655 research outputs found
Facilitating Wolbachia introductions into mosquito populations through insecticide-resistance selection.
Wolbachia infections are being introduced into mosquito vectors of human diseases following the discovery that they can block transmission of disease agents. This requires mosquitoes infected with the disease-blocking Wolbachia to successfully invade populations lacking the infection. While this process is facilitated by features of Wolbachia, particularly their ability to cause cytoplasmic incompatibility, blocking Wolbachia may produce deleterious effects, such as reduced host viability or fecundity, that inhibit successful local introductions and subsequent spatial spread. Here, we outline an approach to facilitate the introduction and spread of Wolbachia infections by coupling Wolbachia introduction to resistance to specific classes of insecticides. The approach takes advantage of very high maternal transmission fidelity of Wolbachia infections in mosquitoes, complete incompatibility between infected males and uninfected females, the widespread occurrence of insecticide resistance, and the widespread use of chemical control in disease-endemic countries. This approach is easily integrated into many existing control strategies, provides population suppression during release and might be used to introduce Wolbachia infections even with high and seasonally dependent deleterious effects, such as the wMelPop infection introduced into Aedes aegypti for dengue control. However, possible benefits will need to be weighed against concerns associated with the introduction of resistance alleles
The statistical mechanics of a polygenic characterunder stabilizing selection, mutation and drift
By exploiting an analogy between population genetics and statistical
mechanics, we study the evolution of a polygenic trait under stabilizing
selection, mutation, and genetic drift. This requires us to track only four
macroscopic variables, instead of the distribution of all the allele
frequencies that influence the trait. These macroscopic variables are the
expectations of: the trait mean and its square, the genetic variance, and of a
measure of heterozygosity, and are derived from a generating function that is
in turn derived by maximizing an entropy measure. These four macroscopics are
enough to accurately describe the dynamics of the trait mean and of its genetic
variance (and in principle of any other quantity). Unlike previous approaches
that were based on an infinite series of moments or cumulants, which had to be
truncated arbitrarily, our calculations provide a well-defined approximation
procedure. We apply the framework to abrupt and gradual changes in the optimum,
as well as to changes in the strength of stabilizing selection. Our
approximations are surprisingly accurate, even for systems with as few as 5
loci. We find that when the effects of drift are included, the expected genetic
variance is hardly altered by directional selection, even though it fluctuates
in any particular instance. We also find hysteresis, showing that even after
averaging over the microscopic variables, the macroscopic trajectories retain a
memory of the underlying genetic states.Comment: 35 pages, 8 figure
Emergence of clones in sexual populations
In sexual population, recombination reshuffles genetic variation and produces
novel combinations of existing alleles, while selection amplifies the fittest
genotypes in the population. If recombination is more rapid than selection,
populations consist of a diverse mixture of many genotypes, as is observed in
many populations. In the opposite regime, which is realized for example in the
facultatively sexual populations that outcross in only a fraction of
reproductive cycles, selection can amplify individual genotypes into large
clones. Such clones emerge when the fitness advantage of some of the genotypes
is large enough that they grow to a significant fraction of the population
despite being broken down by recombination. The occurrence of this "clonal
condensation" depends, in addition to the outcrossing rate, on the heritability
of fitness. Clonal condensation leads to a strong genetic heterogeneity of the
population which is not adequately described by traditional population genetics
measures, such as Linkage Disequilibrium. Here we point out the similarity
between clonal condensation and the freezing transition in the Random Energy
Model of spin glasses. Guided by this analogy we explicitly calculate the
probability, Y, that two individuals are genetically identical as a function of
the key parameters of the model. While Y is the analog of the spin-glass order
parameter, it is also closely related to rate of coalescence in population
genetics: Two individuals that are part of the same clone have a recent common
ancestor.Comment: revised versio
Asymptotic power law of moments in a random multiplicative process with weak additive noise
It is well known that a random multiplicative process with weak additive
noise generates a power-law probability distribution. It has recently been
recognized that this process exhibits another type of power law: the moment of
the stochastic variable scales as a function of the additive noise strength. We
clarify the mechanism for this power-law behavior of moments by treating a
simple Langevin-type model both approximately and exactly, and argue this
mechanism is universal. We also discuss the relevance of our findings to noisy
on-off intermittency and to singular spatio-temporal chaos recently observed in
systems of non-locally coupled elements.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
Species assembly in model ecosystems, II: Results of the assembly process
In the companion paper of this set (Capitan and Cuesta, 2010) we have
developed a full analytical treatment of the model of species assembly
introduced in Capitan et al. (2009). This model is based on the construction of
an assembly graph containing all viable configurations of the community, and
the definition of a Markov chain whose transitions are the transformations of
communities by new species invasions. In the present paper we provide an
exhaustive numerical analysis of the model, describing the average time to the
recurrent state, the statistics of avalanches, and the dependence of the
results on the amount of available resource. Our results are based on the fact
that the Markov chain provides an asymptotic probability distribution for the
recurrent states, which can be used to obtain averages of observables as well
as the time variation of these magnitudes during succession, in an exact
manner. Since the absorption times into the recurrent set are found to be
comparable to the size of the system, the end state is quickly reached (in
units of the invasion time). Thus, the final ecosystem can be regarded as a
fluctuating complex system where species are continually replaced by newcomers
without ever leaving the set of recurrent patterns. The assembly graph is
dominated by pathways in which most invasions are accepted, triggering small
extinction avalanches. Through the assembly process, communities become less
resilient (e.g., have a higher return time to equilibrium) but become more
robust in terms of resistance against new invasions.Comment: 14 pages, 13 figures. Revised versio
From Parasite to Mutualist: Rapid Evolution of Wolbachia in Natural Populations of Drosophila
Wolbachia are maternally inherited bacteria that commonly spread through host populations by causing cytoplasmic incompatibility, often expressed as reduced egg hatch when uninfected females mate with infected males. Infected females are frequently less fecund as a consequence of Wolbachia infection. However, theory predicts that because of maternal transmission, these “parasites” will tend to evolve towards a more mutualistic association with their hosts. Drosophila simulans in California provided the classic case of a Wolbachia infection spreading in nature. Cytoplasmic incompatibility allowed the infection to spread through individual populations within a few years and from southern to northern California (more than 700 km) within a decade, despite reducing the fecundity of infected females by 15%–20% under laboratory conditions. Here we show that the Wolbachia in California D. simulans have changed over the last 20 y so that infected females now exhibit an average 10% fecundity advantage over uninfected females in the laboratory. Our data suggest smaller but qualitatively similar changes in relative fecundity in nature and demonstrate that fecundity-increasing Wolbachia variants are currently polymorphic in natural populations
A simple mathematical model of gradual Darwinian evolution: Emergence of a Gaussian trait distribution in adaptation along a fitness gradient
We consider a simple mathematical model of gradual Darwinian evolution in
continuous time and continuous trait space, due to intraspecific competition
for common resource in an asexually reproducing population in constant
environment, while far from evolutionary stable equilibrium. The model admits
exact analytical solution. In particular, Gaussian distribution of the trait
emerges from generic initial conditions.Comment: 21 pages, 2 figures, as accepted to J Math Biol 2013/03/1
Monte carlo simulations of parapatric speciation
Parapatric speciation is studied using an individual--based model with sexual
reproduction. We combine the theory of mutation accumulation for biological
ageing with an environmental selection pressure that varies according to the
individuals geographical positions and phenotypic traits. Fluctuations and
genetic diversity of large populations are crucial ingredients to model the
features of evolutionary branching and are intrinsic properties of the model.
Its implementation on a spatial lattice gives interesting insights into the
population dynamics of speciation on a geographical landscape and the
disruptive selection that leads to the divergence of phenotypes. Our results
suggest that assortative mating is not an obligatory ingredient to obtain
speciation in large populations at low gene flow.Comment: submitted to Phys.Rev.
Species assembly in model ecosystems, I: Analysis of the population model and the invasion dynamics
Recently we have introduced a simplified model of ecosystem assembly (Capitan
et al., 2009) for which we are able to map out all assembly pathways generated
by external invasions in an exact manner. In this paper we provide a deeper
analysis of the model, obtaining analytical results and introducing some
approximations which allow us to reconstruct the results of our previous work.
In particular, we show that the population dynamics equations of a very general
class of trophic-level structured food-web have an unique interior equilibrium
point which is globally stable. We show analytically that communities found as
end states of the assembly process are pyramidal and we find that the
equilibrium abundance of any species at any trophic level is approximately
inversely proportional to the number of species in that level. We also find
that the per capita growth rate of a top predator invading a resident community
is key to understand the appearance of complex end states reported in our
previous work. The sign of these rates allows us to separate regions in the
space of parameters where the end state is either a single community or a
complex set containing more than one community. We have also built up
analytical approximations to the time evolution of species abundances that
allow us to determine, with high accuracy, the sequence of extinctions that an
invasion may cause. Finally we apply this analysis to obtain the communities in
the end states. To test the accuracy of the transition probability matrix
generated by this analytical procedure for the end states, we have compared
averages over those sets with those obtained from the graph derived by
numerical integration of the Lotka-Volterra equations. The agreement is
excellent.Comment: 16 pages, 8 figures. Revised versio
Cryo-EM structures and binding of mouse and human ACE2 to SARS-CoV-2 variants of concern indicate that mutations enabling immune escape could expand host range.
Investigation of potential hosts of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is crucial to understanding future risks of spillover and spillback. SARS-CoV-2 has been reported to be transmitted from humans to various animals after requiring relatively few mutations. There is significant interest in describing how the virus interacts with mice as they are well adapted to human environments, are used widely as infection models and can be infected. Structural and binding data of the mouse ACE2 receptor with the Spike protein of newly identified SARS-CoV-2 variants are needed to better understand the impact of immune system evading mutations present in variants of concern (VOC). Previous studies have developed mouse-adapted variants and identified residues critical for binding to heterologous ACE2 receptors. Here we report the cryo-EM structures of mouse ACE2 bound to trimeric Spike ectodomains of four different VOC: Beta, Omicron BA.1, Omicron BA.2.12.1 and Omicron BA.4/5. These variants represent the oldest to the newest variants known to bind the mouse ACE2 receptor. Our high-resolution structural data complemented with bio-layer interferometry (BLI) binding assays reveal a requirement for a combination of mutations in the Spike protein that enable binding to the mouse ACE2 receptor
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