Concurrent evolution of resistance and tolerance to pathogens

Recent experiments on plant defenses against pathogens or herbivores have shown various patterns of the association between resistance, which reduces the probability of being infected or attacked, and tolerance, which reduces the loss of fitness caused by the infection or attack. Our study describes the simultaneous evolution of these two strategies of defense in a population of hosts submitted to a pathogen. We extended previous approaches by assuming that the two traits are independent (e.g., determined by two unlinked genes), by modeling different shapes of the costs of defenses, and by taking into account the demographic and epidemiological dynamics of the system. We provide novel predictions on the variability and the evolution of defenses. First, resistance and tolerance do not necessarily exclude each other; second, they should respond in different ways to changes in parameters that affect the epidemiology or the relative costs and benefits of defenses; and third, when comparing investments in defenses among different environments, the apparent associations among resistance, tolerance, and fecundity in the absence of parasites can lead to the false conclusion that only one defense trait is costly. The latter result emphasizes the problems of estimating trade-offs and costs among natural populations without knowledge of the underlying mechanisms.     

Malaria infection increases attractiveness of humans to mosquitoes

Do malaria parasites enhance the attractiveness of humans to the parasite's vector? As such manipulation would have important implications for the epidemiology of the disease, the question has been debated for many years. To investigate the issue in a semi-natural situation, we assayed the attractiveness of 12 groups of three western Kenyan children to the main African malaria vector, the mosquito Anopheles gambiae. In each group, one child was uninfected, one was naturally infected with the asexual (non-infective) stage of Plasmodium falciparum, and one harboured the parasite's gametocytes (the stage transmissible to mosquitoes). The children harbouring gametocytes attracted about twice as many mosquitoes as the two other classes of children. In a second assay of the same children, when the parasites had been cleared with anti-malarial treatment, the attractiveness was similar between the three classes of children. In particular, the children who had previously harboured gametocytes, but had now cleared the parasite, were not more attractive than other children. This ruled out the possibility of a bias due to differential intrinsic attractiveness of the children to mosquitoes and strongly suggests that gametocytes increase the attractiveness of the children.     

Coevolutionary interactions between host and parasite genotypes

More than 20 years after Dawkins introduced the concept of "extended phenotype" (i.e. phenotypes of hosts and parasites result from interactions between the two genomes) and although this idea has now reached contemporary textbooks of evolutionary biology, most studies of the evolution of host-parasite systems still focus solely on either the host or the parasite, neglecting the role of the other partner. It is important to consider that host and parasite genotypes share control of the epidemiological parameters of their relationship. Moreover, not only the traits of the infection but also the genetic correlations among these and other traits that determine fitness might be controlled by interactions between host and parasite genotypes.     

Environmental influence on the genetic basis of mosquito resistance to malaria parasites

The genetic basis of a host's resistance to parasites has important epidemiological and evolutionary consequences. Understanding this genetic basis can be complicated by non-genetic factors, such as environmental quality, which may influence the expression of genetic resistance and profoundly alter patterns of disease and the host's response to selection. In particular, understanding the environmental influence on the genetic resistance of mosquitoes to malaria gives valuable knowledge concerning the use of malaria-resistant transgenic mosquitoes as a measure of malaria control. We made a step towards this understanding by challenging eight isofemale lines of the malaria vector Anopheles stephensi with the rodent malaria parasite Plasmodium yoelii yoelii and by feeding the mosquitoes with different concentrations of glucose. The isofemale lines differed in infection loads (the numbers of oocysts), corroborating earlier studies showing a genetic basis of resistance. In contrast, the proportion of infected mosquitoes did not differ among lines, suggesting that the genetic component underlying infection load differs from the genetic component underlying infection rate. In addition, the mean infection load and, in particular, its heritable variation in mosquitoes depended on the concentration of glucose, which suggests that the environment affects the expression and the evolution of the mosquitoes' resistance in nature. We found no evidence of genotype-by-environment interactions, i.e. the lines responded similarly to environmental variation. Overall, these results indicate that environmental variation can significantly reduce the importance of genes in determining the resistance of mosquitoes to malaria infection.     

Costs and benefits of resistance against antimalarial drugs

Recent experiments have suggested that resistance to antimalarial drugs, in particular chloroquine, is associated with increased transmission. However, epidemiological patterns suggest the opposite: ie. that resistance should be associated with a transmission cost. Here, Jacob Koella reviews the evidence for either a cost or a benefit of chloroquine resistance and proposes ideas from population and evolutionary biology that might explain the apparent contradiction between experimental and epidemiological evidence.     

A correlated response of a parasite’s virulence and life cycle to selection on its host’s life history

We demonstrate a correlated response of the virulence and the mode of transmission of the microsporidian parasite Edhazardia aedis to selection on the age at pupation of its host, the mosquito Aedes aegypti. We selected three lines of mosquitoes each for early or late pupation and exposed the larvae after zero, two and four generations of selection to a low and a high concentration of the parasite’s spores. Before selection the parasites induced a similar level of mortality in the six lines; after four generations of selection mortality was higher in the mosquitoes selected for late pupation than in those selected for early pupation. Overall, parasite-induced mortality was positively correlated with the mean age at pupation of the matching uninfected line. When they died, mosquitoes selected for early pupation harboured mostly binucleate spores, which are responsible for vertical transmission. Mosquitoes selected for late pupation were more likely to harbour uninucleate spores, which are responsible for horizontal transmission. The parasite enhanced this tendency for horizontal transmission by prolonging the larval period in the lines selected for late pupation, but not in the ones selected for early pupation. These results suggest that the genetic basis of the mosquito’s age at pupation helps to determine the parasite’s mode of transmission: parasites in rapidly developing mosquitoes are benign and transmit vertically, while parasites in slowly developing mosquitoes are virulent and transmit horizontally. Thus, as the host’s life history evolves, the parasite’s performance changes, because the host’s evolution changes the environment in which the parasite develops.     

Epidemiological evidence for an association between chloroquine resistance of Plasmodium falciparum and its immunological properties

The appearance of chloroquine-resistant genotypes o f Plasmodium falciparum has thwarted the goal of global eradication of malaria. Although much effort has been put into understanding the molecular mechanisms of chloroquine resistance, many questions about its distribution remain open: Why, some 30 years after the emergence o f chloroquine resistance, have resistant genotypes not taken over the population? Why have many parasites remained sensitive? Why, after its first appearance in Africa, has chloroquine resistance spread so rapidly through sub-Saharan Africa? In this paper Jacob Koella reviews epidemiological data that suggest that an answer to these questions may involve an association between chloroquine resistance and immunological properties o f malaria parasites.     

Optimal Pattern of Replication and Transmission for Parasites with Two Stages in Their Life Cycle

This paper describes how a parasite with distinct stages for replication within its host and for transmission among hosts should schedule the production of the two stages so that it achieves maximal transmission. A mathematical model of the within-host dynamics of a parasite and of its interactions with the immune response predicts that the optimal pattern of investment depends largely on the relationships between the growth rate of the parasite, the rate of increase of immunity against the parasite, and parasite-induced mortality of the host. We consider first a parasite with a constant, time-independent level of investment in transmission. If such a parasite grows rapidly and can therefore reach a density that kills the host before it is cleared by the immune response, it can achieve maximal transmission by producing transmission stages, and thus reducing its effective growth rate, to the extent that its peak density is just below the lethal density. This leads to the prediction that investment in transmission should be positively correlated with growth rate. In contrast, if the parasite grows more slowly and is cleared by the immune system before it can reach lethal density, the level of investment should be negatively correlated with growth rate. If a parasite can vary its investment into transmission during the course of infection, it should delay investment into transmission until it reaches lethal density or until shortly before it is cleared by the host′s immune system. If a parasite grows slowly in comparison with immunity, the optimal pattern of investment is a bang-bang pattern: the investment switches from total production of the replication stage to total production of the transmission stage shortly before the parasite is cleared by the immune response. If a parasite grows much more rapidly than immunity, the parasite initially replicates up to lethal density without producing any transmission stages, then produces transmission stages at the rate that reduces its effective growth rate to zero and thus allows it to be maintained at lethal density, and finally switches to complete investment into transmission stages shortly before it is cleared by the immune system