Genotype and diet affect resistance, survival, and fecundity but not fecundity tolerance

Abstract

Insects are exposed to a variety of potential pathogens in their environment, many of which can severely impact fitness and health. Consequently, hosts have evolved resistance and tolerance strategies to suppress or cope with infections. Hosts utilizing resistance improve fitness by clearing or reducing pathogen loads, and hosts utilizing tolerance reduce harmful fitness effects per pathogen load. To understand variation in, and selective pressures on, resistance and tolerance, we asked to what degree they are shaped by host genetic background, whether plasticity in these responses depends upon dietary environment, and whether there are interactions between these two factors. Females from ten wild-type Drosophila melanogaster genotypes were kept on high-or low-protein (yeast) diets and infected with one of two opportunistic bacterial pathogens, Lactococcus lactis or Pseudomonas entomophila. We measured host resistance as the inverse of bacterial load in the early infection phase. The relationship (slope) between fly fecundity and individual-level bacteria load provided our fecundity tolerance measure. Genotype and dietary yeast determined host fecundity and strongly affected survival after infection with pathogenic P. entomophila. There was considerable genetic variation in host resistance, a commonly found phenomenon resulting from for example varying resistance costs or frequency-dependent selection. Despite this variation and the reproductive cost of higher P. entomophila loads, fecundity tolerance did not vary across genotypes. The absence of genetic variation in tolerance may suggest that at this early infection stage, fecundity tolerance is fixed or that any evolved tolerance mechanisms are not expressed under these infection conditions.

Does mating negatively affect female immune defences in insects?

“…Apart from mating, nutrients can influence female immune defences. In D. melanogaster flies, for example, females infected with bacteria survived better when fed a low amount of sugar or an added-yeast diet than when fed a high amount of sugar (Howick & Lazzaro, 2014) or a reduced-yeast diet (Kutzer et al, 2018). In several insect species, males provide a nuptial gift at mating, including prey items, secretions from male glands, and substances transferred in ejaculates (Thornhill & Alcock, 1983;Vahed, 1998;Gwynne, 2008).…”

“…aeruginosa as a pathogen. The effect of mating on female immune function is likely to depend on the pathogenic strain or species used (Short & Lazzaro, 2010) and genetic variation in host resistance (e.g., Tinsley et al, 2006;Kutzer et al, 2018). Short et al (2012) only used Pr.…”

A multi‐faceted approach testing the effects of previous bacterial exposure on resistance and tolerance

“…Genetic variation for tolerance is well established in animal systems (e.g., Blanchet, Rey, & Loot, ; Graham et al., ; Howick & Lazzaro, ; Parker, Garcia, & Gerardo, ; Råberg et al., ; Sternberg, Li, Wang, Gowler, & de Roode, ), but this is the first time that we have observed such variation for range tolerance in our infection system. This could be due to larger sample sizes than in a previous experiment, the fact that each genotype was selected specifically because they appeared to differ in disease tolerance and resistance, or other experimental design differences (Kutzer et al., ).…”

“…For an overview of the experimental designs, see Figure . The lines were selected based on the results of a previous experiment (Kutzer, Kurtz, & Armitage, ), where we found that each varied in fecundity and in their capacity to resist bacteria. The stocks were maintained by placing adults onto new food every 2 weeks.…”

“…We collected 300–400 larvae for each genotype for both the F1 and F2 generations per replicate following Kutzer et al. (). When the F2 generation (i.e., the experimental animals) had eclosed, we allocated 120 virgin females of each genotype into separate vials in groups of 10 and 120 virgin males of each genotype were allocated to vials in groups of 20.…”