Idiosyncratic effects of bacterial infection on female fecundity in Drosophila melanogaster

Existing theories make different predictions regarding the effect of a
pathogenic infection on the host capacity to reproduce. Terminal
investment theory suggests that due to the increased risk of mortality,
and the associated risk of losing future opportunity to reproduce,
infected individuals would increase their investment towards reproduction.
Life-history theory posits that due to energetic and resource costs
associated with mounting an immune defense, hosts would decrease their
investment towards reproduction, and reallocate resources towards defense
and survival. Additionally, Somatic damage incurred by the host due to the
infection is also expected to compromise the host capacity to reproduce.
We explored these possibilities in Drosophila melanogaster
females experimentally infected with pathogenic bacteria. We tested if the
effect of infection on female fecundity is pathogen specific, determined
by infection outcome, and variable between individual infected females. We
observed that the mean, population level change in post-infection female
fecundity was pathogen specific, but not correlated with mortality risk.
Furthermore, infection outcome, i.e., if the infected female died or
survived the infection, had no effect on fecundity at this level. At
individual resolution, females that died after infection exhibited greater
variation in fecundity compared to ones that survived the infection. This
increased variation was bidirectional, with some females reproducing in
excess while others reproducing less compared to the controls. Altogether,
our results suggest that post-infection female fecundity is unlikely to be
driven by risk of mortality and is probably determined by the precise
physiological changes that an infected female undergoes when infected by a
specific pathogen.