Evolutionary Ecology of Immunity

 

Contrary to the existing practice of using non-pathogenic bacteria, we addressed the issue of evolution of Immune response using natural pathogens of Drosophila melanogaster. Our studies clearly show that bacterial infection can have differential effects on fitness-related traits of either sex. We have found that infections with Serratia, a natural pathogen of Drosophila, significantly reduce the reproductive fitness of females but have no significant effect on the reproductive fitness of males. Longevity is unaffected by infection in both sexes and bacterial growth in males and females is not significantly different. Such sex-specific effects of infection on life-history related traits can have major consequences for the overall fitness of the sexes and thereby alter the evolutionary trajectory of the evolution of antibacterial immune response.

 

Our studies, for the first time, have revealed that the immunity of females can be affected by the phenotype of its mate. We found that females mated to large males have significantly lower immunity and fecundity compared to females mated with smaller males. This difference was evident even after a single mating, indicating that this effect is biochemically mediated. Thus, we have identified an additional cost of mating with large males in terms of reduced immunity. This can have major consequences for the evolution of body size and male-female conflict.

Our studies, again for the first time, have revealed adaptive male mate choice based on female infection status. We have shown that males bias their mating towards uninfected females when given a choice. This mating bias cannot be explained in terms of female mating behaviour, and hence is clearly indicative of adaptive male mate choice. A recent set of studies show that mating may enhance antibacterial immunity among males.

 

Mated males die in lesser numbers and live for longer when infected with a pathogen in comparison with virgin males. However, such beneficial effects of mating were pathogen specific. The duration of cohabitation with the females and the time gap between the last mating event and infection severely affect antibacterial immunity. Singly mated males have the same immunity as virgins and mated males separated from females for 4 days have same immunity as virgin males.

We have addressed the aging of the immune response using long-term laboratory populations as well as fresh caught wild populations held as iso -female lines. Our results indicate that middle-aged flies have better immune response compared to young flies. This difference could indicate altered resource investments at the two ages, with reproductive investment predominating at young age and somatic maintenance dominating at middle age. Such aging is unaffected by sex or mating status of the flies. Parental age seems to have no effect on progeny immunity - a question that has been unanswered so far. We have now created a set of populations of Drosophila melanogaster that are being selected for increased resistance to infection from a natural pathogen. We have seen that resistance evolves within 5 generations of selection with the selected populations being almost twice as resistant as control populations.

 

Analysis of expression of genes known to be involved in immune pathways indicates that the expression levels of specific genes have evolved in the selected population. Further experiments are underway.