Environmental dependence of mutation in microbes is well-known, but most experiments have investigated contexts in which growth rate is greatly reduced below optimum. A new experiment shows ...mutational variability extends to contexts in which growth is near optimum.
Environmental dependence of mutation in microbes is well-known, but most experiments have investigated contexts in which growth rate is greatly reduced below optimum. A new experiment shows mutational variability extends to contexts in which growth is near optimum.
A basic knowledge about mutation rates is central to our understanding of a myriad of evolutionary phenomena, including the maintenance of sex and rates of molecular evolution. Although there is ...substantial evidence that mutation rates vary among taxa, relatively little is known about the factors that underlie this variation at an empirical level, particularly in multicellular eukaryotes. Here we integrate several disparate lines of theoretical and empirical inquiry into a unified framework to guide future studies that are aimed at understanding why and how mutation rates evolve in multicellular species.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The hermaphroditic nematode
has been one of the primary model systems in biology since the 1970s, but only within the last two decades has this nematode also become a useful model for experimental ...evolution. Here, we outline the goals and major foci of experimental evolution with
and related species, such as
and
, by discussing the principles of experimental design, and highlighting the strengths and limitations of
as model systems. We then review three exemplars of
experimental evolution studies, underlining representative evolution experiments that have addressed the: (1) maintenance of genetic variation; (2) role of natural selection during transitions from outcrossing to selfing, as well as the maintenance of mixed breeding modes during evolution; and (3) evolution of phenotypic plasticity and its role in adaptation to variable environments, including host-pathogen coevolution. We conclude by suggesting some future directions for which experimental evolution with
would be particularly informative.
Cu-BTC (also known as HKUST-1) is a well-characterized metal-organic framework material produced in an industrial scale and widely studied for a number of potential applications by the scientific ...community. The co-existence of Cu(+) and Cu(2+) entities has already been observed in this material, but the presence of Cu(+) ions was attributed to oxide impurities. The results presented here clearly demonstrate that Cu(+) ions can be present in high concentrations inside the hybrid structure. Furthermore, switching between the two copper oxidation states can be induced by redox treatments, using vacuum and/or reducing gases at different sample temperatures.
Cellular structures such as the nucleus, Golgi, centrioles, and spindle show remarkable diversity between species, but the mechanisms that produce these variations in cell biology are not known.
Here ...we investigate the mechanisms that contribute to variations in morphology and dynamics of the mitotic spindle, which orchestrates chromosome segregation in all Eukaryotes and positions the division plane in many organisms. We use high-throughput imaging of the first division in nematodes to demonstrate that the measured effects of spontaneous mutations, combined with stabilizing selection on cell size, are sufficient to quantitatively explain both the levels of within-species variation in the spindle and its diversity over ∼100 million years of evolution. Furthermore, our finding of extensive within-species variation for the spindle demonstrates that there is not just one "wild-type" form, rather that cellular structures can exhibit a surprisingly broad diversity of naturally occurring behaviors.
Our results argue that natural selection acts predominantly on cell size and indirectly influences the spindle through the scaling of the spindle with cell size. Previous studies have shown that the spindle also scales with cell size during early development. Thus, the scaling of the spindle with cell size controls its variation over both ontogeny and phylogeny.
The mutational process varies at many levels, from within genomes to among taxa. Many mechanisms have been linked to variation in mutation, but understanding of the evolution of the mutational ...process is rudimentary. Physiological condition is often implicated as a source of variation in microbial mutation rate and may contribute to mutation rate variation in multicellular organisms.Deleterious mutations are an ubiquitous source of variation in condition. We test the hypothesis that the mutational process depends on the underlying mutation load in two groups of Caenorhabditis elegans mutation accumulation (MA) lines that differ in their starting mutation loads. "First-order MA" (O1MA) lines maintained under minimal selection for ∼250 generations were divided into high-fitness and low-fitness groups and sets of "second-order MA" (O2MA) lines derived from each O1MA line were maintained for ∼150 additional generations. Genomes of 48 O2MA lines and their progenitors were sequenced. There is significant variation among O2MA lines in base-substitution rate (µbs), but no effect of initial fitness; the indel rate is greater in high-fitness O2MA lines. Overall, µbs is positively correlated with recombination and proximity to short tandem repeats and negatively correlated with 10 bp and 1 kb GC content. However, probability of mutation is sufficiently predicted by the three-nucleotide motif alone. Approximately 90% of the variance in standing nucleotide variation is explained by mutability. Total mutation rate increased in the O2MA lines, as predicted by the "drift barrier" model of mutation rate evolution. These data, combined with experimental estimates of fitness, suggest that epistasis is synergistic.
Knowledge of mutation processes is central to understanding virtually all evolutionary phenomena and the underlying nature of genetic disorders and cancers. However, the limitations of standard ...molecular mutation detection methods have historically precluded a genome-wide understanding of mutation rates and spectra in the nuclear genomes of multicellular organisms. We applied two high-throughput DNA sequencing technologies to identify and characterize hundreds of spontaneously arising base-substitution mutations in 10 Caenorhabditis elegans mutation-accumulation (MA)-line nuclear genomes. C. elegans mutation rate estimates were similar to previous calculations based on smaller numbers of mutations. Mutations were distributed uniformly within and among chromosomes and were not associated with recombination rate variation in the MA lines, suggesting that intragenomic variation in genetic hitchhiking and/or background selection are primarily responsible for the chromosomal distribution patterns of polymorphic nucleotides in C. elegans natural populations. A strong mutational bias from G/C to A/T nucleotides was detected in the MA lines, implicating oxidative DNA damage as a major endogenous mutagenic force in C. elegans. The observed mutational bias also suggests that the C. elegans nuclear genome cannot be at equilibrium because of mutation alone. Transversions dominate the spectrum of spontaneous mutations observed here, whereas transitions dominate patterns of allegedly neutral polymorphism in natural populations of C. elegans and many other animal species; this observation challenges the assumption that natural patterns of molecular variation in noncoding regions of the nuclear genome accurately reflect underlying mutation processes.
Caenorhabditis elegans strains with the heat-sensitive mortal germline phenotype become progressively sterile over the course of a few tens of generations when maintained at temperatures near the ...upper range of C. elegans' tolerance. Mortal germline is transgenerationally heritable, and proximately under epigenetic control. Previous studies have suggested that mortal germline presents a relatively large mutational target and that mortal germline is not uncommon in natural populations of C. elegans. The mortal germline phenotype is not monolithic. Some strains exhibit a strong mortal germline phenotype, in which individuals invariably become sterile over a few generations, whereas other strains show a weaker (less penetrant) phenotype in which the onset of sterility is slower and more stochastic. We present results in which we (1) quantify the rate of mutation to the mortal germline phenotype and (2) quantify the frequency of mortal germline in a collection of 95 wild isolates. Over the course of ∼16,000 meioses, we detected one mutation to a strong mortal germline phenotype, resulting in a point estimate of the mutation rate UMrt≈ 6×10-5/genome/generation. We detected no mutations to a weak mortal germline phenotype. Six out of 95 wild isolates have a strong mortal germline phenotype, and although quantification of the weak mortal germline phenotype is inexact, the weak mortal germline phenotype is not rare in nature. We estimate a strength of selection against mutations conferring the strong mortal germline phenotype s¯≈0.1%, similar to selection against mutations affecting competitive fitness. The appreciable frequency of weak mortal germline variants in nature combined with the low mutation rate suggests that mortal germline may be maintained by balancing selection.
Genetic and developmental architecture may bias the mutationally available phenotypic spectrum. Although such asymmetries in the introduction of variation may influence possible evolutionary ...trajectories, we lack quantitative characterization of biases in mutationally inducible phenotypic variation, their genotype-dependence, and their underlying molecular and developmental causes. Here we quantify the mutationally accessible phenotypic spectrum of the vulval developmental system using mutation accumulation (MA) lines derived from four wild isolates of the nematodes Caenorhabditis elegans and C. briggsae. The results confirm that on average, spontaneous mutations degrade developmental precision, with MA lines showing a low, yet consistently increased, proportion of developmental defects and variants. This result indicates strong purifying selection acting to maintain an invariant vulval phenotype. Both developmental system and genotype significantly bias the spectrum of mutationally inducible phenotypic variants. First, irrespective of genotype, there is a developmental bias, such that certain phenotypic variants are commonly induced by MA, while others are very rarely or never induced. Second, we found that both the degree and spectrum of mutationally accessible phenotypic variation are genotype-dependent. Overall, C. briggsae MA lines exhibited a two-fold higher decline in precision than the C. elegans MA lines. Moreover, the propensity to generate specific developmental variants depended on the genetic background. We show that such genotype-specific developmental biases are likely due to cryptic quantitative variation in activities of underlying molecular cascades. This analysis allowed us to identify the mutationally most sensitive elements of the vulval developmental system, which may indicate axes of potential evolutionary variation. Consistent with this scenario, we found that evolutionary trends in the vulval system concern the phenotypic characters that are most easily affected by mutation. This study provides an empirical assessment of developmental bias and the evolution of mutationally accessible phenotypes and supports the notion that such bias may influence the directions of evolutionary change.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Because of pleiotropy, mutations affect the expression and inheritance of multiple traits and, together with selection, are expected to shape standing genetic covariances between traits and ...eventual phenotypic divergence between populations. It is therefore important to find if the M matrix, describing mutational variances of each trait and covariances between traits, varies between genotypes. We here estimate the M matrix for six locomotion behavior traits in lines of two genotypes of the nematode Caenorhabditis elegans that accumulated mutations in a nearly neutral manner for 250 generations. We find significant mutational variance along at least one phenotypic dimension of the M matrices, but neither their size nor their orientation had detectable differences between genotypes. The number of generations of mutation accumulation, or the number of MA lines measured, was likely insufficient to sample enough mutations and detect potentially small differences between the two M matrices. We then tested if the M matrices were similar to one G matrix describing the standing genetic (co)variances of a population derived by the hybridization of several genotypes, including the two measured for M, and domesticated to a lab-defined environment for 140 generations. We found that the M and G were different because the genetic covariances caused by mutational pleiotropy in the two genotypes are smaller than those caused by linkage disequilibrium in the lab population. We further show that M matrices differed in their alignment with the lab population G matrix. If generalized to other founder genotypes of the lab population, these observations indicate that selection does not shape the evolution of the M matrix for locomotion behavior in the short-term of a few tens to hundreds of generations and suggests that the hybridization of C. elegans genotypes allows selection on new phenotypic dimensions of locomotion behavior.
PDF
