You are what you eat

Abstract

Gut bacteria is now considered as an additional host organ, and it has been shown that they have important influences on host developmental and physiological processes. More recently gut bacteria have additionally been implicated in behavioral processes. We showed that in two species of Drosophila, gut bacteria variation affects behavior, altering partner investment in copulation in relation to whether individuals were familiar (i.e., having developed in the same environment), or were related. This suggests that gut bacteria play a role in kin recognition in these species. We suggest that this phenomenon is associated with gut bacteria influencing the scent profiles used by Drosophila in mate choice.

Keywords: :

• gut microbiota
• gut bacteria
• behaviour
• kin recognition
• cuticular hydrocarbons
• Drosophila

This article refers to:

In the past ten years there has been a surge in studies of the interactions between gut symbionts and their hosts. The hologenome theory,¹ whereby the host genome is considered to also comprise that of its symbiotic microbiota, is now widely accepted, coinciding with an explosion in publications showing the important influences gut bacteria can have on diverse host developmental and physiological processes, for example the immune response and host metabolism.² Yet there has been relatively little examination of the link between gut bacteria and host behavior, particularly behavior associated with mating behavior. This is despite the fact that as far back as the 1970s, it was suggested that gut bacteria might influence olfactory cues in mammals, and therefore alter the signals used in mate choice.^3,4

We recently examined whether relatedness, familiarity, and food eaten during development, altered copulation investment in three species of Drosophila with diverse ecologies.⁵ In many insect species olfactory cues are used in mate choice, and it was previously shown that manipulation of food eaten, and thus gut bacteria, influences mating preferences in Drosophila melanogaster.⁶ We therefore hypothesized that gut bacteria might similarly alter kin recognition cues, which are important in inbreeding avoidance, and that the effect of gut bacteria might vary depending on the ecologies of the host species. In Drosophila subobscura females only mate once in their lifetime,^7-9 therefore we predicted this species to exhibit a robust kin recognition system to avoid the costs associated with mating with relatives. In Drosophila bifasciata females mate multiply, but the species lives in dense aggregations on sap fluxes with limited dispersal, so here too we predicted evidence of kin recognition. Finally, we also tested D. melanogaster, which, as stated above, had previously been shown to exhibit an effect of gut bacteria on mating preferences.⁶

We utilized a fully factorial design whereby first instar larvae, from each species, originating from the same (sibling) or different (unrelated) pairs of parents were placed in same or different vials (familiarity) containing the same (ASG, a cornmeal-based medium) or a different (banana-based medium) type of food. On reaching adulthood, we then conducted mating trials whereby we assigned pairs where partners were either related or unrelated, developed in the same or different vials, and were reared on the same type of food or different food, again, in a fully factorial design. We then measured mating latency, and copulation duration, both of which are standard proxies for mating preference and investment respectively, in behavioral experiments.^7,10-13

We found that all three species were faster to mate with a partner from the same environment (i.e., vial); relatedness and food type had no effect on mating propensity. Conversely, we found differences in copulation duration, our proxy for investment, across the three species. In the singly mating species D. subobscura, we found a significant decrease in copulation duration when partners were related, indicating kin recognition and avoidance,⁵ while there was no effect of familiarity or food type. In the aggregating species, D. bifasciata partners invested less when mating with individuals coming from the same vial while in D. melanogaster there was a significant negative interaction between food type and relatedness acting on copulation investment.⁵

In D. bifasciata and D. melanogaster, we also examined the effect of gut bacteria, by removing it via the addition of the antibiotic streptomycin to the larval food. We found that in D. bifasciata, removal of the gut bacteria cancelled the effect of familiarity, and in D. melanogaster it cancelled the interaction between food and relatedness. Instead, in the latter, we found a significant effect of relatedness alone, i.e., once the gut bacteria was removed in D. melanogaster, they became capable of recognizing and reducing investment in kin.

Our study provides the first evidence that ecologically determined mechanisms of kin recognition occur in Drosophila, and that these can be affected by gut bacteria. As hypothesized we found a clear reduction in copulation investment, suggesting kin recognition and avoidance in D. subobscura, a species in which females only mate once, and therefore the costs of mating with a related individual are likely to be high. In D. bifasciata we found a reduction in copulation investment when mating with familiar individuals, which also makes sense given their aggregative ecology—avoid investing in something that smells like the environment you were reared in, as they may be related to you. This effect was removed when the food was treated with antibiotics, suggesting that the familiar recognition process is influenced by gut bacteria shared by individuals developing in the same vial. Finally, in D. melanogaster we found an effect of an interaction between food type and relatedness on copulation duration; this was removed when the food was treated with antibiotics, at which point partners exhibited true kin recognition, based only on relatedness. This suggests that potentially two cues are used by D. melanogaster to determine investment in a given mating, the first linked to the type of food eaten during development and ultimately rooted in the gut bacteria, the second based on relatedness. The fact that the gut bacteria masked the ability to recognize kin, suggests that the risk of inbreeding may be low in this highly dispersing species, known to be a generalist frugivore in which gut bacterial communities vary,¹⁴ or that the costs of inbreeding may be insubstantial.

What is likely to be the mechanism underlying this phenomenon? It is well known that in many insects including Drosophila, mating preferences are mediated by changes in the chemical composition of Cuticular Hydrocarbons Compounds (CHCs)¹⁵ on the cuticle of insects, which also play a role in the prevention of desiccation. Potentially therefore, gut bacteria influences the olfactory cues emitted by CHCs, and thus can mediate mating preferences as demonstrated in D. melanogaster.⁶

Might our results suggest a role for gut bacteria in mediating other behaviors, and in other organisms? Indeed, although D. melanogaster is emerging as a model system for studies of the interaction between gut microbiota and host,¹⁶ there are increasing numbers of examples of gut microbiota influencing behavior across diverse animal taxa.^17,18 In addition, studies suggest that not only mating behaviors can be affected by the gut microbiota. It has recently been shown, for example, that gut bacteria can affect anxiety levels in rats¹⁹ and mice.²⁰

Beyond behavior, there is the potential for gut microbiota to have more far-reaching implications for species ecology and evolution. Symbiotic gut bacteria have long been implicated in the evolution of social behavior in insects. In the termite Reticulitermes flavipes for example, gut symbionts necessary for the metabolism of cellulose are acquired by ingestion of the hindgut fluid of nest mates.²¹ The importance of the symbionts, and the manner of their acquisition render these termites obligate social insects. Differences in the composition of gut microbiota are evident between both termite colonies²² and species,²³ and it is suggested that these differences underlie the mechanism by which termites recognize their nest mates.²² Even more fundamentally, gut bacteria have recently been associated with post-zygotic reproductive barriers. Brucker and Bordenstein²⁴ demonstrated that perturbed interactions between gut bacteria communities and their host can lead to hybrid inviability in Nasonia wasps, suggesting that gut bacteria could be implicated in reproductive isolation, and therefore speciation. Finally our results suggest that gut bacteria can alter social and sexual recognition systems. Recognition systems are essential for almost all interactions between organisms, both conspecific and heterospecific. The suggestion that gut bacteria can have impact on recognition systems has important implications for the evolutionary ecology of species, and lends weight to the argument that the gut microbiome should be taken into account when considering the biology of an organism.¹

The traditional anthropocentric view of gut microbiota only being of interest in relation to human nutrition and disease, is being replaced as we come to accept the theory of the “holobiont” or “superorganism,” whereby organismal diversity should be viewed not only at the level of the individual, but also incorporating the microorganisms that live in and on the host.¹ In the past decade there has been an explosion in research investigating the role of the gut microbiome in the developmental and physiological processes of the host, however the role of gut bacteria in mediating behavior, and indeed the evolutionary ecology of species in general, has received relatively little attention. Future studies should attempt to redress this imbalance, and further explore the role of gut microbiota in host evolutionary processes.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

10.4161/gmic.29153