Trust your gut
For honey bees, the trick to telling if another bee is a member of the colony or an outsider appears to reside in their guts
When Cady Heron sits with the Plastics during lunch, they lay out the rules. “You can’t wear a tank top two days in a row, and you can only wear your hair in a pony tail once a week,” Gretchen Wieners says. Jeans and track pants are only worn on Fridays; on Wednesday’s the Plastics wear pink.
“Now if you break any of these rules, you can’t sit with us,” Gretchen warns Cady.
The way the Plastics dress gives them a shared visual identity, one that allows them to quickly assess who’s a member of the group and who isn’t. It’s Wednesday and you’re not wearing pink? Sorry, but you’ll have to sit with the art freaks.
But what if they used a different strategy to define membership? Like their gut microbiome, the collection of all the microorganisms in the gut? A little far-fetched for the Plastics, but this newsletter’s paper suggests it may play a role in helping honey bees differentiate between colony-mates and those that should be banished to the art freaks’ table.
[Photo by Fernanda, A picture of the paper “The gut microbiome defines social group membership in honey bee colonies” by Vernier et al. on my very messy desk. On the left is my laptop, on the right are three pens and a sliver from my wallet. There’s also chocolate wrappers, Tylenol, a cup on a coaster and other assorted items you’d find on a desk that hasn’t been organized in days.]
When we think of group membership cues in terms of the Plastic’s clothing, it’s easy to see such signals as silly, a phase that will pass. In the animal world they’re anything but silly. Membership cues guide decisions about who to mate with, who to share resources with, who to accept into the group and who to kick out.
This is especially true for eusocial insects like ants and bees that live in colonies with thousands of lookalikes. When face-to-face with an identical-looking bee, bees are capable of telling if that bee is a colony-mate or an outsider. To do this they use pheromones, those chemical triggers that waft into the air and modify behaviors.
Covering the cuticle of all insects is a layer of wax, filled with lipids, fatty acids, glycerides, and cuticular hydrocarbons (CHCs). For most insects, the most abundant component in these layers are CHCs, molecules made up of chains of carbon and hydrogen. CHCs have a number of roles, such as keeping insects from drying up, but their most interesting one is related to pheromonal communication.
An insect’s CHC profile, the collection of CHCs in the wax layer, is genetically determined, yet heavily affected by an insects’ diet, role, reproductive status, etc. And, in the course of an insect’s life, their CHC profile can change.
In the Indian jumping ant (Harpegnathos saltator), workers and queens have distinct CHC profiles, with queens producing a longer CHC chain. When a queen dies, a group of worker ants fight for the position. The winner becomes reproductive and starts producing these longer CHC chains. And when she does, all the other ants behave differently towards her: they start treating her like a queen.
CHCs also help the Indian jumping ant distinguish between colony-mates and outsiders, and they have the same role in honey bees (Apis mellifera), the insect of this newsletter’s paper: “The gut microbiome defines social group membership in honey bee colonies.”
Cute and fuzzy, the honey bee is a force of nature. As one researcher wrote in the introduction of their paper on the evolutionary history of honey bees, Apis mellifera is “probably the single most significant pollinator of agricultural crops and wild plants.” In the US alone there are around 2.88 million honey bee colonies. And each colony has its own blend of CHC to help bees determine who is a colony-mate and who isn’t.
The current model for how each member of the colony gets dosed with these membership cues is a gestalt-like process. Each bee produces its own particular CHC blend that gets shared and mixed among the bee in the colony. The result is an average blend carried by every member of the colony. It looks something like this:
[Figure by Fernanda, using BioRender. According to the current model for how a colony specific blend is generated, each honey bee produces its own innate CHC blend (the three honey bees of different shades of gray on the left). These CHCs are transferred between all the bees in the colony (arrows going back-and-forth between the three gray bees in the middle) homogenizing the CHC blends. In the end, all the bees in the colony carry the same, mean CHC blend (three honey bees now in color on the right)]
Though the idea of bees rubbing against each other to create a colony-specific blend is adorable, recent evidence suggests that this isn’t quite the case. Instead, it seems that each bee innately produces the colony-blend. But, because we also know that this blend is highly dependent on the environment, this presents a bit of a conundrum. How can an innate characteristic also be impacted by the environment? The answer, the authors of the paper propose, lies in the gut microbiome of honey bees.
As we’re slipping out of our mothers’ birth canal (or getting sliced out if you’re a C-section baby like me), we’re already acquiring the bacteria that will form our microbiome. Honey bees aren’t too different: as soon as they eclose or emerge from the pupae, bacteria begin colonizing them. Most of this bacteria comes from older bees in the colony.
What this means is that if you take a group of post-eclosion bees from Colony A and divide them between two different colonies, B & C, the gut microbiome of adult bees will be more similar to that of the other bees in the colonies they grew up in.
[Figure by Fernanda, using BioRender. Recently eclosed bees acquire their gut microbiome from adult bees in the colony. So, when newly eclosed bees from colony A (dark gray on the left) are placed in either colony B (tan hive box above) or colony C (brown hive box below), those bees end up with gut microbiomes that are similar to the colony they grew up in (lighter gray bees on the right.]
To see if there’s a pathway from the gut microbiome to the CHC profile to colony membership, the authors of this newsletter’s paper, published in Science Advances in October, start each experiment by modifying the microbiome directly or moving newly-eclosed bees into different colonies. Then they wait and note what happens to the bee’s microbiome, its CHC profile and, lastly, its acceptance by other bees.
Note: There are two experiments that build up to the main one. They’re important, but we don’t need to go through them. Suffice to say that when Vernier et al. disrupted the gut microbiome of bees–with antibiotics, or by inoculating a sort of smoothie of bee gut microbiomes that was either alive or heat-killed–they saw that the CHC profiles were different compared to control bees.
Vernier et al. then fed half of a group of newly eclosed bees Gilliamella apicola, which is a common bacteria in honey bee guts. The other half got Lonsdalea quercina, an opportunistic bacteria, meaning it tends to invade the gut when the microbiome gets disrupted. Sort of like Clostridium difficile that can run havoc when it takes over human guts following a course of antibiotics or another kind of disruption.
G. apicola appears to have been hand chosen by the researchers. Unlike our microbiomes which is pretty diverse, adult honey bees have up to ten main bacterial species in their gut; these make up about 95% of all the bacteria cells in bee guts. Of these, five species are found in all adult honey bee workers independent of their location, their life stage or the season. G. apicola is one of these five core species.
There’s another reason they chose G. apicola: it plays a role in both sugar metabolism and plant polysaccharide digestion, producing molecules that may be precursors to CHCs. The idea is that differences in G. apicola may result in different precursors and, subsequently, different CHC profiles.
Inoculating bees with either G. apicola or L. quercina is a rather blunt way to testing this out. Vernier et al. are not exactly looking at the effect of discrete changes in gut bacteria; they’re looking at what happens when newly eclosed bees are inoculated with only G. apicola or only L. quercina. The adult bees end up with a mix of bacteria in their guts, but this isn’t a very natural experiment. Alas, you need blunt experiments to build up evidence to conduct a more realistic one…and to get grant money to do these more expensive experiments.
As expected, the microbiomes and CHCs of bees inoculated with either Gilliamella or Lonsdalea are different. Now, it’s time to look at acceptance.
A bee acceptance test is kind of like a WWE joust. You take three bees from one colony and put them in a petri dish (think of it as a boxing ring) to which you add an “intruder” bee that’s either from the same colony or a different colony. If, in the next 10 minutes, the intruder doesn’t suffer any aggression from the other bees, you consider them accepted. But if the other bees bite, sting or drag the intruder, it has been rejected.
Bees inoculated with G. apicola accepted about 90% of intruders who had been inoculated with the same bacteria, but rejected 60% of intruders inoculated with L. quercina. Interestingly, bees inoculated with L. quercina don’t appear to be able to distinguish between G. apicola- and L.quercina-inoculated bees; they accepted 60% of both. For Vernier et al., this suggests that only the microbes that are typical of the bee gut microbiome are able to impact colony-mate recognition.
From an evolutionary perspective this makes sense. Many of the bacteria in our microbiome are symbionts who have been living within us (or bees) for a long time, long enough to have roles in our development and functioning. There are four other core bacteria in adult honey bee worker microbiomes and surely other opportunistic bacteria. I’d want to see this experiment repeated at least once with two different bacterial species to see if Vernier et al.’s suggestion has an n greater than 1.
[Figure by Fernanda, using BioRender. Sister honey bees inoculated with G. apicola or L. quercina have different gut microbiomes and different CHC profiles (shown by the different colors). In the final part of the experiment, Vernier et al., looked at who the adult bees accepted into the petri dish and who they bit/stung/dragged. G. apicola bees accepted 90% of G. apicola bee intruders, but only 40% of L. quercina intruders. L. quercina bees on the other hand accepted 60% of all bees, independent of what bacteria they were inoculated with.]
Sister bees are related to each other genetically, so Vernier et al. repeated the entire experiment, but this time inoculated bees that were not related. While the source of the bees influenced their CHC profile, G. apicola bees still accepted > 80% of all bees inoculated with G. apicola while rejecting 60% of L. quercina-inoculated bees.
For Vernier et al., this experiment indicates that the gut microbiome of honey bees is sufficient to drive colony-mate recognition, probably by influencing the development of distinct CHC profiles.
But, remember how the gut microbiome of bees only has up to 10 dominant species and that G. apicola is present in all adult worker bees? How can a bacterial species in essentially all honey bees help them generate a colony-specific CHC blend?
Vernier et al. propose that variations in each bacterial species could be sufficient to modify CHC profiles and, subsequently, group membership recognition. To test this they took four groups of newly eclosed honey bees and inoculated each with a different strain of G. apicola. The CHC profiles of the four bee groups were different and bees accepted about 80 % of bees inoculated with the same strain, but only ~ 50% inoculated with a different strain.
According to Vernier et al., this data indicates that different strains of at least one core bacterial species (G. apicola) are enough to change the CHC profile of honey bees and, subsequently, their membership in the colony. They propose that the differences in the microbiome lead to differences in the amount/quality CHC precursors and/or modify the expression of CHCs.
While it’s true that Vernier et al. establish a chain of consequences that connects disruptions/modifications in the gut microbiome to changes in the CHC profiles and different rates of bee acceptance, this paper isn’t the end to the story. It’s really the beginning, opening up a new niche in insect gut microbiome research.
There are at least two types of research avenues that can come out of this paper. The first will explore if Vernier et al.’s discovery happens in other insects. The second will hammer into the observations and do two things (1) see if this chain of consequences remains in more realistic experiments and (2) try to figure out the nuts and bolts of how the microbiome may be influencing group membership recognition
There’s still some ways to go before we can say with 100% certainty that the gut microbiome of honey bees “defines” social group membership, but that doesn’t take away from Vernier et al.’s work. It’s an incredibly interesting idea and, for me, a completely new one, so I look forward to all the future papers that will either support or refute it.
Further Reading
I chose this paper because with a title like “The gut microbiome defines social group membership in honey bee colonies” how could you not? And while I know a decent amount about microbiomes, I knew next to nothing about social group membership coming in.
What I’m trying to say is that there were a lot of papers collected during the writing of this newsletter. Some have been linked throughout the piece, but there was also this paper about CHCs and their role in distinguishing queen ants and worker ants and this one about how parasites use “chemical insignificance” to invade and remain in host nests.
In Other News
Water! On! The! Moon!
When NASA said they had big news about the Moon, I wasn’t expecting the news to be about water. Or, as Casey Honniball, a post-doc at NASA’s Goddard, put it: “water molecules that are so spread apart that they do not form ice or liquid water.”
It’s been an interesting few weeks for space research. In September, the discovery of phosphine (evidence of possible biological activity) in the clouds that shroud Venus was announced. This month, however, news came out that new analyses of the data now suggest that the discovery might have been a fluke.
The evidence of water molecules on the Moon was described as unambiguous in the New York Times, probably because there’s a body of research (going back to at least the 90s) pointing to evidence of water on the Moon. NASA’s announcement is more a confirmation of previous work than an announcement of discovery. Still… Water! On! The! Moon!