Epigenetic inheritance and the evolution of infectious diseases

Abstract

Genes with identical DNA sequences may show differential expression because of epigenetic marks. These marks in pathogens are key to their virulence and are being evaluated as targets for medical treatment. Where epigenetic marks were created in response to past conditions (epigenetically inherited), they represent a form of memory, the impact of which has not been considered in the evolution of infectious diseases. We fill this gap by exploring the evolution of virulence in pathogens that inherit epigenetic information on the sex of their previous host. We show that memories of past hosts can also provide clues about the sex of present and future hosts when women and men differ in their immunity to infection and/or their interactions with the sexes. These biological and social differences between the sexes are pervasive in humans. We show that natural selection can favour the evolution of greater virulence in infections originating from one sex. Furthermore, natural selection can favour the evolution of greater virulence in infections across sexes (or within sexes). Our results explain certain patterns of virulence in diseases like measles, chickenpox and polio that have puzzled medical researchers for decades. In particular, they address why girls infected by boys (or boys infected by girls) are more likely to die from the infection than girls infected by girls (or boys infected by boys). We propose epigenetic therapies to treat infections by tampering with the memories of infecting pathogens. Counterintuitively, we predict that successful therapies should target pathogens genes that inhibit virulence, rather than those enhancing virulence. Our findings imply that pathogens can carry memories of past environments other than sex (e.g. those related to socioeconomic status) that may condition their virulence and could signify an important new direction in personalised medicine.