A tiny island bird reveals how immune genes shape the gut microbiome

University of East Anglia

Cousin Island is a speck in the Indian Ocean, roughly 27 hectares of tropical forest. Every Seychelles warbler on the island is individually marked with colored leg rings. Researchers can track each bird from birth to death. They know its parents, its territory, its breeding history. And now, thanks to a new study from the University of East Anglia, they know something about its gut bacteria too.

The study, published in the journal Microbiome, demonstrates that a bird's immune genes directly influence which microbes colonize its gut, and what those microbes do once they are there. The finding has implications well beyond ornithology, because the immune genes involved, the major histocompatibility complex (MHC), are shared across virtually all vertebrates, including humans.

The perfect natural laboratory

What makes the Seychelles warbler system exceptional for this kind of research is its isolation. The birds never leave the island. They eat natural diets, maintain natural social structures, and are exposed to natural pathogens. But because every individual is tracked, researchers can collect the kind of detailed longitudinal data that usually requires a controlled laboratory setting.

David Richardson, Professor at UEA's School of Biological Sciences and senior author of the study, described it as the best of both worlds: wild animals living natural lives, with the data quality of a lab population.

Chuen Zhang Lee, who conducted the work as part of his PhD, collected faecal samples from the warblers during fieldwork on Cousin Island. These were used to characterize the birds' gut microbiomes, the communities of bacteria living in their digestive systems.

MHC variation predicts microbiome differences

The MHC is a cluster of genes central to immune defense. MHC molecules present fragments of foreign proteins to immune cells, essentially showing the immune system what to attack. Variation in MHC genes determines which threats an individual can recognize, and different individuals within a population can carry very different MHC profiles.

Using advanced statistical and modeling approaches, the team found that specific regions of the MHC were associated with differences in the gut microbiome. Birds with different immune gene profiles harbored different bacterial communities.

But the team went beyond simply cataloguing which bacteria were present. They also analyzed what those bacteria were doing: whether they were involved in metabolism, nutrient processing, or defense against viral and other infections. This functional analysis revealed that MHC variation influences not just the composition of the microbiome but its actual activity.

A two-way conversation

The findings suggest a reciprocal relationship. Immune genes shape which microbes can establish themselves in the gut, likely by influencing the intestinal immune environment. Those microbes, in turn, help train and calibrate the immune system. Lee described it as a two-way relationship: immune genes influence the gut microbiome, and the microbiome feeds back to influence immune function.

The research also points to evolutionary trade-offs. By shaping the gut microbiome in different ways, different MHC variants may balance the benefits and costs of hosting particular microbes. An immune profile that favors one set of beneficial bacteria might simultaneously permit a different set of potentially harmful ones. These trade-offs could help explain how hosts and their microbial partners coevolve over time.

From warblers to humans

The underlying biology is not specific to birds. The MHC exists in essentially all jawed vertebrates, and the interaction between immune genes and gut microbiota has been hypothesized in humans for years. What this study adds is direct evidence from a wild vertebrate population with the kind of individual-level data that human microbiome studies typically lack.

The researchers stress that while the mechanisms are likely shared across vertebrates, the specific MHC-microbiome associations found in warblers will not directly translate to humans. Human microbiomes are shaped by factors, including diet diversity, medication use, and urbanization, that do not apply to island birds.

One island, one species, many unknowns

The study examines a single species on a single island. The Seychelles warbler population on Cousin Island is genetically bottlenecked, meaning it has lower genetic diversity than most wild populations. This could amplify or obscure MHC-microbiome associations in ways that might not apply to more genetically diverse populations.

Faecal samples provide a proxy for the gut microbiome but do not perfectly reflect the bacterial communities actually attached to intestinal walls, which may be more functionally relevant. The study is cross-sectional in that microbiome samples were taken at one time point, so it cannot track how MHC-microbiome relationships change over an individual's lifetime.

Causation is difficult to establish in observational studies of this kind. While the association between MHC and microbiome is clear, other correlated factors, including territory quality, diet variation, and social interactions, could contribute to the patterns observed.

Still, the study represents one of the most complete demonstrations to date that immune genotype shapes gut microbiome composition and function in a wild vertebrate, a finding that will inform future research across species, including our own.