Philip Agnew

Host life history responses to parasitism

Parasites and their infections can adversely effect a hosts growth, reproduction and survival. These effects are often not immediate, but increase with time since infection. A general prediction from evolutionary biology is that hosts suffering from this type of infection should preferentially allocate resources towards reproduction, even if this is at the expense of their growth and survival. This review illustrates this argument with several empirical studies showing hosts behaving in this manner. These studies indicate that one way for hosts to reduce the costs of parasitism is by altering their life history traits to bring forward their schedule of reproduction.

A minimalist approach to the effects of density-dependent competition on insect life-history traits

Abstract  1. Due to its effects on the phenotypic and genotypic expression of life‐history traits, density‐dependent competition is an important factor regulating the growth of populations. Specifically for insects, density‐dependent competition among juveniles is often associated with increased juvenile mortality, delayed maturity, and reduced adult size.

Virulence reaction norms across a food gradient.

Host–parasite interactions involve competition for nutritional resources between hosts and the parasites growing within them. Consuming part of a hosts resources is one cause of a parasites virulence, i.e. part of the fitness cost imposed on the host by the parasite. The influence of a hosts nutritional conditions on the virulence of a parasite was experimentally tested using the mosquito Aedes aegypti and the microsporidian parasite Vavraia culicis. A condition–dependent expression of virulence was found and a positive relation between virulence and transmissibility was established. Spore production was positively influenced by host food availability, indicating that the parasiteapos;s within–host growth is limited by host condition. We also investigated how the fitness of each partner varied across the nutritional gradient and demonstrated that the sign of the correlation between host fitness and parasite fitness depended on the amount of nutritional resources available to the host.

Costs and benefits of multiple resistance to insecticides for Culex quinquefasciatus mosquitoes

BackgroundThe evolutionary dynamics of xenobiotic resistance depends on how resistance mutations influence the fitness of their bearers, both in the presence and absence of xenobiotic selection pressure. In cases of multiple resistance, these dynamics will also depend on how individual resistance mutations interact with one another, and on the xenobiotics applied against them. We compared Culex quinquefasciatus mosquitoes harbouring two resistance alleles ace-1Rand KdrR(conferring resistance to carbamate and pyrethroid insecticides, respectively) to mosquitoes bearing only one of the alleles, or neither allele. Comparisons were made in environments where both, only one, or neither type of insecticide was present.ResultsEach resistance allele was associated with fitness costs (survival to adulthood) in an insecticide-free environment, with the costs of ace-1Rbeing greater than for KdrR. However, there was a notable interaction in that the costs of harbouring both alleles were significantly less than for harbouring ace-1Ralone. The two insecticides combined in an additive, synergistic and antagonistic manner depending on a mosquitos resistance status, but were not predictable based on the presence/absence of either, or both mutations.ConclusionInsecticide resistance mutations interacted to positively or negatively influence a mosquitos fitness, both in the presence or absence of insecticides. In particular, the presence of the KdrRmutation compensated for the costs of the ace-1Rmutation in an insecticide-free environment, suggesting the strength of selection in untreated areas would be less against mosquitoes resistant to both insecticides than for those resistant to carbamates alone. Additional interactions suggest the dynamics of resistance will be difficult to predict in populations where multiple resistance mutations are present or that are subject to treatment by different xenobiotics.

Coevolution between Parasite Virulence and Host Life-History Traits

Epidemiological models generally explore the evolution of parasite life‐history traits, namely, virulence and transmission, against a background of constant host life‐history traits. However, life‐history models have predicted the evolution of host traits in response to parasitism. The coevolution of host and parasite life‐history traits remains largely unexplored. We present an epidemiological model, based on resource allocation theory, that provides an analysis of the coevolution between host reproductive effort and parasite virulence. This model allows for hosts with either a fixed (i.e., genetic) or conditional (i.e., a phenotypically plastic) response to parasitism. It also considers superinfections. We show that parasitism always favors increased allocation to host reproduction, but because of epidemiological feedbacks, the evolutionarily stable host reproductive effort does not always increase with parasite virulence. Superinfection drives the evolution of parasite virulence and acts on the evolution of the host through parasite evolution, generally leading to higher host reproductive effort. Coevolution, as opposed to cases where only one of the antagonists evolves, may generate correlations between host and parasite life‐history traits across environmental gradients affecting the fecundity or the survival of the host. Our results provide a theoretical framework against which experimental coevolution outcomes or field observations can be contrasted.

Effects of Density and Larval Competition on Selected Life History Traits of Culex pipiens quinquefasciatus (Diptera: Culicidae)

Abstract The effects of larval densities of one to four individuals in standard Drosophila-vials (diameter 25 by 95 mm) on the age at pupation, starved dry weight, and wing length of Culex pipiens quinquefasciatus Say were studied. This approach required relatively few larvae per replicate and included a control treatment, where individual larvae developed in the absence of competition. This design also tested for competitive interactions between male and female larvae. Mosquitoes pupated later, and emerged with lighter starved dry adult weight and shorter wings as larval density increased. The size of adult female mosquitoes, particularly their starved dry weight, was sensitive to larval density and also was influenced by the presence or absence of competition with another female larva. In contrast, the life history traits of males did not vary as a function of competition with female larva. Female larvae were also more likely to die in the highest density treatment. This design confirmed previous results and offered a potentially useful experimental approach to investigate the effects of density-dependent competition among mosquito larvae.

Asexual reproduction: Genetics and evolutionary aspects

Abstract.Reproduction is essential to all organisms if they are to contribute to the next generation. There are various means and ways of achieving this goal. This review focuses on the role of asexual reproduction for eukaryotic organisms and how its integration in a life cycle can influence their population genetics and evolution. An important question for evolutionary biologists as to why some organisms reproduce sexually, as opposed to asexually, is addressed. We also discuss the economic and medical importance of asexual organisms.

PARASITISM INCREASES AND DECREASES THE COSTS OF INSECTICIDE RESISTANCE IN MOSQUITOES

Abstract Adaptations conferring resistance to xenobiotics (antibiotics, insecticides, herbicides, etc.) are often costly to the organisms fitness in the absence of the selecting agent. In such conditions, and unless other mutations compensate for the costs of resistance, sensitive individuals are expected to out‐reproduce resistant individuals and drive resistance alleles to a low frequency, with the rate and magnitude of this decline being proportional to the costs of resistance. However, this evolutionary dynamic is open to modification by other sources of selection acting on the relative fitness of susceptible and resistant individuals. Here we show parasitism not only as a source of selection capable of modifying the costs of organophosphate insecticide resistance in mosquitoes, but also that qualitatively different interactions (increasing or decreasing the relative fitness of resistant individuals) occurred depending on the particular form of resistance involved. As estimates of the parasites fitness also varied according to its hosts form of resistance, our data illustrate the potential for epidemiological feedbacks to influence the strength and direction of selection acting on resistance mutations in untreated environments.

Costs of insensitive acetylcholinesterase insecticide resistance for the malaria vector Anopheles gambiae homozygous for the G119S mutation

BackgroundThe G119S mutation responsible for insensitive acetylcholinesterase resistance to organophosphate and carbamate insecticides has recently been reported from natural populations of Anopheles gambiae in West Africa. These reports suggest there are costs of resistance associated with this mutation for An. gambiae, especially for homozygous individuals, and these costs could be influential in determining the frequency of carbamate resistance in these populations.MethodsLife-history traits of the AcerKis and Kisumu strains of An. gambiae were compared following the manipulation of larval food availability in three separate experiments conducted in an insecticide-free laboratory environment. These two strains share the same genetic background, but differ in being homozygous for the presence or absence of the G119S mutation at the ace-1 locus, respectively.ResultsPupae of the resistant strain were significantly more likely to die during pupation than those of the susceptible strain. Ages at pupation were significantly earlier for the resistant strain and their dry starved weights were significantly lighter; this difference in weight remained when the two strains were matched for ages at pupation.ConclusionsThe main cost of resistance found for An. gambiae mosquitoes homozygous for the G119S mutation was that they were significantly more likely to die during pupation than their susceptible counterparts, and they did so across a range of larval food conditions. Comparing the frequency of G119S in fourth instar larvae and adults emerging from the same populations would provide a way to test whether this cost of resistance is being expressed in natural populations of An. gambiae and influencing the dynamics of this resistance mutation.

Life history interactions with environmental conditions in a host–parasite relationship and the parasite's mode of transmission

The microsporidian parasite Edhazardia aedis is capable of vertical or horizontal transmission among individuals of its host, the mosquito Aedes aegypti, and either mode of transmission may follow the other. We show that following the horizontal infection of host larvae, the parasites subsequent mode of transmission largely depends on host life history traits and their responses to different environmental conditions. In two experiments the intensity of larval exposure to infection and the amount of food available to them were simultaneously manipulated. One experiment followed the dynamics of host development and the parasites production of spores while the other estimated the outcome of their relationship. Host life history traits varied widely across treatment conditions while those of the parasite did not. Of particular importance was the hosts larval growth rate. Horizontal rather than vertical transmission by the parasite was more likely as low food and high dose conditions favoured slower larval growth rates. This pattern of transmission behaviour with host growth rate can be considered in terms of reproductive value: the potential vertical transmission success that female mosquitoes offer the parasite decreases as larval growth rates slow and makes them more attractive to exploitation for horizontal transmission (requiring host mortality). However, the lack of variation in the parasites life history traits gave rise in some conditions to low estimates for both its vertical and horizontal transmission success. We suggest that the unresponsive behaviour of the parasites life history traits reflects a bet-hedging strategy to reduce variance in its overall transmission success in the unpredictable environmental conditions and host larval growth rates that this parasite encounters in nature.