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Biomedical subjects

I M Hastings

Publications and source records attributed to I M Hastings.

At least 19 recordsLinked to original sources
PubMed2006

Complex dynamics and stability of resistance to antimalarial drugs.

A succession of antimalarial drugs has been deployed to treat human falciparum malaria but each has, in turn, been nullified by the spread of drug resistance. The consensus view has always been that, once present, resistance will inevitably rapidly increase to 100%. However, recent field evidence has shown this is not inevitable, and that drug resistance may initially spread and then stabilize at relatively low frequencies. It is proposed that intense competition between separate malaria clones co-infecting the same human can generate complex dynamics capable of explaining this observation. Standard population genetic analysis confirms this assertion. The dynamics underlying the evolution of antimalarial resistance may therefore be much more complex than previously realized, and can resolve the apparent paradox between field data and the underlying theory of the evolution of resistance. This explanation is novel and the results are equally applicable to other parasitic species where multiple infections of the same host are common.

Animals↗
PubMed2001

Reproductive compensation and human genetic disease.

The effects of reproductive compensation on the population genetics of sex-linked recessive lethal mutations are investigated. Simple equations are presented which describe these effects, and so complement existing population genetic theory. More importantly, this type of mutation is responsible for several severe human genetic diseases such as Duchenne muscular dystrophy. It is argued that the applications of three modern reproductive technologies--effective family planning, in utero diagnosis with termination, and embryo sexing--will lead to reproductive compensation. The adoption of any of these technologies may rapidly elevate the frequencies of those mutations which are lethal in childhood. This increase is large, in the order of 33% upwards, and occurs rapidly over two to five generations. It also depends on the source of mutations, the effect being larger if most mutations are paternal. In utero diagnosis and/or embryo sexing increase the frequency of the mutation, but simultaneously decrease disease incidence by preventing the birth of affected offspring. In contrast, effective family planning may rapidly increase both mutation frequency and disease incidence.

Family Planning Services↗
PubMed2001

Modelling parasite drug resistance: lessons for management and control strategies.

Mathematical models of the evolution of drug resistance in infectious diseases are predominantly concentrated in three main areas: antimalarial, antibiotic and anthelmintic resistance. There appears to be little or no cross-reference between them. This literature was examined to identify factors that influence the evolution of drug resistance irrespective of the species and drug under study. The aim is to provide non-technical readers with a basic qualitative understanding of the issues and pitfalls involved in designing drug treatment regimens to minimize the evolution of resistance. The principal factors determining the rate at which resistance evolves appear to be (i) the starting frequency of resistance, (ii) the level and pattern of drug use, (iii) the drug's pharmacokinetic properties, (iv) the number of genes required to encode resistance, (v) the level of sexual recombination in the parasite population, (vi) intrahost dynamics and, in particular, whether 'crowding' effects are present, (vii) the genetic basis of resistance and (viii) the number of individual parasites in an infection. The relative importance of these factors depends on the biology of the organisms under consideration and external factors such as the extent to which the infrastructure of health care delivery constrains the practicalities of drug regimens.

Animals↗
PubMed2000

Modelling a predictable disaster: the rise and spread of drug-resistantmalaria.

The evolution of drug-resistant malaria is one of the most important factors thwarting the development of effective malaria disease control. Several mathematical models have been developed to try and understand the dynamics of this process and how it can be slowed or even avoided. Much of the mathematics describing the evolution of drug resistance in malaria focuses on the derivation and mechanics of the calculations, which can make it inaccessible to experimentalists and field workers. In this article, Ian Hastings and Umberto D'Alessandro describe general model results without recourse to mathematical details, identify the factors that should be considered in the design of drug control programmes, and discuss the crucial parameters that remain unknown and need to be measured in the field or laboratory.

Animals↗
PubMed2000

Models of human genetic disease: how biased are the standard formulae?

Standard theory provides a simple prediction for the frequency of a recessive lethal allele conferring heterozygous protection against an infectious disease (the best-known example being sickle cell protection against malaria). This relationship allows historic disease mortality rates to be estimated. There are, however, hidden biases in this approach. Reproductively active human females in archaic societies normally produce children at intervals of around 4 years. If death of the fetus or young infant (less than around 3 years of age) occurs, then the mother re-enters oestrus and produces another child. This 'reproductive compensation' reduces selection against the agent causing early mortality (the recessive allele or infective agent) and biases our estimates of historic mortality rates. The magnitude of these biases is investigated. Re-conception also constitutes a demographic selective pressure acting alongside natural selection: lethal genetic diseases (or tightly linked loci) will be selected to become ever more virulent, killing at ever decreasing ages, to allow the mother to re-enter oestrus and re-conceive a (hopefully unaffected) sibling; this effect also invalidates statistical tests using the number of alleles to distinguish overdominance from drift as explanations for high allele frequency. The same bias affects calculations of mutation/selection balance: for any given mutation rate, syndromes which kill early in life will reach much higher frequencies than those killing at later ages. An intriguing consequence is that lethal recessive disorders in humans will increase in frequency by up to 45% as a consequence of the recent demographic transition to planned family size.

Genetic Diseases, Inborn↗
PubMed1998

Effects of thyroid hormone deficiency on mice selected for increased and decreased body weight and fatness.

A study was undertaken to test whether the elimination of metabolic pathways strongly involved in growth and fatness, comprising thyroid hormones (TH) and growth hormone (GH), is responsible for a substantial part of the genetic change produced by selection. Lines used in this study have been selected for about 50 generations for high (PH) and low (PL) body weight at 10 weeks and for high (F) and low fat content (L) at 14 weeks, producing a 3-fold difference in body weights and a 5-fold difference in fat content. Thyroid ablation was achieved by repeated backcrossing into the four selection lines of a transgene comprising the HSV1-tk gene coupled to the promoter of the thyroglobulin gene. Hemizygous pregnant dams were treated with ganciclovir leading to thyroid-ablated dams and offspring and therefore to a lack of TH and subsequently of GH. In the absence of TH and GH, lines still differ in body weight over the period studied (10 d to about 100 d; e.g. at the end PH = 32.1 g vs PL = 10.2 g) and in fat content (F = 16.2% vs L = 3.8%); the corresponding values for the wild-type controls were PH = 49.9 g vs PL = 17.4 g and F = 27.5% vs L = 4.8%. The effect of the transgene depended on the genetic background for body weights at most ages and for relative gonadal fat pad weights, but less for fat content. The L line showed the lowest growth depression. The lit gene, which causes GH but not TH deficiency, was also transferred by repeated backcrosses into three of these lines (PH, PL, F). The combined deficiency of TH and GH had bigger effects on body weights at earlier ages than did GH deprivation. The data show that changes in the TH- and GH-systems are not the only cause of line differences in growth and fatness resulting from long-term selection, but both are involved to a significant extent. The interactions between the effects of the transgene and of the lit gene and the genetic background were, nevertheless, relatively small and therefore these results support a polygenic model of selection response.

Animals↗
PubMed1998

The emergence of drug-resistant malaria.

Stochastic processes play a vital role in the early stages of the evolution of drug-resistant malaria. We present a simple and flexible method for investigating these processes and understanding how they affect the emergence of drug-resistant malaria. Qualitatively different predictions can be made depending on the biological and epidemiological factors which prevail in the field. Intense intra-host competition between co-infecting clones, low numbers of genes required to encode resistance, and high drug usage all encourage the emergence of drug resistance. Drug-resistant forms present at the time drug application starts are less likely to survive than those which arise subsequently; survival of the former largely depends on how rapidly malaria population size stabilizes after drug application. In particular, whether resistance is more likely to emerge in areas of high or low transmission depends on malaria intra-host dynamics, the level of drug usage, the population regulation of malaria, and the number of genes required to encode resistance. These factors are discussed in relation to the practical implementation of drug control programmes.

Drug Resistance↗
PubMed1998

Mutation and selection within the individual.

Selection within the individual may have played a critical and creative role in evolution, boosting the survival chances of mutations beneficial to the cell and the individual, hindering the spread of deleterious mutations, and reducing the genetic load imposed on the population. We review the literature and present new results to describe the effects of cell-lineage selection on the rate and fixation probability of new mutations. Cell-lineage selection can alter these quantities by several orders of magnitude. Cell-lineage selection is especially important in the case of rare recessive mutations, which are hidden from selection at the individual level but may be exposed to selection at cellular level. Because selection within the individual acts as a sieve eliminating deleterious mutations and increasing the frequency of beneficial ones, mutations observed among progeny will have been pre-selected and are more likely to increase cell proliferation than would randomly generated mutations. Although many authors have focused on the potential conflict between selection at the cellular and individual levels, it must be much more common that the two levels act concordantly. When selection at the cell and individual levels act in a cooperative manner, increased rather than decreased opportunity for germline selection will be favored by evolution.

Animals↗
PubMed1997

Sex, strains and virulence.

When are populations of infectious agents likely to evolve into distinct strains? Are they likely to differ in their virulence? Will genetically distinct strains or clones remain stable long enough to be useful as epidemiological markers? Sexual recombination can break down the genetic associations that define a strain structure, but if sex is rare or inbreeding is common, can strains persist? In this paper Ian Hastings and Bruce Wedgwood-Oppenheim show how some simple population genetic theory may provide a basis for addressing these questions.

Journal Article↗
PubMed1997

The effect of testosterone in mice divergently selected on fat content or body weight.

Lines of mice have been divergently selected on one of two traits: (i) estimated fat content at 14 weeks of age, which has resulted in a 5-fold divergence, and (ii) body weight at 10 weeks of age, which has resulted in a 3-fold divergence. Individuals from each line were castrated or sham operated at 10 days of age and subsequently given either exogenous testosterone or the appropriate control from 14 days of age. Castration increased fat content and decreased lean weight in all lines, an effect which was not reversed by administration of testosterone. Body weight was reduced by around 10% as a result of castration and this effect was at least partially reversed by exogenous testosterone. Analysis of body weight, fat content and lean mass at 10 weeks of age failed to detect any interaction between these treatments and genetic background. It is therefore concluded that testosterone metabolism has not contributed disproportionately to the response to artificial selection in spite of its known effects on growth and body composition.

Adipose Tissue↗
PubMed1997

A model for the origins and spread of drug-resistant malaria.

A general method of investigating parasite population genetics is presented and used to investigate the evolution of drug resistance in Plasmodium. The most important biological factor is the nature of the control, presumably through host immunity, of the malarial infection. Two models are examined: a 'generalized immunity' (GI) model in which immunity regulates the overall level of infection, and a 'specific immunity' (SI) model in which each clone within the infection is regulated independently. These models are used to investigate 3 critical factors in the evolution of resistance: (i) the frequency of resistant alleles in the population prior to drug use, (ii) the dynamics of resistance following drug application and (iii) the magnitude of threshold frequencies below which resistance will not evolve. These analyses also identify the implicit assumptions made in several previous models, reconcile their differing conclusions and allow a more informed debate about the practical application of drugs.

Alleles↗
PubMed1996

Population genetics and the detection of immunogenic and drug-resistant loci in Plasmodium.

Host immunity will result in increased homozygosity at immunogenic loci if they encode products that elicit allele-specific immune responses. This increased homozygosity can be detected by comparison to 'neutral' (i.e. unselected) loci in the same genome which act as controls for the various epidemiological factors, such as biting rate, which affect homozygosity. Numerical results suggest that homozygosity should be 20 to 500% higher at immunogenic loci, a result which is robust to changes in host death rate and the degree of linkage between the immunogenic and neutral loci. The same logic applies to loci which encode drug resistance: treated individuals with resistant infections will transmit zygotes homozygous for the resistance allele. It is argued that this increased homozygosity at putative immunogenic loci can act as a diagnostic feature to test using field data. The method also serves as a potentially very powerful method of identifying immunogenic and drug-resistant loci in laboratory studies of species such as the murine malaria Plasmodium chabaudi.

Alleles↗
PubMed1995

Behavioural changes as a correlated response to selection.

Lines of mice have been selected for up to 50 generations on the following traits: high body weight, low body weight, high fat content or low fat content. The lines selected for high or low body weight differ by a factor of 2.5 and those selected for high or low fat content differ by a factor of five, both traits measured in 10 week old males. A set of behavioural traits was measured to ascertain whether this selection had caused correlated responses in behaviour: studies included feeding behaviour, open field behaviour, ultrasound calling rates of pups, and the response to the introduction of a novel physical object. Alterations in behavioural patterns which were expected a priori were observed but there appeared to be no changes in behaviour associated with any one selection criterion. Estimates of the genetic correlations between selected and behavioural traits were, with one exception, generally less than 0.1 in magnitude and not significantly different from zero (the exception was food intake in lines selected on body weight). Assuming that mice are accurate models for commercial species, then these results have important implications for animal welfare: they demonstrate that large scale behavioural changes do not arise as an inevitable consequence of intense long-term selection on traits of economic importance in commercial species.

Adipose Tissue↗
PubMed1994

Manifestations of sexual selection may depend on the genetic bases of sex determination.

A variant of the 'handicap' model of sexual selection is described which predicts that the evolution of ornate male traits occurs more easily in species where females are the heterogametic sex. The process occurs even when the alleles conferring high paternal 'fitness' remain advantageous for only a short time due to a rapidly changing physical or biotic environment: the timescale of this advantage may approach the gestation time of the organism. This provides an explanation as to why sexual selection in species where females are heterogametic (such as birds) occurs mainly by the elaboration of ornate male secondary sexual characteristics, whereas in species where females are homogametic (such as mammals) sexual selection results predominantly in inter-male rivalry and the evolution of traits such as horns, antlers and large body size. An analogy between the evolution of elaborate male traits and the evolution of warning coloration is noted.

Alleles↗
PubMed1994

Selfish DNA as a method of pest control.

The inheritance of most genes is tightly controlled, governed by the rules of mendelian inheritance if nuclear or uniparental inheritance if cytoplasmic. A few notable genes and cytoplasmic genomes have escaped this regulation. Such genes may spread by increasing their own rate of transmission despite reducing host fitness and may be regarded as 'selfish'. Their population genetics are described and it appears they may impose a significant genetic load on the host population. Modern molecular techniques may enable similar loads to be imposed on pest species either by transferring selfish genes between species, or by linking deleterious genes to a selfish locus. Alternatively, 'modifier' genes that eliminate the virulent, or disease vectorial capacity, of the pest population may be introduced by linkage to a selfish locus. Selfish elements present in multiple copies may be preferable to single-copy elements as the former are capable of a larger reduction in host fitness. The practical application of these agents depends on five factors: (i) the rate of 'reversion' to a non-selfish form; (ii) the evolution of host repressor systems; (iii) their effect on host fitness, which determines their rate of invasion; (iv) the mechanism regulating host population size in the field; and (v) their ease of manipulation in the laboratory. The first two factors are the most uncertain in most systems, but should be amenable to experimental analysis. It is proposed that the development of such techniques may result in powerful new methods of population control which may be applied to both agricultural pests and disease vectors.

Alleles↗
PubMed1993

The role of growth hormone in lines of mice divergently selected on body weight.

An understanding of the physiological and genetic changes which determine the response to selection is critical for both evolutionary theory and to assess the application of new molecular techniques to commercial animal breeding. We investigated an aspect of physiology, growth hormone (GH) metabolism, which might a priori have been expected to play a large part in the response of mouse lines selected for high or low body weight. Disruption of endogenous GH or addition of exogenous GH had similar proportionate effects on body weight in both lines of mice (although differences in body composition arose) suggesting that neither the production of GH nor receptor sensitivity to GH had been altered as a result of selection. This supports a 'pleiotropic model' of the response to selection: that many genes with diverse metabolic roles all contribute to the divergent phenotype. This result has significant commercial implications as it suggests that artificial selection, transgenic technology and environmental manipulation may be synergistic rather than antagonistic strategies.

Animals↗