Strong Evolutionary Forces Research Articles

On the Possible Link Between Vitamin D Deficiency and Cardiovascular Disease

We were all supposedly black in the beginning. As man migrated out from the equator, a progressive depigmentation took place. For this to occur, strong evolutionary forces in favor of less pigmentation must have been present in regions with low solar exposure. We do not know toward what end such an advantage might have been working, but a main hypothesis is that in regions with low sun ultraviolet (UV) radiation, less pigmented individuals would need less time in the sun to avoid vitamin D deficiency. This, in turn, might have helped avoid respiratory tract infections, tuberculosis, and influenza—conditions that have proven lethal over the course of time.1 Because, in most populations, females have less pigmented skin and approximately twice the risk of vitamin D deficiency as males, female reproduction may be involved in an evolutionary advantage, that is, improved fertility, fewer miscarriages, or fewer pregnancy complications. Even a small increase in successful reproduction per generation will make a great difference in the long run, favoring skin pigmentation adapted to a particular region. Individuals with heart failure, hypertension, stroke, and other cardiovascular diseases (CVD) tend to have lower vitamin D levels than others.2 In addition, manifestations of CVD are more common during the winter season, when vitamin D levels are …

Natural Competence and the Evolution of DNA Uptake Specificity

Many bacteria are naturally competent, able to actively transport environmental DNA fragments across their cell envelope and into their cytoplasm. Because incoming DNA fragments can recombine with and replace homologous segments of the chromosome, competence provides cells with a potent mechanism of horizontal gene transfer as well as access to the nutrients in extracellular DNA. This review starts with an introductory overview of competence and continues with a detailed consideration of the DNA uptake specificity of competent proteobacteria in the Pasteurellaceae and Neisseriaceae. Species in these distantly related families exhibit strong preferences for genomic DNA from close relatives, a self-specificity arising from the combined effects of biases in the uptake machinery and genomic overrepresentation of the sequences this machinery prefers. Other competent species tested lack obvious uptake bias or uptake sequences, suggesting that strong convergent evolutionary forces have acted on these two families. Recent results show that uptake sequences have multiple "dialects," with clades within each family preferring distinct sequence variants and having corresponding variants enriched in their genomes. Although the genomic consensus uptake sequences are 12 and 29 to 34 bp, uptake assays have found that only central cores of 3 to 4 bp, conserved across dialects, are crucial for uptake. The other bases, which differ between dialects, make weaker individual contributions but have important cooperative interactions. Together, these results make predictions about the mechanism of DNA uptake across the outer membrane, supporting a model for the evolutionary accumulation and stability of uptake sequences and suggesting that uptake biases may be more widespread than currently thought.

The characteristic trajectory of a fixing allele: a consequence of fictitious selection that arises from conditioning.

This work is concerned with the historical progression, to fixation, of an allele in a finite population. This progression is characterized by the average frequency trajectory of alleles that achieve fixation before a given time, T. Under a diffusion analysis, the average trajectory, conditional on fixation by time T, is shown to be equivalent to the average trajectory in an unconditioned problem involving additional selection. We call this additional selection "fictitious selection"; it plays the role of a selective force in the unconditioned problem but does not exist in reality. It is a consequence of conditioning on fixation. The fictitious selection is frequency dependent and can be very large compared with any real selection that is acting. We derive an approximation for the characteristic trajectory of a fixing allele, when subject to real additive selection, from an unconditioned problem, where the total selection is a combination of real and fictitious selection. Trying to reproduce the characteristic trajectory from the action of additive selection, in an infinite population, can lead to estimates of the strength of the selection that deviate from the real selection by >1000% or have the opposite sign. Strong evolutionary forces may be invoked in problems where conditioning has been carried out, but these forces may largely be an outcome of the conditioning and hence may not have a real existence. The work presented here clarifies these issues and provides two useful tools for future analyses: the characteristic trajectory of a fixing allele and the force that primarily drives this, namely fictitious selection. These should prove useful in a number of areas of interest including coalescence with selection, experimental evolution, time series analyses of ancient DNA, game theory in finite populations, and the historical dynamics of selected alleles in wild populations.

Rapid Divergent Evolution of Male Genitalia Among Populations of Drosophila buzzatii

Fil: Soto, Ignacio Maria. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires; Argentina

Micro‐evolutionary change in relation to insecticide resistance in the peach–potato aphid,Myzus persicae

1. Phenotypic diversity is the fuel that powers evolution.2. Asexual organisms rely on mutation whereas sexual organisms combine mutation with recombination.3. Few organisms provide examples of species that are both sexual and asexual, but aphids do.4. To examine evolution on perceptible timescales requires strong evolutionary forces and, as Darwin noted, agricultural practices provide strong selection. In the case of aphids, insecticides provide a considerable force in the elimination of genotypes.5. Insecticide resistance inMyzus persicae(Sulzer) has arisen independently through point mutation and gene amplification on a number of occasions and at different times. Resistance to organophosphates, pyrethroids, and pirimicarb (a dimethyl carbamate) is now widespread.6. In this paper, we examine these three elements: sexual recombination, clonal expansion, and insecticide selection in the peach–potato aphidM. persicaein relation to the evolution of insecticide resistance and survival of the fittest clone.

Seasonal dynamics of foliar antioxidative enzymes and total anthocyanins in natural populations of Iris pumila L.

Aims Plants in their natural habitats frequently cope with a multitude of abiotic stresses, such as high light intensity, extreme temperatures and water deficit, which often co-occur during periods of drought, especially in semi-arid and arid regions. Exposure of plants to stressful environmental conditions usually induce overproduction of reactive oxygen species (ROS) that, as highly toxic derivatives of O2, can assault all cell macromolecules, leading to the disruption of cellular homeostasis and, consequently, the uncoupling of major metabolic processes, the photosynthesis and photorespiration. In order to minimize ROS-mediated cellular damage, plants have evolved highly efficient antioxidative defense systems that include both enzymatic and non-enzymatic components. Since abiotic stress can also operate as a strong evolutionary force that shapes adaptations in natural plant populations, the aim of this study was to examine the seasonal variation patterns of two distinct antioxidative systems, ROS-scavenging enzymes and anthocyanin pigments, in the leaf tissue of a steppe plant, Iris pumila, as expressed under contrasting light conditions that the species regularly experiences in the wild. Methods We selected two natural populations of I.pumila inhabiting the alternative radiation environments in the Deliblato Sands, a sunexposed dune site and a woodland understory. The specific activity of three antioxidative enzymes, superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) and the content of total anthocyanins were examined in leaves of I.pumila plants collected from each of the 31 Iris clones (17 in the exposed population and 14 in the shaded population) once during each of the three seasons, spring, summer and autumn in 2004. Specifically, a fully expanded leaf was cut from each clonal plant between 15:00 and 16:00 h, immediately frozen in liquid nitrogen and stored at 70 C until preparation. Important Findings Generally, all three antioxidative enzymes were up-regulated in summer-harvested leaves compared to their spring or autumn counterparts, as was observed for the concentration of foliar anthocyanins, indicating that strengthening of antioxidant systems was the key mechanism for long-term acclimatization of I.pumila plants to stressful environmental conditions within their natural ecological niches. When plants from contrasting radiation environments were compared, SOD and CAT activities appeared to be greater in shade-exposed than in sun-exposed leaves. Conversely, POD activity and the content of foliar anthocyanins were notably higher in foliage experiencing full sunlight relative to those developed under vegetation canopy, suggesting the synergistic function of these two molecules in protecting leaf cells against photoinhibitory and photooxidative effects of strong light.

Segregation distortion and its effect on genetic mapping in plants

AbstractSegregation distortion, identified as a strong evolutionary force, is common in genetic mapping research. In plants, the percentage, degree, origin and genetic effects of segregation distortion vary significantly with species, population types, crosses and marker types. The exhibition, common features, causes, methods of mapping segregation distortion loci, effects of segregation distortion on map construction and corresponding mapping strategies in plants, mainly in crops, are reviewed in this paper.

Score-based prediction of genomic islands in prokaryotic genomes using hidden Markov models.

BackgroundHorizontal gene transfer (HGT) is considered a strong evolutionary force shaping the content of microbial genomes in a substantial manner. It is the difference in speed enabling the rapid adaptation to changing environmental demands that distinguishes HGT from gene genesis, duplications or mutations. For a precise characterization, algorithms are needed that identify transfer events with high reliability. Frequently, the transferred pieces of DNA have a considerable length, comprise several genes and are called genomic islands (GIs) or more specifically pathogenicity or symbiotic islands.ResultsWe have implemented the program SIGI-HMM that predicts GIs and the putative donor of each individual alien gene. It is based on the analysis of codon usage (CU) of each individual gene of a genome under study. CU of each gene is compared against a carefully selected set of CU tables representing microbial donors or highly expressed genes. Multiple tests are used to identify putatively alien genes, to predict putative donors and to mask putatively highly expressed genes. Thus, we determine the states and emission probabilities of an inhomogeneous hidden Markov model working on gene level. For the transition probabilities, we draw upon classical test theory with the intention of integrating a sensitivity controller in a consistent manner. SIGI-HMM was written in JAVA and is publicly available. It accepts as input any file created according to the EMBL-format.It generates output in the common GFF format readable for genome browsers. Benchmark tests showed that the output of SIGI-HMM is in agreement with known findings. Its predictions were both consistent with annotated GIs and with predictions generated by different methods.ConclusionSIGI-HMM is a sensitive tool for the identification of GIs in microbial genomes. It allows to interactively analyze genomes in detail and to generate or to test hypotheses about the origin of acquired genes.

Transposable element dynamics in two sibling species: Drosophila melanogaster and Drosophila simulans.

Transposable elements (TEs) in the two sibling species, Drosophila melanogaster and D. simulans, differ considerably in amount and dynamics, with D. simulans having a smaller amount of TEs than D. melanogaster. Several hypotheses have been proposed to explain these differences, based on the evolutionary history of the two species, and claim differences either in the effective size of the population or in genome characteristics. Recent data suggest, however, that the higher amount of TEs in D. melanogaster could be associated with the worldwide invasion of D. melanogaster a long time ago while D. simulans is still under the process of such geographical spread. Stresses due to new environmental conditions and crosses between migrating populations could explain the mobilization of TEs while the flies colonize. Colonization and TE mobilization may be strong evolutionary forces that have shaped and are still shaping the eukaryote genomes.

Pollination genetics of hybridization in sympatric populations of asclepias exaltata and a. syriaca (asclepiadaceae)

Enzyme electrophoresis of pollinia and seeds was used to identify pollinia on pollinators, interspecific pollinations, and hybrid seed production in sympatric populations of Asclepias exaltata and A. syriaca. The frequency of mixed pollinia loads on pollinators was low. Only 4.1% (N= 169 pollinia) and 8% (N= 244 pollinia) of the pollinia collected from pollinators of A. syriaca and A. exaltata, respectively, were identified as pollinia from the other species. Natural levels of interspecific pollination, 3.6% (N = 166 pollinia) on A. syriaca and 3.1% (N = 228 pollinia) on A. exaltata, were typically lower than the number of foreign pollinia carried by pollinators. Hybrid seeds were identified in only two of 208 A. syriaca fruits and one of 178 A. exaltata fruits. Hybrid seeds were largely underdeveloped in the single A. exaltata fruit. High fruit set (34.7%) and near normal seed set following hand‐pollination of A. syriaca with A. exaltata pollinia suggest that hybridization is more likely between these taxa when A. syriaca is the maternal parent. Our findings indicate that the likelihood of hybridization between A. exaltata and A. syriaca is remote. Nevertheless, introgressive hybridization has been documented in these species, which suggests that hybridization remains a strong evolutionary force even for species that infrequently interbreed.

Detecting clinal and balanced selection using spatial autocorrelation analysis under kin‐structured migration

Recently spatial autocorrelation has been employed to infer microevolutionary processes from patterns of genetic variation. In theory, different processes should show characteristic signature correlograms; e.g., clinal selection should produce correlograms decreasing from positive to negative autocorrelation, whereas uniform balanced selection should lead to no spatial autocorrelation. The ability of a statistical method such as spatial autocorrelation analysis to distinguish between these selective regimes or even to detect departures from neutrality is dependent on the strength of the evolutionary force and the population structure. Weak selection or migration will not be apparent against the expected background of stochastic noise. Moreover, the population structure may generate sufficient stochastic variation such that even strong evolutionary forces may fail to be detected. This study uses computer simulation to examine the effects of kin-structured migration and three different selective regimes on the shape of spatial correlograms to assess the ability of this technique to detect different microevolutionary processes. Genetic variation among 8 loci is simulated in a linear set of 25 artificial populations. Kin-structured stepping-stone migration among adjacent populations is modeled; directional, balanced, and clinal selection, as well as neutral loci are considered. These experiments show that strong selection produces correlograms of the predicted shape. However, with an anthropologically reasonable population structure, considerable stochastic variation among correlograms for different alleles may still exist. This suggests the need for caution in inferring genetic process from spatial patterns.

A model of extranuclear genomes and the substitution rate under within-generation selection

SummaryA single locus model of extranuclear genomes is developed under the assumption of the complete action of within-generation drift which is caused by random transmission of multiple copy genomes during cell division in a generation. Within-generation drift segregates different copy genomes in a cell into different cells, resulting in homoplasmic cells. Under some conditions, the present model reduces to that for haploid nuclear genomes. A point overlooked in previous models is that the multiplicity also admits of the possibility of selection occurring within a cell or between cells in an individual (within-generation selection). If there is selection mediated by, for instance, differential proliferation of genomes, then a haploid model no longer explains the dynamics of extranuclear genomes. Rather a model analogous to biased gene conversion at a single locus (Nagylaki, 1983; Walsh, 1983) is more appropriate. An application of this model to either the fixation probability or substitution rate of new mutations shows that strictly maternal inheritance does not allow the fullest use of mutations, as it obscures the effect of within-generation selection. But if there is appreciable paternal contribution, within-generation selection could be a strong evolutionary force to which nuclear genomes are never exposed.

MICROSPATIAL GENETIC HETEROGENEITY IN POCKET GOPHERS: NON-RANDOM BREEDING AND DRIFT.

In the absence of selection, the genetic structure of populations will reflect the interaction of population size, amount of gene exchange, and breeding patterns. Populations characterized by high gene flow and random mating will be much less structured than those in which the reverse conditions hold. Thus, it will be in those species that are subdivided into small, discrete breeding units, or demes, that genetic drift and demic extinction can serve as strong evolutionary forces. If demes are sufficiently small and temporally closed, sampling error can override all but the strongest selection regimes (see, for example, Wright, 1931, 1969; Kimura, 1955). Moreover, local extinctions, while they may enhance the probability of drift by increasing the likelihood of inbreeding within surviving demes, may also act to maintain genetic heterogeneity among subdivisions of a population (Wright, 1940). Examples of species with highly structured populations in which the structure results, at least in part, from small population size and constraints to movement due to social factors and/or inherently low vagility include house mice (Petras, 1967; Selander, 1970; but see Baker, 1981) and land snails (Selander and Kaufman, 1975; Jones et al., 1977). Pocket gophers of the North American rodent family Geomyidae provide an example of how population size, local differential selection pressures, and genetic drift can interact to form populations highly structured on both a local and regional level. The Thomomys bottae complex is comprised of more than 200 described races (Hall and Kelson, 1959), based primarily on adaptive color variation and body size, and exhibits a concomitantly greater degree of chromosomal variability than any other known mammal (Patton, 1972). While selection regimes may be partly responsible for this variation (i.e., local cryptic matching of pelage color and soil color), the general biological attributes of gophers which stem from their fossorial habitus (such as low vagility, male and female exclusive territories, relatively low population densities, and patchy distribution) provide substrates for the action of stochastic factors in population divergence (Patton, 1972; Penney and Zimmerman, 1976). Indeed, in recent summaries of macrogeographic patterns in genic (=electromorphic) variability in this complex, the major determinants of within and between population variability were hypothesized to be gene flow and the temporal stability of populations (Patton and Yang, 1977; Patton and Feder, 1978). In order to further understand both the extent and controlling factors of among population heterogeneity, the genetic structure of pocket gopher (Thomomys bottae) populations has been examined in detail at Hastings Natural History Reservation, Carmel Valley, California, using electrophoretic techniques. At this site, gophers are generally confined to grassland fields of small size, varyingly isolated from one another by road cuts, stream courses, or patches of vegetation largely uninhabited by gophers (Fig. 1). Densities of animals in this area are moderate to high and no human-induced perturbations

Tridacnin, a potent anti-galactan precipitin from the hemolymph of Tridacna maxima (Röding).

In discussing the evolution of the immune process, Burnet (1) stated that invertebrate hemocytes possess at least a limited capacity to recognise foreignness and that agglutinins from body fluids demonstrate pseudoimmunological capacities. This, he concluded, provided the basis from which the more sophisticated antibody system of vertebrates could be developed given strong enough evolutionary forces. Many invertebrate species have been shown to be capable of phagocytosing pathogens and other materials (2) and, although a degree of specificity has been observed in the agglutinins present in the hemolymph of some invertebrates (3), phagocytosis may be the most important defense against infection in the organisms. As pointed out by Hildemann(4), invertebrates have obviously resisted infection for many millions of years without the aid of the more sophisticated adaptive immune mechanisms found in vertebrates. The ability to recognise and distinguish self from foreignness is a characteristic of all animal species and this capacity is well developed in invertebrates (4). Two good examples of this capacity amongst the invertebrates are the aggregation of sponge cells (5) and the species specificity demonstrated by fusing tunicate colonies (6). Specific interactions such as these appear to be due to glycoproteins on the cell surfaces (7, 8) and, as suggested by Marchalonis and Cone (9), vertebrate immunological mechanisms may have evolved from such invertebrate recognition phenomena.KeywordsHouse Dust MiteAscaris LumbricoidesSponge CellBlood Group SubstanceTridacna MaximumThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.