Gut microbiome analyses uncover a surprising correlation.
You contain multitudes. Researchers estimated that our bodies house at least as many bacterial cells as human cells (see this paper for the calculation). The preponderance of bacteria has sparked many questions about the evolutionary interplay between hosts and microbiomes. Numerous studies have documented so-called phylosymbiosis, in which microbiome similarity decreases with evolutionary distance. In other words, different species house different microbiomes. But what about hybrids?
On the one hand, hybrid microbiomes could be dysfunctional if they contain deleterious combinations of bacterial species that are specific to the parental species (similar to Dobzhansky-Muller incompatibilities on the genomic level). On the other hand, hybrid microbiomes might accommodate beneficial combinations of bacterial species, providing them with an adaptive advantage.
In a recent study, Marcella Baiz and her colleagues explored the gut microbiomes of Blue-winged Warblers (Vermivora cyanoptera), Golden-winged Warblers (Vermivora chrysoptera), and their hybrids. Do the hybrid microbiomes house deleterious or beneficial combinations of bacteria?
Species-specific Bacteria
The researchers sampled parental species and hybrids across two hybrid zones in North America: the Appalachians and the Great Lakes. Statistical analyses revealed no significant effect of host species on gut microbiome diversity, although there were a few species-specific bacteria. One species in the genus Jatrophihabitans was associated with admixed individuals, and two other bacterial species were overrepresented in one of the parental species: Kineococcus in Blue-winged Warblers and Rickettsia in Golden-winged Wablers.
The lack of clear differences between the microbiomes of these warblers can be explained by their recent divergence and high levels of hybridization. Blue-winged and Golden-winged Warblers diverged only one million years ago and hybridization has prevented clear genomic differentiation. The authors suggest that “limited genome-wide divergence constrains structuring of the gut microbiome between these parental species.” An alternative explanation concerns the common environment of these species. Because they use similar habitats and consume similar prey, the gut microbiomes of these warblers might not be able to differentiate.
Carotenoids
Subsequent analyses uncovered a surprising link between the gut microbiome and plumage coloration. The researchers noticed that the abundance of several bacterial species was elevated in hybrid individuals with more yellow plumage. A closer look at the functional diversity of these bacteria pointed to the “carotenoid biosynthesis pathway”. Could these bacteria play a key role in the uptake and processing of these pigments? The authors certainly think so.
Warblers are insectivores and almost exclusively consume arthropods. A straightforward mechanism for carotenoid uptake is via the consumption of herbivorous arthropods as a source of plant-derived carotenoids. Bacteria-derived carotenoids have been overlooked as a source of plumage pigments, but our results and other recent work suggest the gut microbiome could be directly linked to avian coloration.
An exciting discovery that requires further investigation. Given the importance of carotenoid-pigmented plumage in mate choice, the microbiome might be indirectly involved in the sexual selection.
References
Baiz, M. D., Wood, A. W., & Toews, D. P. (2024). Association between the gut microbiome and carotenoid plumage phenotype in an avian hybrid zone. Proceedings of the Royal Society B, 291(2021), 20240238.
Featured image: Golden-winged warbler (Vermivora chrysoptera) © Bettina Arrigoni | Wikimedia Commons
Avian Hybrids 