were 0.098 and 0.009 per population, respectively. Population size displayed a significant positive correlation with proportion of polymorphic loci, but not with observed heterozygosity. Low levels of genetic differentiation among populations were demonstrated by an F-statistic analysis and the computation of genetic similarity values. A hierarchical analysis of gene diversity revealed that only about 2% of the total gene diversity in W. filifera resides as among-population diversity. Climatic and geological changes since the Pliocene have eliminated widespread palm populations, and the species is presently restricted to isolated locations around the Colorado Desert. Existing populations in southern California are either relicts or recent recolonizations resulting from the dispersal of seeds from a refugium population in Baja California, Mexico. The observed patterns of low within- and low among-population genic diversity seem most consistent with a recent colonization by fan palms. It is hypothesized that stochastic processes reduced levels of genic variability in this refugium population during its formation. Dispersal of seeds from this refugium into suitable habitats in the Colorado Desert would produce populations with low variability and high genetic similarity because of their common ancestry. However, low intrapopulation variability and genetic homogeneity across populations could be the product of uniform selection pressures favoring a narrow array of specialized genotypes in either relict or colonizing populations.     

Experimental island invasion of house mice

The ability of invasive species to recurrently establish populations from small numbers of founders, while threatened species struggle at the same low population sizes, is a paradox in conservation biology. Little is known about the mechanisms contributing to the post-arrival success of low density invasive populations as most invasive species research focuses on established, high density populations. Experimental studies are powerful, but generally limited to laboratory or invertebrate experiments. Here, we experimentally demonstrate that vertebrate mammal invasion from a very small (n = 2) number of founders follows relatively simple deterministic predictions. An intentional island invasion of introduced house mice (Mus musculus Linnaeus) from one founding pair closely tracked the density dependent logistic growth curve and reached the seasonal carrying capacity of a previously extant population in only 5 months. Carrying capacity reflected both density dependent and independent processes. In contrast to the previously incumbent population, the invading population retained a marked genetic signal of its recent founder event, but the populations were otherwise demographically indistinguishable. Stochastic events such as individual variability, supplemental immigration and ecological release, but not Allee effects, played important roles during colonisation, but following establishment dynamics rapidly became deterministic, with little demographic impact of reduced genetic diversity. The small population paradigm appears to have little influence on the population dynamics of highly successful invasive species.     

Extinction,recolonization, and the genetic structure of tidepool copepod populations

Summary Extinction and recolonization in metapopulations may either increase or decrease genetic differentiation among populations, but recent genetic models predict increased differentiation under most circumstances of recolonization. I examine this prediction empirically using tidepool populations of the marine tidepool copepodTigriopus californicus. The probability of extinction of tidepool populations was sufficiently high to invoke the model's predictions, but varied among populations. Nearly 75% of colonizing groups consisted of 10 or fewer individuals. The genetic effective size of colonizing groups might be as high as 18, depending on assumptions, but colonists probably originated from a subset of local populations. In contrast to my predictions, genetic differentiation was smaller among younger tidepool populations than among older populations on each of three rock outcrops, suggesting that genetic differentiation was reduced by metapopulation dynamics. The discrepancy between the prediction and the results may be explained by the unmet assumptions of classical metapopulation structure underlying the genetic models.     

Genetic and phenotypic consequences of early domestication in black soldier flies (Hermetia illucens)

The black soldier fly, Hermetia illucens, is an emerging biotechnological agent with its larvae being effective converters of organic waste into usable bio-products including protein and lipids. To date, most operations use unimproved commercial populations produced by mass rearing, without cognisance of specific breeding strategies. The genetic and phenotypic consequences of these commercial practices remain unknown and could have a significant impact on long-term population viability and productivity. The aim of this study was thus to assess the genetic and phenotypic changes during the early phases of colony establishment and domestication in the black soldier fly. An experimental colony was established from wild founder flies and a new microsatellite marker panel was developed to assess population genetic parameters along with the phenotypic characteristics of each generational cohort under captive breeding. The experimental colony was characterised by a small effective population size, subsequent loss of genetic diversity and rapid genetic and phenotypic differentiation between the generational cohorts. Ultimately, the population collapsed by the fifth generation, most likely owing to the adverse effect of inbreeding depression following the fixation of deleterious alleles. Species with r-selected life history characteristics (e.g. short life-span, high fecundity and low larval survival) are known to pose particular challenges for genetic management. The current study suggests that sufficient genetic and phenotypic variations exist in the wild population and that domestication and strain development could be achieved with careful population augmentation and selection during the early stages of colony establishment.     

Morphological versus genetic diversity of Viola reichenbachiana and V. riviniana (sect. Viola,Violaceae) from soils differing in heavy metal content

Morphological characters, AFLP markers and flow cytometry were used to investigate the morphological and genetic variability and differentiation of Viola reichenbachiana and V. riviniana in non‐metallicolous (NM) and metallicolous (M) populations. The aims were to clarify the taxonomic status of plants occurring in ore‐bearing areas, to determine any relationship in V. reichenbachiana and V. riviniana from sites not polluted with heavy metals, and to examine the genetic variability and differentiation of M and NM populations of both species. Multivariate analyses based on morphological characters showed significant differences between V. reichenbachiana and V. riviniana from non‐polluted sites, high levels of intra‐ and inter‐population variability, and the occurrence of inter‐specific hybrids. Plants from M populations showed hybrid characters but also fell within the range of V. riviniana or V. reichenbachiana. There were no significant differences in relative genome size between plants from polluted areas and V. riviniana from NM populations. Bayesian analysis of population genetic structure based on AFLP markers distinguished two main groups: V. reichenbachiana and V. riviniana together with the M populations. That analysis also revealed the occurrence of populations of inter‐specific hybrids from non‐polluted areas. Further Bayesian analysis of V. riviniana including NM and M populations separated all the studied M populations from NM populations. We conclude that plants forming the M populations are well adapted to a metal‐polluted environment, and could be considered as stabilised introgressive forms resulting from unidirectional (asymmetric) introgression toward V. riviniana.     

Coexistence of two competing corixid species (Heteroptera) in an archipelago of temporary rock pools

Summary Coexistence of Callicorixa producta and Arctocorisa carinata in an archipelago of rock pools appears to be facilitated by division of the environment on the time axis. To test this, a hypothesis is built on r-K theory. As A. carinata is twice the size of C. producta it is expected that the former species is more K-selected living in more permanent pools where food resources are limiting and competition strong. The smaller C. producta is expected to prefer newly refilled pools with abundant food, and thus to be more r-selected. Gathered test data on 17 life history and population ecological features agree with the prediction. It is also shown that the both rock-pool corixids are more r-selected than other species of the family.     

Gene flow between populations of two invertebrates in springs

1. Using allozymes, we analysed genetic structure of the freshwater gastropod Bythinella dunkeri and the freshwater flatworm Crenobia alpina. The two species are habitat specialists, living almost exclusively in springs. The sampled area in Hesse (Germany) covers a spatial scale of 20 km and includes two river drainages. From the biology of the two species we expected little dispersal along rivers. However, the possibility exists that groundwater provide suitable pathways for dispersal. 2. In B. dunkeri heterozygosity decreased from west to east. For some alleles we found clines in this geographic direction. These clines generated a positive correlation between geographic distance and genetic differentiation. Furthermore patterns of genetic variation within populations suggested that populations may have been faced with bottlenecks and founder effects. If populations are not in population genetic equilibrium, such founder effects would also explain the rather high amount of genetic differentiation between populations (10%). 3. For C. alpina the mean number of alleles decreased with increasing isolation of populations. Genetic differentiation between populations contributed 19% to the total genetic variation. Genetic differentiation was not correlated to geographic distance, but compared with B. dunkeri variability of pairwise differentiation between pairs of populations was higher in C. alpina. 4. Overall B. dunkeri appears to be a fairly good disperser, which may use groundwater as dispersal pathway. Furthermore populations seem to be not in equilibrium. In contrast C. alpina forms rather isolated populations with little dispersal between springs and groundwater seems to play no important role for dispersal.     

ADAPTIVE STRATEGIES OF COLONIZING ANIMAL SPECIES

1. An understanding of the adaptive strategies of colonizing animal species depends upon an integration of population genetics and ecology, but behavioural components should not be ignored especially higher in the phylogenetic series than insects. 2. An ecologically marginal habitat from which colonists are derived can be regarded as one in which physical stresses of climatic origin tend to be variable and extreme, so that resources are unpredictable and short lived. Prerequisites for genetic analysis are therefore phenotypes relatable to the r K continuum of adaptive strategies. These can be called ‘ecological phenotypes’. 3. Ecological phenotypes include tolerances to environmental stresses, development time, and resource utilization variability. Such phenotypes enable distinctions to be made between colonizing species and non-colonists. For example, colonizing species have ecological phenotypes incorporating high resistance to physical stress, rapid development time, and the exploitation of an array of food resources. They are ecologically versatile generalists. This includes the use of ethanol as a resource in Drosophila. 4. There is a substantial literature on variation in central and marginal populations based upon gene and chromosome polymorphisms. Most data show a reduction of chromosome polymorphisms and of lethals and semilethals towards the margins, but no equivalent reduction in enzyme polymorphisms. Widespread species tend to have low levels of chromosome polymorphisms as in marginal populations, but enzyme polymorphism levels vary too much among species for meaningful interpretations. Since these are genotypic assessments not directly relatable to the field situation, the somewhat unsatisfactory nature of these data from the interpretative point of view is understandable. In addition, the fundamental issue is not the variability of the genome, but the nature and role of loci controlling ecological phenotypes. 5. Ecological phenotypes can be analyzed at the population level with isofemale strains as the starting material. In theory, genetic activity can then be localized to the chromosomal and even genic levels in a species such as D. melanogaster. Isofemale strain studies in D. melanogaster are interpretable in terms of the r K continuum, and so reflect adaptive strategies in nature. 6. It is highly likely that the genetic architecture of ecological phenotypes of marginal populations mainly comprises a few additive genes of relatively large effect. This is an architecture permissive of rapid adaptation to new habitats, provided that the appropriate genes are present. Discussions of speciation via the founder principle, a colonization event in itself, have invoked a similar explanation.     

The impact of species-specific traits and phylogenetic relatedness on allozyme diversity inCarex sect.Phyllostachys (Cyperaceae)

Allozyme variation was examined inCarex sect.Phyllostachys (Cyperaceae) to study the effects of species-specific traits and phylogenetic relatedness on genetic structure. In contrast to the findings of similar studies, genetic variability in thePhyllostachys is poorly correlated with geographic range and putative differences in breeding systems (as inferred from morphology). This suggests that other patterns of evolution, colonization, and gene flow characterize the species found in this section. Fixation indices are negative for all populations suggesting that mechanisms such as disassortative mating and selection are maintaining heterozygous excess within populations. Closely related taxa often exhibit different genetic variability statistics. In some instances, however, clades (e.g.C. jamesii andC. juniperorum) display very similar levels of genetic variability despite marked differences in species-specific traits. Recent speciation coupled with the ability to maintain historical levels of variation within populations may be factors accounting for this phenomenon. Contrary to similar studies, species restricted to known glacial refugia have lower genetic diversity than those species that underwent mass migrations in response to deglaciation. Narrowly endemic species were found to partition their genetic diversity within, as opposed to between populations. The opposite trend was evident in wider ranging congeners.     

Obligate association with gut bacterial symbiont in Japanese populations of the southern green stinkbug <Emphasis Type="Italic">Nezara viridula</Emphasis> (Heteroptera: Pentatomidae)

The southern green stinkbug Nezara viridula (Linnaeus) has a number of sac-like outgrowths, called crypts, in a posterior section of the midgut, wherein a specific bacterial symbiont is harbored. In previous studies on N. viridula from Hawaiian populations, experimental elimination of the symbiont caused few fitness defects in the host insect. Here we report that N. viridula from Japanese populations consistently harbors the same gammaproteobacterial gut symbiont, but, in contrast with previous work, experimental sterilization of the symbiont resulted in severe nymphal mortality, indicating an obligate host–symbiont relationship. Considering worldwide host–symbiont association and these experimental data, we suggest that N. viridula is generally and obligatorily associated with the gut symbiont, but that the effect of the symbiont on host biology may be different among geographic populations. Possible environmental factors that may affect the host–symbiont relationship are discussed.     

Population and species boundaries in the South American subterranean rodent Ctenomys in a dynamic environment

Subterranean rodents of the genus Ctenomys are an interesting system to assess the effects of habitat instability on the genetic structure of populations. The perrensi group is a complex of three species (C. roigi, C. perrensi and C. dorbignyi) and several forms of uncertain taxonomic status, distributed in the vicinity of the Iberá wetland in Argentina. Because of limited availability of suitable dry habitat, Ctenomys populations are distributed patchily around a vast mosaic of marshes, swamps and lagoons and become connected or isolated over time, depending particularly on the precipitation regime. Genetic variation at 16 microsatellite loci in 169 individuals collected in the area revealed eight clusters of populations which are thought to be evolutionary units, but which do not fit previous species limits. We interpret this lack of congruence between taxonomy and genetic structure as the result of a dynamic population structure. Where populations become connected, hybridization is possible. Where populations become isolated, rapid genetic divergence may occur. In the perrensi group, it appears that both of these factors disrupt the association between different genetic and morphological characters. The study of multiple characters is crucial to the understanding of the recent evolutionary history for dynamic systems such as this. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 368–383.