Sex-specific effects of antagonistic coevolution: Insights from an insect host and a bacterial pathogen coevolution system

Experimental host-pathogen coevolution provides an opportunity to
understand the long-term consequences of adaptive interactions between
hosts and pathogens. Studies using prokaryotic and eukaryotic hosts and
their pathogens have explored the changing dynamics of antagonistic
interactions with time, evolution of generalists or specialists, and
potential costs associated with coevolution. However, the dependence of
host-pathogen coevolutionary responses on sex of the host remains
unexplored. To address this, we chose a host species which allow us to
compare coevolved traits between male and female hosts with their native
pathogens. Towards this end, we conducted a laboratory experiment where we
coevolved insect host Drosophila melanogaster with its bacterial pathogen
Pseudomonas entomophila. Apart from the host-pathogen coevolution regimen,
the experimental design included three other selection regimes - host
adaptation to a non-evolving ancestral pathogen, and two control regimes.
To study coevolved traits in hosts in pathogens across time and sex, we
measured host survivorship post infection against pathogens from three
evolutionary time points – ancestor, pathogen from the past, and present
coevolution cycle. Our results showed that the coevolved hosts exhibited
higher survivorship when exposed to pathogens, relative to the hosts
adapted to non-evolved pathogen and control hosts. These results are true
against pathogenic exposure from all three time points. Coevolved
pathogens from the present time exhibited the highest virulence, which was
variable across the replicate pathogen populations. We also observed that
despite comparable mortality, the two sexes differ in the onset of
mortality in the control regimes, a response not observed in the coevolved
hosts. Taken together, our results showed that pathogens and hosts had the
greatest success against each other as they coevolved together, but the
susceptibility of naïve hosts was sex-specific. These results provide
important insights into the process of host-pathogen coevolution.