Non-lethal Mutations Research Articles

Callus Induction, Regeneration of Rice Variety BRRI Dhan29 and Determination of Lethal Dose Of Ethyl Methanesulfonate

Rice is an essential meal for a large section of the world's population including Bangladesh. BRRI Dhan29 is known as an excellent rice variety in Bangladesh but it is now susceptible to several diseases and pests, which might reduce rice yields. Therefore, it becomes necessary to explore and optimize different approach with the aim of enhancing yield, diseases resistance and overall genetic variability of this important rice variety. Ethyl methanesulphonate (EMS) produces a large number of non-lethal point mutations by alkylates guanine bases and leads to mispairing-alkylated guanine pairs with thiamine instead of cytosine. The LD50 of EMS is frequently used as a general indicator of a substance's acute toxicity and a lower LD50 is indicative of increased toxicity. This study aimed to develop a comprehensive protocol for callus induction, regeneration for the BRRI dhan29 rice and to determine the LD50% of EMS on callus survival for further genetic improvement. Mature embryos were used to induce callus by utilizing different concentrations (0.00, 0.5, 1.00, 1.50, 2.00, 2.50, 3.00 and 3.50 mg/L) of 2,4-Dichlorophenoxyactic acid (2, 4-D), while callus regeneration was studied using varying combine concentrations of 1-naphthaleneacetic acid (NAA: 0, 5, 10 and 15 μg/L) and kinetin (0.00, 1.00, 2.00 and 3.00 ml/L). The optimal concentrations for callus induction was 2.0 mg/L 2, 4-D and a combination of 10 μg/L NAA and 2.0 mg/L kinetin were identified for efficient callus regeneration. The LD50 of EMS on BRRI dhan29 calli was 0.31%, indicating its potential mutagen for creating genetic variations. Regenerated plants were successfully acclimatized at field conditions. These findings might provide valuable insights for tissue culture-based studies and genetic improvement efforts for the BRRI dhan29. Bangladesh J. Nuclear Agric, 37(1): 1-10, 2023

The impact of Fisher's reproductive compensation on raising equilibrium frequencies of semidominant, nonlethal mutations under mutation/selection balance.

Fisher's reproductive compensation (fRC) occurs when a species' demography means the death of an individual results in increased survival probability of his/her relatives, usually assumed to be full sibs. This likely occurs in many species, including humans. Several important recessive human genetic diseases cause early foetal/infant death allowing fRC to act on these mutations. The impact of fRC on these genetic conditions has been previously calculated and shown to be substantial as quantified by ω, the fold increase in equilibrium frequencies of the mutation under fRC compared with its absence, i.e. ω = 1.22 and ω = 1.33 for autosomal and sex-linked loci, respectively. However, the impact of fRC on the frequency of the much larger class of semidominant, nonlethal mutations is unknown. This is calculated here as ω = 2 - h*s for autosomal loci and ω up to 2 for sex-linked loci where h is dominance (varied between 0.05 and 0.95) and s is selection coefficient (varied between 0.05 and 0.9). These results show that the actions of fRC can almost double the equilibrium frequency of deleterious mutations with low values of h and/or s (noting that "low" is s∼0.05 to 0.1). It is noted that fRC may act differentially across the genome with genes expressed early in life being fully exposed to fRC while those expressed later in life may be unaffected; this could lead to systematic differences in deleterious allele frequency across the genome.

Reducing PHYTOENE SYNTHASE activity fine-tunes the abundance of a cis-carotene-derived signal that regulates the PIF3/HY5 module and plastid biogenesis.

PHYTOENE SYNTHASE (PSY) is a rate-limiting enzyme catalysing the first committed step of carotenoid biosynthesis, and changes in PSY gene expression and/or protein activity alter carotenoid composition and plastid differentiation in plants. Four genetic variants of PSY (psy-4, psy-90, psy-130, and psy-145) were identified using a forward genetics approach that rescued leaf virescence phenotypes and plastid abnormalities displayed by the Arabidopsis CAROTENOID ISOMERASE (CRTISO) mutant ccr2 (carotenoid and chloroplast regulation 2) when grown under a shorter photoperiod. The four non-lethal mutations affected alternative splicing, enzyme-substrate interactions, and PSY:ORANGE multi-enzyme complex binding, constituting the dynamic post-transcriptional fine-tuning of PSY levels and activity without changing localization to the stroma and protothylakoid membranes. psy genetic variants did not alter total xanthophyll or β-carotene accumulation in ccr2, yet they reduced specific acyclic linear cis-carotenes linked to the biosynthesis of a currently unidentified apocarotenoid signal regulating plastid biogenesis, chlorophyll biosynthesis, and photomorphogenic regulation. ccr2 psy variants modulated the PHYTOCHROME-INTERACTING FACTOR 3/ELONGATED HYPOCOTYL 5 (PIF3/HY5) ratio, and displayed a normal prolamellar body formation in etioplasts and chlorophyll accumulation during seedling photomorphogenesis. Thus, suppressing PSY activity and impairing PSY:ORANGE protein interactions revealed how cis-carotene abundance can be fine-tuned through holoenzyme-metabolon interactions to control plastid development.

Genetic basis of the historical iron-accumulating dgl and brz mutants in pea.

The Pisum sativum (pea) mutants degenerate leaves (dgl) and bronze (brz) accumulate large amounts of iron in leaves. First described several decades ago, the two mutants have provided important insights into iron homeostasis in plants but the underlying mutations have remained unknown. Using exome sequencing we identified an in-frame deletion associated with dgl in a BRUTUS homolog. The deletion is absent from wild type and the original parent line. BRUTUS belongs to a small family of E3 ubiquitin ligases acting as negative regulators of iron uptake in plants. The brz mutation was previously mapped to chromosome 4, and superimposing this region to the pea genome sequence uncovered a mutation in OPT3, encoding an oligopeptide transporter with a plant-specific role in metal transport. The causal nature of the mutations was confirmed by additional genetic analyses. Identification of the mutated genes rationalizes many of the previously described phenotypes and provides new insights into shoot-to-root signaling of iron deficiency. Furthermore, the non-lethal mutations in these essential genes suggest new strategies for biofortification of crops with iron.

Hyperphosphorylated PTEN exerts oncogenic properties

PTEN is a multifaceted tumor suppressor that is highly sensitive to alterations in expression or function. The PTEN C-tail domain, which is rich in phosphorylation sites, has been implicated in PTEN stability, localization, catalytic activity, and protein interactions, but its role in tumorigenesis remains unclear. To address this, we utilized several mouse strains with nonlethal C-tail mutations. Mice homozygous for a deletion that includes S370, S380, T382 and T383 contain low PTEN levels and hyperactive AKT but are not tumor prone. Analysis of mice containing nonphosphorylatable or phosphomimetic versions of S380, a residue hyperphosphorylated in human gastric cancers, reveal that PTEN stability and ability to inhibit PI3K-AKT depends on dynamic phosphorylation-dephosphorylation of this residue. While phosphomimetic S380 drives neoplastic growth in prostate by promoting nuclear accumulation of β-catenin, nonphosphorylatable S380 is not tumorigenic. These data suggest that C-tail hyperphosphorylation creates oncogenic PTEN and is a potential target for anti-cancer therapy.

Bulk segregant analysis-sequencing and RNA-Seq analyses reveal candidate genes associated with albino phenotype in Brassica napus.

Inheritable albino mutants are excellent models for exploring the mechanism of chloroplast biogenesis and development. However, only a few non-lethal albino mutations have been reported to date in Brassica species. Here, we describe a resynthesized Brassica napus mutant, whose leaf, stem, and silique tissues showed an inheritable albino phenotype under field conditions after the bud stage but green phenotype in the greenhouse during the whole growing season, indicating that the albino phenotype depends on environmental conditions. Compared with the green leaves of the field-grown wild-type (GL) and greenhouse-grown mutant (WGL) plants, white leaves of the field-grown mutant (WL) showed significantly lower chlorophyll contents and structural defects in chloroplasts. Genetic analysis revealed that the albino phenotype of WL is recessive and is controlled by multiple genes. Bulk segregant analysis-sequencing (BSA-Seq) indicated that the candidate regions responsible for the albino phenotype spanned a total physical distance of approximately 49.68 Mb on chromosomes A03, A07, A08, C03, C04, C06, and C07. To gain insights into the molecular mechanisms that control chloroplast development in B. napus, we performed transcriptome (RNA-Seq) analysis of GL, WGL, and WL samples. GO and KEGG enrichment analyses suggested that differentially expressed genes (DEGs) associated with leaf color were significantly enriched in photosynthesis, ribosome biogenesis and chlorophyll metabolism. Further analysis indicated that DEGs involved in chloroplast development and chlorophyll metabolism were likely the main factors responsible for the albino phenotype in B. napus. A total of 59 DEGs were screened in the candidate regions, and four DEGs (BnaC03G0522600NO, BnaC07G0481600NO, BnaC07G0497800NO, and BnaA08G0016300NO) were identified as the most likely candidates responsible for the albino phenotype. Altogether, this study provides clues for elucidating the molecular mechanisms underlying chloroplast development in B. napus.

Functional Properties of Two Distinct PTH1R Mutants Associated With Either Skeletal Defects or Pseudohypoparathyroidism.

ABSTRACTConsistent with a vital role of parathyroid hormone (PTH) receptor type 1 (PTH1R) in skeletal development, homozygous loss‐of‐function PTH1R mutations in humans results in neonatal lethality (Blomstrand chondrodysplasia), whereas such heterozygous mutations cause a primary failure of tooth eruption (PFE). Despite a key role of PTH1R in calcium and phosphate homeostasis, blood mineral ion levels are not altered in such cases of PFE. Recently, two nonlethal homozygous PTH1R mutations were identified in two unrelated families in which affected members exhibit either dental and skeletal abnormalities (PTH1R‐V204E) or hypocalcemia and hyperphosphatemia (PTH1R‐R186H). Arg186 and Val204 map to the first transmembrane helix of the PTH1R, and thus to a critical region of this class B G protein‐coupled receptor. We used cell‐based assays and PTH and PTH‐related protein (PTHrP) ligand analogs to assess the impact of the R186H and V204E mutations on PTH1R function in vitro. In transiently transfected HEK293 cells, PTH1R‐R186H mediated cyclic adenosine monophosphate (cAMP) responses to PTH(1‐34) and PTHrP(1‐36) that were of comparable potency to those observed on wild‐type PTH1R (PTH1R‐WT) (half maximal effective concentrations [EC50s] = 0.4nM to 1.2nM), whereas the response‐maxima were significantly reduced for the PTH1R‐V204E mutant (maximum effect [Emax] = 81%–77% of PTH1R‐WT, p ≤ 0.004). Antibody binding to an extracellular hemagglutinin (HA) tag was comparable for PTH1R‐R186H and PTH1R‐WT, but was significantly reduced for PTH1R‐V204E (maximum binding level [Bmax] = 44% ± 11% of PTH1R‐WT, p = 0.002). The potency of cAMP signaling induced by a PTH(1‐11) analog was reduced by ninefold and threefold, respectively, for PTH1R‐R186H and PTH1R‐V204E, relative to PTH1R‐WT, and a PTH(1‐15) radioligand analog that bound adequately to PTH1R‐WT exhibited little or no specific binding to either mutant receptor. The data support a general decrease in PTH1R surface expression and/or function as a mechanism for PFE and a selective impairment in PTH ligand affinity as a potential PTH1R‐mutation‐based mechanism for pseudohypoparathyroidism. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Domain-specific cadherin order in desmosomes is conserved across isoforms

Desmosomes are intercellular adhesion complexes responsible for maintaining the balance between mechanical integrity and plasticity in tissues that are continuously subjected to large forces, such as epithelia and cardiac muscle. Indeed, desmosomes are required for all vertebrate life and non-lethal mutations of their component proteins often lead to severe diseases of the heart and skin. Many of these mutations are predicted to disrupt the architecture of the desmosomal cadherins (DCs): Ca2+-dependent transmembrane proteins that form the adhesive interface.

Emergence and evolution of novel G2b-like porcine epidemic diarrhea virus inter-subgroup G1b recombinants.

Porcine epidemic diarrhea virus (PEDV) is a fatal epizootic swine coronavirus that presents a financial threat to the global swine industry. Since the discovery of the low-pathogenic genotype 1b (G1b) in 2014, it has been responsible for sporadic outbreaks in South Korea. In this study, we identified novel G1b variants arising from the natural recombination of a major pandemic-like G2b virus and a minor G1b virus currently circulating in the domestic field. The whole-genome sequences of two 2018–19 G1b recombinants, KNU-1808 and KNU-1909, were determined. A genomic comparison showed that these two viruses share the highest nucleotide sequence similarity with the 2017 G1b strain but share less similarity with the 2014 G1b emergent strain KNU-1406. However, the putative recombination breakpoints spanning the first 1,170 nucleotides of the spike (S) gene were almost identical among the emergent and contemporary G1b strains. Recombination detection indicated that the inter-subgroup G1b recombinant first emerged in 2017 by introducing the N-terminal domain of S from KNU-1406 into the backbone of KNU-1703, possibly leading to antigenic shift. It then evolved into KNU-1808 and KNU-1909 through genetic drift, moving toward a more G2b-like genotype. Phylogenetic analysis revealed that the 2018–2019 G1b recombinants belong to a cluster containing other G1b strains but form a new branch. This study provides an important advance warning in regard to the emergence and prevalence of new genotypes or variants that can result from genetic recombination between two different PEDV genotypes circulating in endemic areas and continuous non-lethal mutations essential for viral fitness in the host environment.Electronic supplementary materialThe online version of this article (10.1007/s00705-020-04767-4) contains supplementary material, which is available to authorized users.

Genomic characterization of classical swine fever virus LOM variants with 3'-UTR INDELs from pigs on Jeju Island, South Korea.

Classical swine fever virus (CSFV) reemerged in naïve pig herds on Jeju Island, South Korea, due to the accidental introduction of the LOM vaccine strain in 2014. Since this reemergence, the previously CSFV-free region has experienced numerous outbreaks, causing the virus to become endemic in provincial herds. In this study, we determined the complete genome sequences and investigated the molecular characteristics of LOM-derived field CSFV strains with unique insertion-deletion (INDEL) mutations in the 3'-untranslated region (UTR) that were responsible for ongoing sporadic outbreaks on Jeju Island in 2019. The Jeju LOM-derived variants that emerged in 2019 had their own INDEL signatures in the 3'-UTR, resulting in changes to the predicted secondary stem-loop structures. The genomes of these strains were 12,297-12,302 nucleotides in length, one nucleotide (nt) shorter or one, two, or four nt longer than the reference LOM strain. The 3'-UTR INDEL variants shared 98.8-99.0% and 98.3-98.6% identity with the LOM strain at the polyprotein and full-genome level, respectively. The total number of genetic variations between the LOM vaccine strain and the 3'-UTR INDEL isolates ranged from 161 to 202 and 37 to 45 at the nucleotide and amino acid level, respectively. These mutations were broadly dispersed throughout the genome and particularly clustered in NS2 and the 3'-UTR, possibly triggering a reversion to low virulence and allowing the virus to adapt to improve its persistence in the field. This study provides important information about the genetic evolution of LOM-derived CSFV circulating in the free region, and suggests that it arose from continuous non-lethal mutations to ensure viral fitness in host animals.

Implication of Serum and Salivary Levels of Albumin as a Diagnostic Tool in Oral Premalignancy and Malignancy

Background: Reactive oxygen and nitrogen species are involved in the initiation and promotion of the multistep carcinogenesis. In neoplastic conditions many biochemical substances many biochemical substances including albumin show altered values .The aim of the present study was to investigate whether serum and salivary albumin levels can be used as diagnostic tools in oral premalignant and malignant leisons. Methods: The study includes a total of 45 subjects with 15 each of normal healthy, pre-malignancy and malignancy. Five milliliter of venous blood was drawn and unstimulated saliva was collected in the early morning. Albumin levels were estimated by Bromocresol Green Method. Results: Serum albumin levels were found to decrease in pre-malignancy and malignancy as compared to healthy individuals. Salivary Albumin levels were increased in pre-malignancy and malignancy as compared to healthy individuals. Conclusion: Tobacco consumption is one of the most important risk factors for the development of oral cancer. Burst of reactive oxygen species which cause oxidative stress in tobacco users is prime form of damage brought to the genetic material leading to non-lethal mutations causing pre-malignancy and malignancy. Serum protein has long been implicated to have anti-oxidant properties due to rich concentration of free thiol group. Albumin is a potent extracellular anti oxidant defense and a reliable marker of oxidative stress in the body. It could be helpful in the early identification of various oral pre malignancy and malignant leisons.

Conformational analysis of the riboflavin-responsive ETF:QO-p.Pro456Leu variant associated with mild multiple acyl-CoA dehydrogenase deficiency

Multiple-CoA dehydrogenase deficiency (MADD) is an inborn disorder of fatty acid and amino acid metabolism caused by mutations in the genes encoding for human electron transfer flavoprotein (ETF) and its partner electron transfer flavoprotein:ubiquinone oxidoreductase (ETF:QO). Albeit a rare disease, extensive newborn screening programs contributed to a wider coverage of MADD genotypes. However, the impact of non-lethal mutations on ETF:QO function remains scarcely understood from a structural perspective. To this end, we here revisit the relatively common MADD mutation ETF:QO-p.Pro456Leu, in order to clarify how it affects enzyme structure and folding. Given the limitation in recombinant expression of human ETF:QO, we resort to its bacterial homologue from Rhodobacter sphaeroides (Rs), in which the corresponding mutation (p.Pro389Leu) was inserted. The in vitro biochemical and biophysical investigations of the Rs ETF:QO-p.Pro389Leu variant showed that, while the mutation does not significantly affect the protein α/β fold, it introduces some plasticity on the tertiary structure and within flavin interactions. Indeed, in the p.Pro389Leu variant, FAD exhibits a higher thermolability during thermal denaturation and a faster rate of release in temperature-induced dissociation experiments, in comparison to the wild type. Therefore, although this clinical mutation occurs in the ubiquinone domain, its effect likely propagates to the nearby FAD binding domain, probably influencing electron transfer and redox potentials. Overall, our results provide a molecular rational for the decreased enzyme activity observed in patients and suggest that compromised FAD interactions in ETF:QO might account for the known riboflavin responsiveness of this mutation.

Using CRISPR/Cas9 to produce haploid inducers of carrot through targeted mutations of centromeric histone H3 (CENH3)

The generation of haploids is one of the most powerful means to accelerate the plant breeding process. Based on recent results published for Arabidopsis thaliana, manipulating the centromeres of the chromosomes has been proposed as a novel breeding tool for the production of haploid crop plants. By this way, haploids can be generated through cross-fertilizations after manipulating a single centromere protein, the centromere-specific histone H3 variant CENH3, in one of the parents designated as ‘haploid inducer’. In our carrot research, we are using the CRISPR/Cas9 technique in combination with an Agrobacterium rhizogenes-mediated transformation system to induce mutations in the carrot CENH3 gene. We are investigating two different approaches for CENH3-based uniparental genome elimination called as ‘1-Step’ and ‘2-Step’ strategies. In the ‘1-Step’ approach we are testing regenerated carrot plants carrying non-lethal mutations within the endogenous CENH3 gene for putative haploid inducer effects. The ‘2-Step’ strategy is based on a fully knocked-out carrot CENH3 gene which is rescued by an alien CENH3 gene isolated from a related plant species. Co-transformations of CRISPR/Cas9-based knockout constructs together with a ginseng (Panax ginseng) CENH3 gene were performed by using A. rhizogenes. We show that we could successfully target the carrot CENH3 gene and that the ginseng CENH3 protein is accumulated inside the kinetochore region. We also discuss preliminary results from crosses with putative haploid inducer genotypes developed by the ‘2-step’ approach.

The fitness cost of mismatch repair mutators in Saccharomyces cerevisiae: partitioning the mutational load.

Mutational load is the depression in a population's mean fitness that results from the continual influx of deleterious mutations. Here, we directly estimate the mutational load in a population of haploid Saccharomyces cerevisiae that are deficient for mismatch repair. We partition the load in haploids into two components. To estimate the load due to nonlethal mutations, we measure the competitive fitness of hundreds of randomly selected clones from both mismatch-repair-deficient and -proficient populations. Computation of the mean clone fitness for the mismatch-repair-deficient strain permits an estimation of the nonlethal load, and the histogram of fitness provides an interesting visualization of a loaded population. In a separate experiment, in order to estimate the load due to lethal mutations (i.e. the lethal mutation rate), we manipulate thousands of individual pairs of mother and daughter cells and track their fates. These two approaches yield point estimates for the two contributors to load, and the addition of these estimates is nearly equal to the separately measured short-term competitive fitness deficit for the mismatch-repair-deficient strain. This correspondence suggests that there is no need to invoke direct fitness effects to explain the fitness difference between mismatch-repair-deficient and -proficient strains. Assays in diploids are consistent with deleterious mutations in diploids tending towards recessivity. These results enhance our understanding of mutational load, a central population genetics concept, and we discuss their implications for the evolution of mutation rates.

Complete genome sequences of novel S-deletion variants of porcine epidemic diarrhea virus identified from a recurrent outbreak on Jeju Island, South Korea.

Porcine epidemic diarrhea virus (PEDV) is a deadly epizootic swine coronavirus that is of importance to the world pork industry. Since the re-emergence of the virulent genotype 2b (G2b) in 2014, Jeju Island in South Korea has faced periodic outbreaks, leading to the occurrence of endemics in provincial herds. In this study, we examined the complete genome sequences and molecular characteristics of novel G2b PEDV variants with a two-amino-acid deletion in the neutralizing epitope of the spike (S) gene, which were concurrently identified on a re-infected farm and its neighboring farm on Jeju Island. Whole-genome sequencing of the Jeju S-DEL isolates KNU-1829 and KNU-1830 revealed the presence of a continuous 9-nucleotide deletion within the nonstructural protein coding region. Their genomes were 28,023 nucleotides in length, 15 nucleotides shorter than those of the classical G2b PEDV strains. The two S-DEL isolates had 96.4–99.2% and 98.3–99.7% identity at the S-gene and full-genome level, respectively, to other global G2b PEDV strains. Genetic and antigenic analyses indicated that the S-DEL isolates are most closely related to the primary strain identified from the initial exposure at the same farm, but the virus appears to undergo continuous evolution, possibly leading to antigenic drift under recurrent or endemic pressure. This study provides important information about the antigenic diversity of PEDV circulating in the endemic areas, which arises from continuous non-lethal mutations to ensure viral fitness in the host environment.Electronic supplementary materialThe online version of this article (10.1007/s00705-019-04360-4) contains supplementary material, which is available to authorized users.

The Photoperiodic Flowering Time Regulator FKF1 Negatively Regulates Cellulose Biosynthesis.

Cellulose synthesis is precisely regulated by internal and external cues, and emerging evidence suggests that light regulates cellulose biosynthesis through specific light receptors. Recently, the blue light receptor CRYPTOCHROME 1 (CRY1) was shown to positively regulate secondary cell wall biosynthesis in Arabidopsis (Arabidopsis thaliana). Here, we characterize the role of FLAVIN-BINDING KELCH REPEAT, F-BOX 1 (FKF1), another blue light receptor and well-known photoperiodic flowering time regulator, in cellulose biosynthesis. A phenotype suppression screen using a cellulose deficient mutant cesa1aegeus,cesa3ixr1-2 (c1,c3), which carries nonlethal point mutations in CELLULOSE SYNTHASE A 1 (CESA1) and CESA3, resulted in identification of the phenotype-restoring large leaf (llf) mutant. Next-generation mapping using the whole genome resequencing method identified the llf locus as FKF1 FKF1 was confirmed as the causal gene through observation of the llf phenotype in an independent triple mutant c1,c3,fkf1-t carrying a FKF1 T-DNA insertion mutant. Moreover, overexpression of FKF1 in llf plants restored the c1,c3 phenotype. The fkf1 mutants showed significant increases in cellulose content and CESA gene expression compared with that in wild-type Columbia-0 plants, suggesting a negative role of FKF1 in cellulose biosynthesis. Using genetic, molecular, and phenocopy and biochemical evidence, we have firmly established the role of FKF1 in regulation of cellulose biosynthesis. In addition, CESA expression analysis showed that diurnal expression patterns of CESAs are FKF1 independent, whereas their circadian expression patterns are FKF1 dependent. Overall, our work establishes a role of FKF1 in the regulation of cell wall biosynthesis in Arabidopsis.

Hormesis Promotes Evolutionary Change.

Exposure to moderate environmental stress is one important source of evolutionary change. This evidence would support the hypothesis that hormesis is an evolutionary expectation. In this short review, I discuss relevant examples of genetic and phenotypic responses to moderate stress exposure that are compatible with hormesis and with paradigms of evolutionary theory such as evolutionary rescue or phenotypic plasticity. Genetic recombination, nonlethal mutations, activity of transposable elements, or gene expression are some of the molecular mechanisms through which hormesis might enable organisms to maintain or even increase evolutionary fitness in stressful environments. These mechanisms span the tree of life from plants to vertebrates.

Genetic diversity and geographic distribution of Mycobacterium bovis from cattle in Mexico

Se obtuvieron patrones moleculares (espoligotipos) de 878 aislados de Mycobacterium bovis de ganado de diferentes regiones de México entre los años 2009 y 2010. Setenta y dos por ciento (72 %) de los espoligotipos cayeron en nueve grupos y 27 % de los aislados dentro de sólo dos espoligotipos; 149 fueron espoligotipos individuales. Los dos espoligotipos predominantes, arbitrariamente identificados como SP1 y SP2, se distribuyen en la mayor parte del territorio nacional, en especial en la zona centro de México en ganado especializado en producción de leche. A pesar de la amplia distribución geográfica de los espoligotipos de mayor frecuencia, algunos muestran cierta localización, en especial los encontrados en zonas geográficas distantes, como es el caso de Chihuahua y Baja California. Aunque pocos, algunos espoligotipos muestran patrones moleculares distintos a los mostrados por los espoligotipos de mayor frecuencia, sugiriendo fuentes de infección desconocida. La mayoría de los Estados con ganadería predominantemente lechera muestran espoligotipos comunes, lo que sugiere intercambio regional frecuente de ganado. Algunos espoligotipos son comunes en ganado para leche y ganado para carne, lo que sugiere transmisión entre estas dos poblaciones; se desconoce, sin embargo, si los animales de carne infectados provienen de explotaciones extensivas o si son de engordas ubicadas dentro de la explotación lechera. Se propone la tipificación rutinaria de aislados de M. bovis obtenidos en todos los laboratorios de diagnóstico y mejorar la captura de información epidemiológica de los casos, para hacer mejores conclusiones epidemiológicas de la distribución espacial de las cepas de este agente en el territorio nacional.

Mutation dynamics and fitness effects followed in single cells.

Mutations have been investigated for more than a century but remain difficult to observe directly in single cells, which limits the characterization of their dynamics and fitness effects. By combining microfluidics, time-lapse imaging, and a fluorescent tag of the mismatch repair system in Escherichia coli, we visualized the emergence of mutations in single cells, revealing Poissonian dynamics. Concomitantly, we tracked the growth and life span of single cells, accumulating ~20,000 mutations genome-wide over hundreds of generations. This analysis revealed that 1% of mutations were lethal; nonlethal mutations displayed a heavy-tailed distribution of fitness effects and were dominated by quasi-neutral mutations with an average cost of 0.3%. Our approach has enabled the investigation of single-cell individuality in mutation rate, mutation fitness costs, and mutation interactions.

Power provides protection: Genetic robustness in yeast depends on the capacity to generate energy.

The functional basis of genetic robustness, the ability of organisms to suppress the effects of mutations, remains incompletely understood. We exposed a set of 15 strains of Saccharomyces cerevisiae form diverse environments to increasing doses of the chemical mutagen EMS. The number of the resulting random mutations was similar for all tested strains. However, there were differences in immediate mortality after the mutagenic treatment and in defective growth of survivors. An analysis of gene expression revealed that immediate mortality was lowest in strains with lowest expression of transmembrane proteins, which are rich in thiol groups and thus vulnerable to EMS. A signal of genuine genetic robustness was detected for the other trait, the ability to grow well despite bearing non-lethal mutations. Increased tolerance of such mutations correlated with high expression of genes responsible for the oxidative energy metabolism, suggesting that the negative effect of mutations can be buffered if enough energy is available. We confirmed this finding in three additional tests of the ability to grow on (i) fermentable or non-fermentable sources of carbon, (ii) under chemical inhibition of the electron transport chain and (iii) during overexpression of its key component, cytochrome c. Our results add the capacity to generate energy as a general mechanism of genetic robustness.