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The regulation of amidase synthesis in P. aeruginosa is under positive control. This review describes the experimental evolution of amidase and its regulator protein for the hydrolysis of novel substrates and experiments to elucidate the mechanism of the control system.
Computed tomographic (CT) and high-resolution ultrasound (HRUS) imaging of experimental brain abscess were correlated with neuropathologic findings in nine mongrel dogs. The HRUS scan was more sensitive to different histologic features than the CT scan but both accurately delineated the evolution of the experimental brain abscess. All stages of abscess evolution were characterized by an appearance of an echogenic rim with a hypoechoic center. In the early stages the echogenicity of the abscess was related primarily to marked cellular infiltration, while in the late stages extensive collagen deposition correlated closely with the echo pattern. The size of the abscess in the cerebritis stages appeared smaller on the HRUS scan than on the CT scan because the latter modality detected the extensive cerebritis around the developing necrotic center whereas the HRUS scan did not. This discrepancy disappeared in the capsule stages. The HRUS scan provided a more accurate depiction of the neuropathologic characteristics of the necrotic than did the CT scan. Healing of the abscess, indicated by a decrease in size of the hypoechoic center, was accurately detected by the HRUS scan.
Polyploidization and regional gene duplication have occurred frequently during vertebrate evolution, providing the genetic material necessary for creating evolutionary novelties. Mammals, including man, can be regarded as diploid species with a polyploid history of evolution. Polyploidization steps during the phylogeny of mammals probably took place in the genomes of amphibian- or fish-like mammalian ancestors. The polyploid status has subsequently been shaped by the process of diploidization, leading to genomes that are polyploid with respect to the amount of genetic material and the number of gene copies, and diploid with respect to the level of gene expression and chromosomal characteristics. Phylogenetically tetraploid amphibian and teleost species together with their diploid close relatives can be used as a model system to study the effect of polyploidization and the mechanisms of diploidization of a parallel event during early mammalian evolution. Experimental evidence permits the assumption that the diploidization of gene expression in tetraploid cyprinid fish may be functionally correlated with structural modifications of the ribosomal components, RNA and protein. These findings are discussed in the light of reduced protein synthesis in diploidized tetraploid species and a mechanism to explain diploidization during mammalian evolution.
Microbial populations strongly shape their environment, which can re-route adaptation toward organism-generated fitness optima. However, the conditions that promote these eco-evolutionary feedbacks are unclear. Here, we used experimental evolution to test whether high population density, by strengthening niche construction, drives eco-evolutionary feedbacks in the bacterial pathogen Pseudomonas aeruginosa (Pa) MPAO1. We tested for adaptation to organism-modified environments by measuring the relative performance of ancestral and endpoint populations in filtrate generated by each evolutionary line sampled across generations. Contrary to expectations, we found that endpoint populations had higher performance than the ancestral strain in filtrate across nearly all evolutionary lines regardless of population density. This was caused by the emergence of hyperactive filamentous bacterio(phage) mutants during experimental passaging that inhibited the ancestral strain but not endpoint populations in modified media. Hyperactive phages emerged from one of two avirulent prophages in MPAO1's genome (Pf4 or Pf6). Hyperactive phages drove the evolution of phage resistance in bacterial populations via mutations in the type IV pilus (TIVP), the phage's binding receptor. In a follow-up experiment, we showed that these TIVP mutations pleiotropically reduced motility and conferred resistance to a TIVP-targeting virulent phage, both of which are important traits for Pa infection and treatment. Overall, this work suggests that filamentous phage evolution can drive eco-evolutionary feedbacks in bacterial populations, causing phenotypic and genetic changes that would not be anticipated from adaptation to the extrinsic environment alone.
The evolution of a deoxyribonucleotide synthesizing ribonucleotide reductase might have initiated the transition from the ancient RNA world into the prevailing DNA world. At least five classes of ribonucleotide reductases have evolved. The ancient enzyme has not been identified. A reconstruction of the first ribonucleotide reductase requires knowledge of contemporary enzymes and of microbial evolution. Experimental work on the former focuses on few organisms, whereas the latter is now well understood on the basis of ribosomal RNA sequences. Deoxyribonucleotide formation has not been investigated in many evolutionary important microorganisms. This review covers our knowledge on deoxyribonucleotide synthesis in microorganisms and the distribution of ribonucleotide reductases in nature. Ecological constraints on enzyme evolution and knowledge deficiencies emerge from complete coverage of the phylogenetic groups.
Evolutionary repair refers to the compensatory evolution that follows perturbations in cellular processes. While evolutionary trajectories are often reproducible, other studies suggest they are shaped by genotype-by-environment (GxE) interactions. Here, we test the predictability of evolutionary repair in response to DNA replication stress-a severe perturbation impairing the conserved mechanisms of DNA synthesis, resulting in genetic instability. We conducted high-throughput experimental evolution on Saccharomyces cerevisiae experiencing constitutive replication stress, grown under different glucose availability. We found that glucose levels impact the physiology and adaptation rate of replication stress mutants. However, the genetics of adaptation show remarkable robustness across environments. Recurrent mutations collectively recapitulated the fitness of evolved lines and are advantageous across macronutrient availability. We also identified a novel role of the mediator complex of RNA polymerase II in adaptation to replicative stress. Our results highlight the robustness and predictability of evolutionary repair mechanisms to DNA replication stress and provide new insights into the evolutionary aspects of genome stability, with potential implications for understanding cancer development.
In the present article we examine clonality in virus evolution. Most viruses retain an active recombination machinery as a potential means to initiate new levels of genetic exploration that go beyond those attainable solely by point mutations. However, despite abundant recombination that may be linked to molecular events essential for genome replication, herein we provide evidence that generation of recombinants with altered biological properties is not essential for the completion of the replication cycles of viruses, and that viral lineages (near-clades) can be defined. We distinguish mechanistically active but inconsequential recombination from evolutionarily relevant recombination, illustrated by episodes in the field and during experimental evolution. In the field, recombination has been at the origin of new viral pathogens, and has conferred fitness advantages to some viruses once the parental viruses have attained a sufficient degree of diversification by point mutations. In the laboratory, recombination mediated a salient genome segmentation of foot-and-mouth disease virus, an important animal pathogen whose genome in nature has always been characterized as unsegmented. We propose a model of continuous mutation and recombination, with punctuated, biologically relevant recombination events for the survival of viruses, both as disease agents and as promoters of cellular evolution. Thus, clonality is the standard evolutionary mode for viruses because recombination is largely inconsequential, since the decisive events for virus replication and survival are not dependent on the exchange of genetic material and formation of recombinant (mosaic) genomes.
The central structural feature of natural proteins is a tightly packed and highly ordered hydrophobic core. If some measure of exquisite, native-like core packing is necessary for enzymatic function, this would constitute a significant obstacle to the development of novel enzymes, either by design or by natural or experimental evolution. To test the minimum requirements for a core to provide sufficient structural integrity for enzymatic activity, we have produced mutants of the ribonuclease barnase in which 12 of the 13 core residues have together been randomly replaced by hydrophobic alternatives. Using a sensitive biological screen, we find that a strikingly high proportion of these mutants (23%) retain enzymatic activity in vivo. Further substitution at the 13th core position shows that a similar proportion of completely random hydrophobic cores supports enzyme function. Of the active mutants produced, several have no wild-type core residues. These results imply that hydrophobicity is nearly a sufficient criterion for the construction of a functional core and, in conjunction with previous studies, that refinement of a crudely functional core entails more stringent sequence constraints than does the initial attainment of crude core function. Since attainment of crude function is the critical initial step in evolutionary innovation, the relatively scant requirements contributed by the hydrophobic core would greatly reduce the initial hurdle on the evolutionary pathway to novel enzymes. Similarly, experimental development of novel functional proteins might be simplified by limiting core design to mere specification of hydrophobicity and using iterative mutation-selection to optimize core structure.
1. Structural changes in the thymus during the evolution experimental renal hypertension were investigated to determine their possible role in the genesis of hypertensive vascular disease. 2. The thymus, adrenal glands and the progression of hypertensive vascular lesions were investigated in rats during the first 30 days after occlusion of the aorta between the two renal arteries. 3. Hypertension was initially accompanied by marked atrophy of the thymus, most pronounced 9 days after operation. During this time, the adrenal glands doubled in size and the heart became enlarged. 4. After 21 days the thymus regenerated and became hypertrophic. Histological features of hyperactivity accompanied by infiltration of plasma cells were evident, while the adrenal glands remained enlarged. 5. The observed structural changes of the regenerated thymus in the presence of sustained adrenal hypertrophy indicate that the thymus may contribute to the production of hypertensive vascular disease.
Males and females experience differences in the strength and direction of selection but discerning the type of genes that are targets of sex differences in selection is complicated by their shared genome. We used experimental evolution in Drosophila melanogaster to partially separate the gene pools of males and females for 130 generations. In six replicate populations, we forced one pool of genetically variable Chromosome 2s to experience patrilinear inheritance (segregating like a Y-chromosome) and male-limited selection. The alternative pool segregated like an X-chromosome and experienced female-biased selection. This allowed alleles which are differentially selected for between the sexes to diverge between these pools, enabling us to gain insight into the type of genes subject to such selection. We find that genes which diverge between these pools have an elevated intersexual genetic correlation(rMF) for expression on average, consistent with the idea that high genetic correlations may hinder sex-specific adaptation under normal inheritance. Diverged genes were also enriched for moderately male-biased genes whereas female-biased genes were underrepresented. At the SNP level, we find an overrepresentation of diverged SNPs involved in splicing or occurring in the 5'UTR and an underrepresentation of missense or synonymous SNPs, suggesting sex differences in selection for isoform usage.
Despite widespread biodiversity loss, our understanding of how species and populations will respond to accelerated climate change remains limited. In this study, we integrate population genomics, experimental evolution, and environmental modeling to elucidate the evolutionary responses to climate change in Populus lasiocarpa, a key alpine forest tree species primarily distributed in the mountainous regions of a global biodiversity hotspot. Over historical timescales, our findings demonstrate that demographic dynamics, divergent selection, and long-term balancing selection have shaped and maintained genetic variation within and between populations. In examining genomic signatures of contemporary climate adaptation, we found that haplotype blocks, potentially caused by inversion polymorphisms that suppress recombination, are linked to enriched combinations of locally adaptive environmental variations. We further assessed the relative contributions of environmentally induced plastic responses, constitutive expression divergence between genetic clusters, and their interactions in driving gene expression variation and divergence. Notably, we observed a strong correlation between sequence divergence and constitutive differential expression among genetic clusters. Finally, by incorporating genetic adaptation, migration, and genetic load into our predictions of population-level climate change risks, we identified western populations-primarily distributed in the Hengduan Mountains, a region known for its environmental heterogeneity and significant biodiversity-as the most vulnerable to climate change. These populations should be prioritized for conservation and management. Overall, our study advances the understanding of the relative roles of long-term natural selection, local environmental adaptation, and immediate plastic expression changes in shaping the responses of natural populations of keystone species to climate change.
A family of mutant amidases has been derived by experimental evolution of the aliphatic amidase of Pseudomonas aeruginosa strain PAC1. Mutation amiE16, in the structural gene for the enzyme, results in the production of the mutant B amidase by strain B6. This strain, unlike the wild-type, can utilize butyramide for growth. Strain B6 gave rise by a single mutational event to strain V9, utilizing valeramide, and strain PhB3, utilizing phenylacetamide. Strain V9 was not itself able to utilize phenylacetamide but gave rise by mutation to the phenylacetamide-utilizing mutant PhV1. Peptide 108 was isolated from chymotryptic digests of mutant amidases from strains B6, PhB3 and PhV1, but could not be detected in chymotryptic digests of the wild-type amidase. The sequence of peptide 108 was established as Met-Arg-His-Gly-Asp-Ile-Phe. Thermolytic digests of mutant amidases from strains B6, PhB3, PhV1 and V9 were compared with digests of the wild-type amidase. A peptide of the composition Met, Arg, His, Gly2, Asp3, Ile, Ser3, Thr, Val was found in the digest of the wild-type amidase and was replaced in the digests of the mutant amidases by a peptide of the composition Met, Arg, His, Gly2, Asp3, Ile, Ser3, Thr, Val, Phe. Mutation amiE16 is common to the four mutant enzymes and can be accounted for by the mutation Ser leads to Phe. The sequence of the chymotryptic peptide corresponds with the N-terminal sequence of the amidase protein, and can also be related to the thermolysin peptides. It is concluded that mutation amiE16 is a Ser leads to Phe change at position 7 from the N-terminus and the effect of this on the enzyme conformation is discussed.
We have previously described a system of experimental evolution in which many of the mutants of Klebsiella aerogenes selected for faster growth on xylitol ('evolvants') synthesized elevated levels of ribitol dehydrogenase and have presented genetic evidence implicating gene duplication in the enzyme superproduction in some of the evolvants. Here we describe a physical approach to the screening for gene duplications and subsequent structure determination. Nick-translated, cloned ribitol operon (rbt) DNA was used as a hybridization probe to identify fragments containing rbt operon sequences in restriction digests of total bacterial DNA. Whilst several of the evolvants probably harbour duplications spanning the entire rbt operon, one of the spontaneously arising evolvants (strain A3) was shown to harbour a small (5.8 kilobase pairs) direct DNA repeat which encodes the dehydrogenase (but not the kinase) of the closely linked D-arabitol operon as well as the dehydrogenase (but not the kinase) of the rbt operon. The hybridization data suggest that there are 4 to 5 copies of the repeat arranged contiguously on the chromosome. The genetic instability of strain A3, the rbt fragment hybridization pattern of an A3 segregant and the activities of the pentitol catabolic enzymes in A3 are all consistent with the proposed gene duplication structure.
Molecular investigations of evolutionary history are being used to study subjects as diverse as the epidemiology of acquired immune deficiency syndrome and the origin of life. These studies depend on accurate estimates of phylogeny. The performance of methods of phylogenetic analysis can be assessed by numerical simulation studies and by the experimental evolution of organisms in controlled laboratory situations. Both kinds of assessment indicate that existing methods are effective at estimating phylogenies over a wide range of evolutionary conditions, especially if information about substitution bias is used to provide differential weightings for character transformations.
A small percentage of species in the fungal kingdom can cause devastating infections in humans, with Candida albicans reigning as a leading cause of systemic disease. One of the key virulence phenotypes for pathogenic fungi is the ability to survive at host body temperature; however, a comprehensive understanding of the mechanisms that orchestrate thermal adaptation in fungi remains incomplete. In this study, we expand the largest functional genomics resource in C. albicans, reaching 71.3% coverage of the entire genome, and perform screens under six different temperatures to identify genes important for temperature-dependent fitness. We describe the function of genes involved in translation (GAR1), splicing (C1_11680C or YSF3), and cell cycle progression (C6_00110C or RHT1) in enabling fungal survival at both low and high temperatures. Through experimental evolution, we also show that C. albicans can rapidly overcome deleterious mutations and adapt to extreme temperature environments. Overall, our study highlights the transformative potential of genome-wide functional genomics to uncover critical vulnerabilities in pathogenic fungi.
Pathways of intrinsic resistance in bacteria are promising targets for novel antibiotics and resistance breakers. Here, we used a genome-wide screen to identify single gene knockouts of Escherichia coli that were hypersusceptible to trimethoprim and chloramphenicol, two chemically diverse broad-spectrum antibiotics. Among the hits from our screen, knockouts of acrB, an efflux pump, and rfaG or lpxM, both involved in cell envelope biogenesis, were hypersensitive to multiple antimicrobials and could sensitize genetically resistant E. coli strains to antibiotics. Using experimental evolution under trimethoprim pressure, we show that high drug selection regimes drove these knockouts to extinction more frequently than wild type. Among them, ΔacrB was most compromised in its ability to evolve resistance, establishing it as a promising target for "resistance proofing." At a sub-inhibitory trimethoprim concentration, however, all three knockouts adapted to the antibiotic and consequently recovered from hypersensitivity, albeit to different extents. This recovery was driven by mutations in drug-specific resistance pathways, rather than compensatory evolution, frequently involving upregulation of the drug target. Notably, resistance-conferring mutations could by-pass defects in cell wall biosynthesis more effectively than efflux even though resistant mutations did not directly engage either pathway. Since inhibiting drug-efflux emerged as a better strategy, we tested the ability of chlorpromazine, an efflux pump inhibitor (EPI), to resistance proof E. coli against trimethoprim. While qualitatively similar in the short term, genetic and pharmacological inhibition differed dramatically on an evolutionary time scale due to evolution of resistance to the EPI. Further, adaptation to the EPI-antibiotic pair also led to multidrug adaptation. The lack of concordance between genetic and pharmacological inhibition revealed a crucial lacuna in our understanding of the mutational repertoires that facilitate adaptation to antibiotics in bacteria. We propose that while intrinsic resistance mechanisms are effective targets for antibiotic sensitization, rapid evolutionary recovery may significantly limit their utility.
Infective endocarditis (IE) is a life-threatening disease most often caused by blood-borne bacteria that infect previously damaged cardiac tissue. Despite the importance of this disease, the genetic basis for IE-associated fitness remains poorly defined. Here, we present the first genome-wide in vivo analysis of bacterial fitness in a vertebrate model of IE. We identified 146 genes in Streptococcus sanguinis required for IE fitness, the majority of which had not previously been linked to endocarditis. These determinants cluster into conserved metabolic, cell envelope, transport, and regulatory pathways, representing a vast reservoir of potential targets for novel antimicrobial intervention. A subset of these genes was examined in Streptococcus mutans; all were found to be essential for IE fitness in this distantly related oral species as well, suggesting broad conservation. Using experimental evolution, we further show that disruption of key fitness pathways triggers reproducible compensatory "bypass" mechanisms. Together, these findings provide a comprehensive, genome-wide map of the bacterial niche-requirements for streptococcal infective endocarditis.
Xenogeneic silencers, such as histone-like nucleoid structuring protein (H-NS), are critical for maintaining horizontally acquired genes in bacterial genomes and minimizing fitness costs associated with inappropriate expression. For bacterial pathogens, this has enabled the acquisition of costly virulence regulons, with H-NS balancing the need for tight silencing with rapid expression in host environments. For Salmonella enterica serovar Typhimurium (STm), survival in these environments relies on phenotypic heterogeneity in virulence gene expression and evolutionary adaptation. Although complete loss of hns is highly deleterious in STm, how subtle impairments to this global silencer disrupt heterogeneity in virulence gene expression and alter adaptation to host environments remains poorly understood. Here, we identify an STm hns hypomorph strain and find that its reduced H-NS DNA-binding affinity increases the proportion of virulence-expressing cells, resulting in enhanced epithelial cell infection in vitro. Furthermore, through experimental evolution in intracellular-like conditions in vitro, we demonstrate that both wild-type and mutant populations converge on disrupting the SPI-2 virulence regulon to improve fitness; however, the mutant population also acquires distinct adaptive mutations to resolve the underlying dysregulation in gene expression. These results suggest that H-NS sets single-cell virulence activation thresholds and that even minor disruptions to its silencing function impact pathogen adaptation, highlighting its role as a critical evolutionary buffer.