Company, Kinship and the Sable Island Horse Microbiome
An exciting new preprint from our lab examining how host genetics and social relationships jointly shape gut microbiome traits associated with survival and reproduction in Sable Island horses.
A new preprint led by Mason R. Stothart, with Philip D. McLoughlin, Alastair J. Wilson and Jocelyn Poissant, draws on 2,394 faecal samples from 794 individually known Sable Island horses collected between 2013 and 2019. By combining metagenomic data with long-term records of survival, reproduction, pedigree and social association, the authors ask whether fitness-associated microbiome traits can be inherited—and whether that inheritance is genetic, maternal or social.
The authors first identified microbiome dimensions associated with overwinter survival. These same dimensions also tracked reproductive success, showing that the signal was not confined to a single measure of fitness. Variation could be partitioned among additive genetic effects, persistent differences among individuals and social relationships. Together, these effects explained about 47% of the fitness-associated microbiota dimension and 38% of the corresponding microbial gene-family dimension. Social effects were two to four times stronger than additive genetic effects on average, while the models found no repeatable maternal-identity effect.
The contrast between microbial identity and function is especially intriguing. Microbial gene-family profiles were more strongly associated with survival and female reproductive success, while taxonomic community composition showed clearer heritability. In other words, what the microbiome can do may be more closely connected to fitness, while the composition of the community may carry a clearer host-genetic signal.
The survival-associated axes captured only 1% of variation in taxonomic composition and 0.1% of variation in microbial gene-family profiles, yet were strongly associated with fitness. This is a useful reminder that a biologically important signal does not need to dominate the total variation in a complex system.
News from Science highlighted the work as evidence from a wild population that heritable, fitness-associated microbiome variation could contribute to host evolution. That is an exciting possibility, but the wording matters: this observational preprint does not show that the microbiome causes better survival or reproduction, and social association does not prove direct microbial transmission. Shared environments may also contribute, functional profiles measure potential rather than activity, and the manuscript has not yet been peer reviewed.
How This Connects to My Sable Island Horse Project
This preprint comes from the Poissant Lab and uses the same long-term Sable Island study system that underpins my work on inbreeding depression. It shows what becomes possible when microbiome profiles, pedigrees, social histories, survival and reproduction can be studied for the same known individuals.
For my project, the central lesson is that genomic effects on fitness do not operate in isolation. As I connect genomic pedigrees and runs of homozygosity with health and life-history records, the microbiome may be one pathway through which genetic, ecological and social processes meet. That makes it a potential mediator to test, rather than background variation to ignore.