Pathogens, or disease producing organisms, will inevitably encounter hosts that differ in their resistance to infection and theory suggests that this will affect pathogen evolution. If a pathogen routinely only encounters one type of host resistance, it will develop a specialist strategy that dictates the severity of infection. It is likely that the age and sex of a host will be a source of variation governing the evolution of infectious disease.
As part of his PhD, Stephen Gipson has used a series of experimental infection trials to test these ideas by investigating how males and females of different ages would respond to infection by two strains of a pathogen. Stephen and his PhD supervisor, Matt Hall, predicted that mortality rates and virulence (severity of infection) would increase with the age at exposure as a consequence of the aging process and, in addition, that these increasing costs would be felt most strongly by the less resistant sex.
Stephen used the freshwater crustacean Daphnia magna and its common pathogen Pasteuria ramosa as a model system to test these ideas. Daphnia can produce genetically identical male and female clones where the females live longer and are less resistant to infection. The pathogen P. ramosa invades the host via attachment to the oesophagus and reproduces within the haemolymph (fluid analogous to blood in the in the arthropod circulatory system) of the infected Daphnia, filling the body with transmission spores, which are released when the host dies.
What Stephen and Matt found was actually a bit more complicated than their predictions. Mortality rates were higher in females (the less resistant sex in Daphnia) as predicted but mortality rates only increased with age in females. They proposed that the patterns observed might have more to do with the exploitation potential of males and females to a pathogen rather than an ageing immune system; the larger size and longer life span of females may provide more resources and more time for exploitation by the pathogen.
Stephen also found that when infecting females, the two pathogen genotypes displayed a range of relationships between spore loads (transmission potential) and virulence (relative reduction in lifespan), in contrast with males, where patterns of virulence and transmission were the same for both strains of pathogen (see graphs).
In general, evolution favours pathogens which strike a balance between transmission and virulence. Stephen and Matt have shown how complex interactions between host-sex and the age at which a host encounters a pathogen can facilitate greater variation in the evolution of infectious disease. Understanding a population’s sex ratio and age structure may be crucial in predicting the severity and spread of disease.