Fruit flies medicate offspring after seeing parasites

Abstract

Hosts have numerous defenses against parasites, of which behavioral immune responses are an important but underappreciated component. Here we describe a behavioral immune response that Drosophila melanogaster uses against endoparasitoid wasps. We found that when flies see wasps, they switch to laying eggs in alcohol-laden food sources that protect hatched larvae from infection. This change in oviposition behavior, mediated by neuropeptide F, is retained long after wasps are removed. Flies respond to diverse female larval endoparasitoids but not to males or pupal endoparasitoids, showing that they maintain specific wasp search images. Furthermore, the response evolved multiple times across the genus Drosophila. Our data reveal a behavioral immune response based on anticipatory medication of offspring and outline a nonassociative memory paradigm based on innate parasite recognition by the host.

Fig. 1

D. melanogaster medicates offspring with alcohol after exposure to wasps. (A) Standard oviposition preference setup. (B) Proportion eggs laid on 6% ethanol dishes for three wasp treatments, at two time points. ***P < 0.001. (C) Proportion eggs laid on dishes with increasing ethanol (EtOH) concentrations, depending on wasp presence. P < 0.001 for distribution comparisons at both time points. (D) Proportion wasp-exposed fly offspring that eclose when laid in cages with different combinations of oviposition dishes. (E) Proportion unexposed offspring that eclose. For (D to E), letters indicate significance groups at P < 0.01. For (B to E), error bars represent 95% confidence intervals (n = 4).

Fig. 2

Sight and NPF signaling control fly ability to sense and respond to wasps. Proportion eggs laid on ethanol dishes by (A) smell and sight mutants, (B) NPF and NPFR1 over-expression mutants, and (C) NPF and NPFR1 knockdown mutants in the presence and absence of wasps. For (A to C), y-axis is the same; error bars represent 95% confidence intervals (*P < 0.05, ***P < 0.001, n = 4). (D) NPF immunostain of an unexposed fly brain. * = NPF-expressing neurons, FSB = fan-shaped body, Lat = lateral regions, SEG = subesophageal ganglion, OL = optic lobes. (E to H) NPF immunostained fan-shaped bodies from control and sight mutant flies unexposed or exposed to wasps.

Fig. 3

Flies form long-term memories of seeing wasps. Proportion eggs laid on ethanol dishes (A) following wasp pre-exposure on different food types, (B) in the presence of wasps by the long-term memory mutant Adf1^nal, and (C) after wasp pre-exposure (on molasses food) in Adf1^nal mutants. For A–C, all y-axes are the same, error bars represent 95% confidence intervals (*P < 0.05, ***P < 0.001, n = 4).

Fig. 4

Breadth of fly search images and evolution of medication behavior. (A) Proportion eggs laid on ethanol dishes in response to different wasp species. Error bars represent 95% confidence intervals (*P < 0.05, ***P < 0.001, n = 4). (B) Correlation between phylogenetically independent contrasts for ethanol tolerance (LD₅₀) and ethanol oviposition preference index across seven Drosophila species.