Biomedical subjects

P Rohani

Publications and source records attributed to P Rohani.

7 recordsLinked to original sources

PubMed1999

Dispersal-induced instabilities in host-parasitoid metapopulations.

We present a general host-parasitoid metapopulation model and, using analytical techniques (supported by numerical simulations), investigate the effects of dispersal on the equilibrium stability of local populations. As has been demonstrated previously, if the intrinsic dynamics of local populations are unstable, then passive dispersal cannot stabilise. Extreme asymmetry in the dispersal fractions between the two species can, however, destabilise the metapopulation equilibrium state. Our key conclusion is that the precise effects of dispersal on stability depend critically on the underlying ecology of the interaction within each population. The presence of regulatory mechanisms, be they in the form of density-dependent host reproduction or the presence of host refugia, decreases the likelihood of observing dispersal-induced instabilities. Indeed, if the stabilising effects are sufficiently strong, then dispersal cannot be destabilising, no matter how asymmetric the dispersal fractions are. On the other hand, positive feedbacks arising from threshold effects in host reproduction or inversely density-dependent patterns of parasitism (due to, for example, long handling times or egg limitation) amplify the destabilising effects of dispersal.

Animals↗

PubMed1998

Population dynamic interference among childhood diseases.

Epidemiologists usually study the interaction between a host population and one parasitic infection. However, different parasite species effectively compete, in an ecological sense, for the same finite group of susceptible hosts, so there may be an indirect effect on the population dynamics of one disease due to epidemics of another. In human populations, recovery from any serious infection is normally preceded by a period of convalescence, during which infected individuals stay at home and are effectively shielded from exposure to other infectious diseases. We present a model for the dynamics of two infectious diseases, incorporating a temporary removal of susceptibles. We use this model to explore population-level consequences of a temporary insusceptibility in childhood diseases, the dynamics of which are partly driven by differences in contact rates in and out of school terms. Significant population dynamic interference is predicted and cannot be dismissed in the limited case-study data available for measles and whooping cough in England before the vaccination era.

Child↗

PubMed1998

Persistence, chaos and synchrony in ecology and epidemiology.

The decline of species in natural habitats concerns ecologists, who view extinction as a danger and conservation of biological diversity as a goal. In contrast, the proliferation of 'undesirable' species is the principal concern of epidemiologists, who view persistence as a problem and eradication as an achievement. While ecologists and epidemiologists have essentially opposite goals, the mathematical structure of the population dynamics that they study is very similar. We briefly review the similarities and differences between these two fields, emphasizing recent work in both areas on the effects of spatial synchrony and dynamical chaos. We hope to stimulate further cross-fertilization of ideas between the disciplines.

Animals↗

PubMed1998

Population Floors and the Persistence of Chaos in Ecological Models.

Chaotic dynamics have been observed in a wide range of population models. Here we describe the effects of perturbing several of these models so as to introduce a non-zero minimum population size. This perturbation generally reduces the likelihood of observing chaos, in both discrete and continuous time models. The extent of this effect depends on whether chaos is generated through period-doubling, quasiperiodicity, or intermittence. Chaos reached via the quasiperiodic route is more robust against the perturbation than period-doubling chaos, whilst the inclusion of a population floor in a model exhibiting intermittent chaos may increase the frequency of population bursts although these become non-chaotic. Copyright 1998 Academic Press.

Journal Article↗

PubMed1998

Receptor-like genes in the major resistance locus of lettuce are subject to divergent selection.

Disease resistance genes in plants are often found in complex multigene families. The largest known cluster of disease resistance specificities in lettuce contains the RGC2 family of genes. We compared the sequences of nine full-length genomic copies of RGC2 representing the diversity in the cluster to determine the structure of genes within this family and to examine the evolution of its members. The transcribed regions range from at least 7.0 to 13.1 kb, and the cDNAs contain deduced open reading frames of approximately 5. 5 kb. The predicted RGC2 proteins contain a nucleotide binding site and irregular leucine-rich repeats (LRRs) that are characteristic of resistance genes cloned from other species. Unique features of the RGC2 gene products include a bipartite LRR region with >40 repeats. At least eight members of this family are transcribed. The level of sequence diversity between family members varied in different regions of the gene. The ratio of nonsynonymous (Ka) to synonymous (Ks) nucleotide substitutions was lowest in the region encoding the nucleotide binding site, which is the presumed effector domain of the protein. The LRR-encoding region showed an alternating pattern of conservation and hypervariability. This alternating pattern of variation was also found in all comparisons within families of resistance genes cloned from other species. The Ka /Ks ratios indicate that diversifying selection has resulted in increased variation at these codons. The patterns of variation support the predicted structure of LRR regions with solvent-exposed hypervariable residues that are potentially involved in binding pathogen-derived ligands.

Amino Acid Sequence↗

PubMed1997

Self-reinforcing spatial patterns enslave evolution in a host-parasitoid system.

Spatially structured models of host-parasitoid interactions exhibit self-structuring into spatial patterns such as spiral waves and turbulence. We discuss the consequences of these patterns in an eco-evolutionary model of host-parasitoid interactions with evolution of the parasitoids' ability to disperse towards dense populations of hosts (termed the aggregation strength). It turns out that the direction of, and the time-scale over which the evolutionary selection pressure acts depends on the type of spatial pattern a parasitoid finds itself in. Evolution tends to reinforce the existence of the prevalent local pattern. Moreover, there is also competition between the patterns that ultimately determines the eco-evolutionary attractor. It is the interaction between multiple processes across spatial and temporal scales that leads to the rich meso-scale behaviour. Predicting the evolutionary outcome from statistical measures and subprocesses is shown to give incorrect and conflicting answers. Comparison with the behaviours of the complex Ginzburg-Landau equation shows striking similarities on which we comment.

Animals↗

PubMed1996

Metapopulations and equilibrium stability: the effects of spatial structure.

Recently, there has been a great deal of interest in the dynamics of metapopulations, where a number of local populations are coupled via dispersal. The importance of movement for the persistence of an ensemble of locally unstable patches has been established in many studies. In this paper, we present analytical and simulation results concerning the effects of spatial structure on the equilibrium stability of individual populations. We conclude that for general single-species and two-species competition models, the introduction of the spatial dimension in a biologically sensible way has no effect on the overall stability properties. In host-parasitoid models, however, strong host or parasitoid over-dispersal may be destabilizing.

Animals↗