# Endemic within endemics: the microbiota of the Galapagos marine iguanas

## TL;DR

This study explores the gut microbes of Galápagos marine iguanas and finds that their microbiota is shaped by dispersal and selection, leading to unique microbial lineages.

## Contribution

The study identifies novel, host-restricted Clostridia clades in marine iguanas that diversified within the host and are maintained by strong selection.

## Key findings

- Fecal microbial composition in marine iguanas is dominated by Clostridia and follows a neutral dispersal model.
- Novel Clostridia clades are host-restricted and show diversification within marine iguanas.
- Strong purifying selection leads to microbiota homogenization across host populations.

## Abstract

The ecological processes shaping host-associated microbial communities in geographically isolated ecosystems remain poorly understood—particularly the interplay between dispersal, selection, and microbial speciation. Here, we characterize the fecal microbiota of the Galápagos marine iguana (Amblyrhynchus cristatus), an iconic endemic vertebrate that depends on its microbiota to digest an algae-based diet. We analyzed fecal samples from 111 individuals across three remote colonies and found that fecal microbial composition is dominated by Clostridia, closely following a neutral dispersal model. Yet, ecological and phylogenetic analyses revealed novel, host-restricted Clostridia clades—spanning species to family level—that appear to have diversified within marine iguanas. These lineages are consistently retained across host populations through strong purifying selection, resulting in striking microbiota homogenization. Our findings demonstrate that endemic hosts can support microbially distinct lineages shaped by stochastic dispersal and parallel selection, advancing our understanding of microbial community assembly in obligate host–microbiota systems.