563 research outputs found
Minimax Estimation of Nonregular Parameters and Discontinuity in Minimax Risk
When a parameter of interest is nondifferentiable in the probability, the
existing theory of semiparametric efficient estimation is not applicable, as it
does not have an influence function. Song (2014) recently developed a local
asymptotic minimax estimation theory for a parameter that is a
nondifferentiable transform of a regular parameter, where the nondifferentiable
transform is a composite map of a continuous piecewise linear map with a single
kink point and a translation-scale equivariant map. The contribution of this
paper is two fold. First, this paper extends the local asymptotic minimax
theory to nondifferentiable transforms that are a composite map of a Lipschitz
continuous map having a finite set of nondifferentiability points and a
translation-scale equivariant map. Second, this paper investigates the
discontinuity of the local asymptotic minimax risk in the true probability and
shows that the proposed estimator remains to be optimal even when the risk is
locally robustified not only over the scores at the true probability, but also
over the true probability itself. However, the local robustification does not
resolve the issue of discontinuity in the local asymptotic minimax risk
When virulence originates from non-agricultural hosts: New insights into plant breeding
Monogenic plant resistance breakdown is a model for testing evolution in action in pathogens. As a rule, plant pathologists argue that virulence â the allele that allows pathogens to overcome resistance â is due to a new mutation at the avirulence locus within the native/endemic population that infects susceptible crops. In this article, we develop an alternative and neglected scenario where a given virulence pre-exists in a non-agricultural host and might be accidentally released or introduced on the matching resistant cultivar in the field. The main difference between the two scenarios is the divergence time expected between the avirulent and the virulent populations. As a consequence, population genetic approaches such as genome scans and Approximate Bayesian Computation methods allow explicit testing of the two scenarios by timing the divergence. This review then explores the fundamental implications of this alternative scenario for plant breeding, including the invasion of virulence or the evolution of more aggressive hybrids, and proposes concrete solutions to achieve a sustainable resistance
Magic traits drive the emergence of pathogens
An important branch of evolutionary biology strives to understand how divergent selection for an ecologically important trait can foster the emergence of new species specialized on different niches. Such ecological speciation is usually difficult to achieve because recombination between different subsets of a population that are adapting to different environments counteracts selection for locally adapted gene combinations. Traits pleiotropically controlling adaptation to different environments and reproductive isolation are therefore the most favourable for ecological speciation, and are thus called âmagic traitsâ. We used genetic markers and cross-inoculations to show that pathogenicity-related loci are responsible for both host adaptation and reproductive isolation in emerging populations of Venturia
inaequalis, the fungus causing apple scab disease. Because the fungus mates within its host and because the pathogenicity-related loci prevent infection of the non-host trees, host adaptation pleiotropically maintains genetic differentiation and adaptive allelic combinations between sympatric populations specific to different apple varieties. Such âmagic traitsâ are likely frequent in fungal pathogens, and likely drive the emergence of new diseases.
The genetic structure of a Venturia inaequalis population in a heterogeneous host population composed of different Malus species
Adaptation, which induces differentiation between populations in relation to environmental conditions, can initiate divergence. The balance between gene flow and selection determines the maintenance of such a structure in sympatry. Studying these two antagonistic forces in plant pathogens is made possible because of the high ability of pathogens to disperse and of the strong selective pressures exerted by their hosts. In this article, we analysed the genetic structure of the population of the apple scab fungus, Venturia inaequalis, in a heterogeneous environment composed of various Malus species. Inferences were drawn from microsatellite and AFLP data obtained from 114 strains sampled in a single orchard on nine different Malus species to determine the forces that shape the genetic structure of the pathogen
Les barriĂšres gĂ©nĂ©tiques endogĂšnes chez les pathogĂšnes : la virulence-Vf de Venturia inaequalis Ă lâĂ©preuve des scans gĂ©nomiques
Differential selection pressures exerted by host resistance quantitative trait loci on a pathogen population: a case study in an apple Ă Venturia inaequalis pathosystem
Understanding how pathogens evolve according to pressures exerted by their plant hosts is essential for the derivation of strategies aimed at the durable management of resistant cultivars. The spectrum of action of the resistance factors in the partially resistant cultivars is thought to be an important determinant of resistance durability. However, it has not yet been demonstrated whether the pressures exerted by quantitative resistance are different according to their spectrum of action.To investigate selection pressures exerted by apple genotypes harbouring various resistance quantitative trait loci (QTLs) on a mixed inoculum of the scab disease agent, Venturia inaequalis, we monitored V. inaequalis isolate proportions on diseased apple leaves of an F1 progeny using quantitative pyrosequencing technology and QTL mapping. Broad-spectrum resistances did not exert any differential selection pressures on the mixed inoculum, whereas narrow-spectrum resistances decreased the frequencies of some isolates in the mixture relative to the susceptible host genotypes. Our results suggest that the management of resistant cultivars should be different according to the spectrum of action of their resistance factors. The pyramiding of broad-spectrum factors or the use of a mixture of apple genotypes that carry narrow-spectrum resistance factors are two possible strategies for the minimization of resistance erosion
Emergence of novel fungal pathogens by ecological speciation: importance of the reduced viability of immigrants
Expanding global trade and the domestication of ecosystems have greatly accelerated the rate of emerging infectious fungal diseases, and host-shift speciation appears to be a major route for disease emergence. There is therefore an increased interest in identifying the factors that drive the evolution of reproductive isolation between populations adapting to different hosts. Here, we used genetic markers and cross-inoculations to assess the level of gene flow and investigate barriers responsible for reproductive isolation between two sympatric populations of Venturia inaequalis, the fungal pathogen causing apple scab disease, one of the fungal populations causing a recent emerging disease on resistant varieties. Our results showed the maintenance over several years of strong and stable differentiation between the two populations in the same orchards, suggesting ongoing ecological divergence following a host shift. We identified strong selection against immigrants (i.e. host specificity) from different host varieties as the strongest and likely most efficient barrier to gene flow between local and emerging populations. Cross-variety disease transmission events were indeed rare in the field and cross-inoculation tests confirmed high host specificity. Because the fungus mates within its host after successful infection and because pathogenicity-related loci prevent infection of nonhost trees, adaptation to specific hosts may alone maintain both genetic differentiation between and adaptive allelic combinations within sympatric populations parasitizing different apple varieties, thus acting as a âmagic traitâ. Additional intrinsic and extrinsic postzygotic barriers might complete reproductive isolation and explain why the rare migrants and F1 hybrids detected do not lead to pervasive gene flow across years
- âŠ