Genetically based adaptive trait shifts at an expanding mangrove range margin

Hydrobiologia - Neotropical black mangrove (Avicennia germinans) is expanding poleward into temperate salt marsh along Atlantic Florida, USA, with field evidence of trait shifts consistent with...     

Formation of Streptococcus pneumoniae non-phase-variable colony variants is due to increased mutation frequency present under biofilm growth conditions

In this report, we show that biofilm formation by Streptococcus pneumoniae serotype 19 gives rise to variants (the small mucoid variant [SMV] and the acapsular small-colony variant [SCV]) differing in capsule production, attachment, and biofilm formation compared to wild-type strains. All biofilm-derived variants harbored SNPs in cps19F. SCVs reverted to SMV, but no reversion to the wild-type phenotype was noted, indicating that these variants were distinct from opaque- and transparent-phase variants. The SCV-SMV reversion frequency was dependent on growth conditions and treatment with tetracycline. Increased reversion rates were coincident with antibiotic treatment, implicating oxidative stress as a trigger for the SCV-SMV switch. We, therefore, evaluated the role played by hydrogen peroxide, the oxidizing chemical, in the reversion and emergence of variants. Biofilms of S. pneumoniae TIGR4-ΔspxB, defective in hydrogen peroxide production, showed a significant reduction in variant formation. Similarly, supplementing the medium with catalase or sodium thiosulfate yielded a significant reduction in variants formed by wild-type biofilms. Resistance to rifampin, an indicator for mutation frequency, was found to increase approximately 55-fold in biofilms compared to planktonic cells for each of the three wild-type strains examined. In contrast, TIGR4-ΔspxB grown as a biofilm showed no increase in rifampin resistance compared to the same cells grown planktonically. Furthermore, addition of 2.5 and 10 mM hydrogen peroxide to planktonic cells resulted in a 12- and 160-fold increase in mutation frequency, respectively, and gave rise to variants similar in appearance, biofilm-related phenotypes, and distribution of biofilm-derived variants. The results suggest that hydrogen peroxide and environmental conditions specific to biofilms are responsible for the development of non-phase-variable colony variants.     

The paternal-age effect in Apert syndrome is due,in part,to the increased frequency of mutations in sperm

A paternal-age effect and the exclusive paternal origin of mutations have been reported in Apert syndrome (AS). As the incidence of sporadic AS births increases exponentially with paternal age, we hypothesized that the frequency of AS mutations in sperm would also increase. To determine the frequency of two common FGFR2 mutations in AS, we developed allele-specific peptide nucleic acid-PCR assays. Analyzing sperm DNA from 148 men, age 21-80 years, we showed that the number of sperm with mutations increased in the oldest age groups among men who did not have a child with AS. These older men were also more likely to have both mutations in their sperm. However, this age-related increase in mutation frequency was not sufficient to explain the AS-birth frequency. In contrast, the mutation frequency observed in men who were younger and had children with AS was significantly greater. In addition, our data suggest selection for sperm with specific mutations. Therefore, contributing factors to the paternal-age effect may include selection and a higher number of mutant sperm in a subset of men ascertained because they had a child with AS. No age-related increase in the frequency of these mutations was observed in leukocytes. Selection and/or quality-control mechanisms, including DNA repair and apoptosis, may contribute to the cell-type differences in mutation frequency.     

The fate of mutations surfing on the wave of a range expansion

Many species, including humans, have dramatically expanded their range in the past, and such range expansions had certainly an impact on their genetic diversity. For example, mutations arising in populations at the edge of a range expansion can sometimes surf on the wave of advance and thus reach a larger spatial distribution and a much higher frequency than would be expected in stationary populations. We study here this surfing phenomenon in more detail, by performing extensive computer simulations under a two-dimensional stepping-stone model. We find that the probability of survival of a new mutation depends to a large degree on its proximity to the edge of the wave. Demographic factors such as deme size, migration rate, and local growth rate also influence the fate of these new mutations. We also find that the final spatial and frequency distributions depend on the local deme size of a subdivided population. This latter result is discussed in the light of human expansions in Europe as it should allow one to distinguish between mutations having spread with Paleolithic or Neolithic expansions. By favoring the spread of new mutations, a consequence of the surfing phenomenon is to increase the rate of evolution of spatially expanding populations.     

The importance of physiological limits in determining biogeographical range shifts due to global climate change: the heat-shock response

Physiological processes that set an organism's thermal limits are in part determining recent shifts in biogeographic distribution ranges due to global climate change. Several characteristics of the heat-shock response (HSR), such as the onset, maximal, and upper limit of heat-shock protein (Hsp) synthesis, contribute to setting the acute upper thermal limits of most organisms. Aquatic animals from stable, moderately variable, or highly variable thermal environments differ in their HSR. Some animals living in extremely stable thermal environments lack the response altogether. In contrast, rocky intertidal animals that experience highly variable thermal conditions start synthesizing Hsps, that is, the onset of synthesis, below the highest temperatures that they experience. Thus, these organisms experience thermal conditions in their environment that are close to the upper thermal limits in which they can defend themselves against cellular thermal insults by employing the HSR. Subtidal animals are characterized by moderately variable thermal environments, and their cells start synthesizing Hsps above the highest temperatures that they experience. The upper thermal limits against which they can defend themselves are thus much higher than the highest body temperatures they currently experience. Furthermore, the ability to acclimate to changing thermal conditions seems greatest among animals from moderately variable environments and limited in animals from stable and highly variable environments. Thus, these findings suggest that organisms with the narrowest (stenothermal) and the widest (highly eurythermal) temperature tolerance ranges live closest to their thermal limits and have a limited ability to acclimate, suggesting that they will be most affected by global climate change.     

The Pseudomonas aeruginosa reference strain PA14 displays increased virulence due to a mutation in ladS

Pseudomonas aeruginosa is a pathogen that causes acute and chronic infections in a variety of hosts. The pathogenic potential of P. aeruginosa is strain-dependent. PA14 is a highly virulent strain that causes disease in a wide range of organisms, whereas PAO1 is moderately virulent. Although PA14 carries pathogenicity islands that are absent in PAO1, the presence or absence of specific gene clusters is not predictive of virulence. Here, we show that the virulent strain PA14 has an acquired mutation in the ladS gene. This mutation has a deleterious impact on biofilm, while it results in elevated type III secretion system (T3SS) activity and increased cytotoxicity towards mammalian cells. These phenotypes can be reverted by repairing the ladS mutation on the PA14 genome. The RetS/LadS/GacS signaling cascade is associated with virulence and the switch between acute and chronic infections. RetS is a sensor that down-regulates biofilm formation and up-regulates the T3SS. Mutations in retS are acquired in strains isolated from chronically infected cystic fibrosis patients and lead to hyperbiofilm formation and reduced cytotoxicity. Conversely, the LadS sensor promotes biofilm formation and represses the T3SS. We conclude that the ladS mutation is partly responsible for the high cytotoxicity of PA14, and our findings corroborate the central role of RetS and LadS in the switch between acute and chronic infections. Given the extensive use of the reference strain PA14 in infection and virulence models, the bias caused by the ladS mutation on the observed phenotypes will be crucial to consider in future research.     

UV-mutagenesis at a cloned target sequence: converted suppressor mutation is insensitive to mutation frequency decline regardless of the gene orientation

Premutational lesions produced by ultraviolet radiation in the Gln2 tRNA genes of E. coli B/r show differing sensitivities to a mutation avoidance phenomenon known as mutation frequency decline (MFD). A mutation event that changes the wild-type gene to an amber (UAG) suppressor is normally sensitive to MFD. Mutation of this amber suppressor to an ochre (UAA) suppressor is not sensitive to MFD. These two mutation events occur in the same anticodon region of the DNA. The dissimilarity of MFD sensitivity between these two mutations may result because the respective premutational photoproducts for the two are located in opposite strands of duplex DNA. To examine the effect of strand position of the premutational lesions on MFD, recombinant lambda phage were constructed that contained the amber suppressor as a mutation target in the two possible orientations. Comparison of MFD in bacterial lysogens containing either of the two types of recombinant prophage indicated that reversing the orientation of the target sequence relative to adjacent bacterial DNA had no effect on MFD. Since rotational inversion of the target sequence did not alter the sensitivity to MFD of mutation occurring at the cloned target gene, the antimutation process inherent to MFD can not be attributed to an asymmetrical interaction between the template strands and the DNA-replication complex.     

The response of two butterfly species to climatic variation at the edge of their range and the implications for poleward range shifts

To predict changes in species' distributions due to climate change we must understand populations at the poleward edge of species' ranges. Ecologists generally expect range shifts under climate change caused by the expansion of edge populations as peripheral conditions increasingly resemble the range core. We tested whether peripheral populations of two contrasting butterflies, a small-bodied specialist (Erynnis propertius) and a large-bodied generalist (Papilio zelicaon), respond favorably to warmer conditions. Performance of populations related to climate was evaluated in seven peripheral populations spanning 1.2 degrees latitude (160 km) using: (1) population density surveys, an indirect measure of site suitability; and (2) organismal fitness in translocation experiments. There was evidence that population density increased with temperature for P. zelicaon whose population density declined with latitude in 1 of 3 sample years. On the other hand, E. propertius showed a positive relationship of population density with latitude, apparently unrelated to climate or measured habitat variables. Translocation experiments showed increased larval production at increased temperatures for both species, and in P. zelicaon, larval production also increased under drier conditions. These findings suggest that both species may increase at their range edge with warming but the preference for core-like conditions may be stronger in P. zelicaon. Further, populations of E. propertius at the range boundary may be large enough to act as sources of colonists for range expansions, but range expansion in this species may be prevented by a lack of available host plants further north. In total, the species appear to respond differently to climate and other factors that vary latitudinally, factors that will likely affect poleward expansion.     

The rate of beneficial mutations surfing on the wave of a range expansion

Many theoretical and experimental studies suggest that range expansions can have severe consequences for the gene pool of the expanding population. Due to strongly enhanced genetic drift at the advancing frontier, neutral and weakly deleterious mutations can reach large frequencies in the newly colonized regions, as if they were surfing the front of the range expansion. These findings raise the question of how frequently beneficial mutations successfully surf at shifting range margins, thereby promoting adaptation towards a range-expansion phenotype. Here, we use individual-based simulations to study the surfing statistics of recurrent beneficial mutations on wave-like range expansions in linear habitats. We show that the rate of surfing depends on two strongly antagonistic factors, the probability of surfing given the spatial location of a novel mutation and the rate of occurrence of mutations at that location. The surfing probability strongly increases towards the tip of the wave. Novel mutations are unlikely to surf unless they enjoy a spatial head start compared to the bulk of the population. The needed head start is shown to be proportional to the inverse fitness of the mutant type, and only weakly dependent on the carrying capacity. The precise location dependence of surfing probabilities is derived from the non-extinction probability of a branching process within a moving field of growth rates. The second factor is the mutation occurrence which strongly decreases towards the tip of the wave. Thus, most successful mutations arise at an intermediate position in the front of the wave. We present an analytic theory for the tradeoff between these factors that allows to predict how frequently substitutions by beneficial mutations occur at invasion fronts. We find that small amounts of genetic drift increase the fixation rate of beneficial mutations at the advancing front, and thus could be important for adaptation during species invasions.     

Limits to the niche and range margins of alien species

We discuss the apparent paradox that while introduced populations often adapt rapidly to conditions in the new range, it is normally assumed that the species’ niche remains unchanged. Focusing on plants, we argue that studies of the niche dynamics of alien species are useful for understanding the constraints acting on species in their native ranges, and vice versa. Most hypotheses about species ecological range margins are more consistent with there being a niche shift than niche stasis in the new range. After reviewing the evidence for niche shifts in alien species, we suggest that the probability of a shift occurring depends primarily upon the ecological and genetic processes limiting the species in its native range. For example, a fundamental niche shift might occur if introduced individuals are released from maladaptive gene flow from central populations, or if genetic diversity is increased by the mixing of individuals from different sources. In addition, other factors such as species characteristics, introduction history and conditions in the new range may also influence whether a niche shift occurs. Based on these considerations, we propose conditions under which niche shifts are most likely. Such understanding is important for predicting and mitigating current and future anthropogenic impacts on species ranges.     

Phase shifts of human circadian rhythms due to shifts of artificial Zeitgebers

In special isolation units, circadian rhythms of human subjects have been investigated under the influence of artificial 24-h Zeitgebers, with 6-h advance and 6-h delay shifts of the Zeitgeber simulating time zone shifts. In most cases, the biological rhythms follow the Zeitgeber shifts in the course of several days: in rare cases, advancing Zeitgeber shifts are followed by delaying shifts of the biological rhythms, either of all variables or, partitioning, of only some of the variables. The rhythm of activity is re-entrained after both Zeitgeber shifts within a few days, independent of the shift direction. The rhythm of rectal temperature needs more time for re-entrainment than the activity rhythm; the rate of re-entrainment is consistently higher after advance than after delay shifts ('direction asymmetry'). Mean value and amplitude of the rectal temperature rhythm are, for some days, reduced after the advance but not after the delay Zeitgeber shift; among the different subjects, the reduction in amplitude is significantly correlated with the direction asymmetry. The rhythm of psychomotor performance (computation speed) re-entrains in parallel to that of rectal temperature; i.e. the performance level is decreased after advance but not after delay shifts. The direction asymmetry in the re-entrainment rates seems to contradict findings in flight experiments where this rate is mostly higher after westward than after eastward flights. Careful considerations, however, show that differences in the re-entrainment behavior after real and simulated time zone shifts disappear when the experimental designs are approximated and when identical procedures of analyzing the data are applied. The results of the time shift experiments are, in all respects tested, in agreement with theoretical postulations; hence, they confirm once more properties of the circadian system deduced earlier. On the other hand, the results are of practical importance since they state significant correlations between the re-entrainment behavior and rhythm parameters measured before the Zeitgeber shifts; this behavior, therefore, can be predicted from data obtained already before the Zeitgeber has been changed in any way: The duration of re-entrainment is correlated with the amplitude, and the decrement in performance with the phase of the rectal temperature rhythm. These practical implications may also apply to shift work.     

Apparent longevity-related adaptation of mitochondrial amino acid content is due to nucleotide compositional shifts

Recent studies across animal phyla have suggested a possible link between amino acid compositional shifts and adaptive evolution across mitochondrial proteomes enabling longer lifespans. These studies examined associations of a gradual loss of cysteine (Cys) residues, increased usage of methionine (Met), and increased usage of threonine (Thr), with the evolution of longevity. Here, we examine all three hypotheses in a framework that considers nucleotide composition. We find that nucleotide composition is strongly correlated across codon positions, and with the above amino acid frequency patterns. We also find that the ND6 gene, which in vertebrates is the only mitochondrial gene situated on the “light-strand” shows no significant pattern for any of the amino acid associations. We also reasoned that if the mitochondrially-encoded proteins of oxidative phosphorylation (OXPHOS) were under selection for such shifts, then nuclear-encoded components should also reflect such pressure. However, we found non-correspondence of these patterns in the nuclear genes when compared to the mitochondrial genes previously associated with positive selection. These results are strongly suggestive of mutational bias, or less efficient purifying selection, as the primary driver of whole proteome shifts in amino acid composition.     

The evolution of growth rates on an expanding range edge

Individuals in the vanguard of a species invasion face altered selective conditions when compared with conspecifics behind the invasion front. Assortment by dispersal ability on the expanding front, for example, drives the evolution of increased dispersal, which, in turn, leads to accelerated rates of invasion. Here I propose an additional evolutionary mechanism to explain accelerating invasions: shifts in population growth rate (r). Because individuals in the vanguard face lower population density than those in established populations, they should (relative to individuals in established populations) experience greater r-selection. To test this possibility, I used the ongoing invasion of cane toads (Bufo marinus) across northern Australia. Life-history theory shows that the most efficient way to increase the rate of population growth is to reproduce earlier. Thus, I predict that toads on the invasion front will exhibit faster individual growth rates (and thus will reach breeding size earlier) than those from older populations. Using a common garden design, I show that this is indeed the case: both tadpoles and juvenile toads from frontal populations grow around 30 per cent faster than those from older, long established populations. These results support theoretical predictions that r increases during range advance and highlight the importance of understanding the evolution of life history during range advance.     

A method for simulating patterns of habitat availability at static and dynamic range margins

The dynamics of populations inhabiting range margins are likely to be critically important in determining the response of species to climate change. Despite this, there is a lack of both empirical and theoretical work that examines the behaviour of these populations. Populations living on the edge of a species' range frequently inhabit a more patchily distributed habitat than those that live closer to the centre of the range. This difference is likely to play an important role in determining the dynamics of range margin populations, both when the range is static and when it is dynamic, for example shifting in response to climate change. Here, we present a simple method that simulates the distribution of suitable habitat sites at the edge of a range. Habitat availability is determined as a function of both latitudinal and local environmental variability, and the relative importance of the two can be adjusted. The method is readily extended to describe shifting habitat availability during a period of climate change. We suggest that there is a need for a greater effort to examine the ecology of range margin populations, and believe that the method presented here could be of considerable use in future theoretical studies.     

Body fat-lowering effect of conjugated linoleic acid is not due to increased lipolysis

The ability of conjugated linoleic acid (CLA) to reduce adiposity may be due to changes in energy expenditure and/or direct effects on adipocyte lipid metabolism. The aim of the present work was to analyse if CLA supplementation modifies lipolytic activity in adipose tissue from hamsters fed on high-fat diet. Hamsters were divided into two groups and fed on diets supplemented with either 0.5% linoleic acid (control) or 0.5% trans-10,cis-12 CLA. After 6 weeks, animals were fasted overnight and adipose tissues were dissected and weighed. Adipocytes were isolated by collagenase digestion and incubated in Krebs-Ringer bicarbonate buffer with or without several agents acting at different levels of the lipolytic cascade. Adipocyte diameters were measured by microscopy. Adipose tissue DNA content was assessed by spectrophotometry. Animals fed on CLA diet showed significantly reduced adipose tissue mass. No differences between both groups was found for basal lipolysis, lipolytic effects of isoproterenol, forskolin, dibutyryl-cAMP and isobutylmethylxanthine, and pD2 for isoproterenol. A similar total DNA amount was found in adipose tissue of both groups, showing that CLA diet had no effect on total cell number per fat pad. Although DNA content per gram tissue, an indirect reverse index of cell size, was significantly increased in CLA fed hamsters, microscopy did not reveal differences in medium mature adipocyte diameter, nor in cell size distribution between both groups. These results suggest that adipose tissue size reduction induced by trans-10,cis-12 CLA intake is not due to changes in lipolysis. Reduced preadipocyte differentiation into mature adipocytes may account for this fat-lowering effect.     

Susceptibility of the male fitness phenotype to spontaneous mutation

Adult reproductive success can account for a large fraction of male fitness, however, we know relatively little about the susceptibility of reproductive traits to mutation-accumulation (MA). Estimates of the mutational rate of decline for adult fitness and its components are controversial in Drosophila melanogaster, and post-copulatory performance has not been examined. We therefore separately measured the consequences of MA for total male reproductive success and its major pre-copulatory and post-copulatory components: mating success and sperm competitive success. We also measured juvenile viability, an important fitness component that has been well studied in MA experiments. MA had strongly deleterious effects on both male viability and adult fitness, but the latter declined at a much greater rate. Mutational pressure on total fitness is thus much greater than would be predicted by viability alone. We also noted a significant and positive correlation between all adult traits and viability in the MA lines, suggesting pleiotropy of mutational effect as required by 'good genes' models of sexual selection.