INFERRING ANCESTRAL CROCODYLAMORPHA-MICROBE SYMBIOSES BASED ON THE AMERICAN ALLIGATOR (ALLIGATOR MISSISSIPPIENSIS) GUT MICROBIOME

The vertebrate gastrointestinal (GI) microbiome represents a complex symbiotic network developed over geological time, specialized in each host for specific host-dependent metabolic processes. Recent advances in gene sequencing now allow for nearly exhaustive evaluation of bacterial 16S rRNA genes from distinct regions along vertebrate GI tracts. The GI bacterial phyla from diverse animals (predominately farm-raised pigs, chickens, and pythons) are dominated by Firmicutes and Bacteroidetes. Despite knowing that these modern tetrapods and their ancestors have occupied a wide range of present and past ecological niches, and have affected biogeochemical cycles and ecosystem level processes in their habitats as a consequence of dietary requirements, notably absent are studies of wild animals or investigations of Reptilian symbioses. Reptiles, crocodylians in particular, have persisted in semi-tropical to tropical regions for over 200 million years, leaving behind a diverse and prolific fossil record. Therefore, this study focused on understanding the GI microbiome of the wild American alligator (A. mississippiensis), an extant member of Crocodylamorpha, found today across the southeast of North America. The phylum-level microbiome composition of A. mississippiensis (averaged for all samples from the mouth to the colon) was strikingly different than previously examined animals: Proteobacteria (~61%), Fusobacteria (~22%), Bacteroidetes (~13%), Firmicutes (~3%). Changes to gut bacterial composition occur as a consequence of changes in diet and environment. The dominance of Proteobacteria (Gammaproteobacteria) and Fusobacteria is unique among studied vertebrates, and may indicate a suite of ancestral symbioses present in basal crocodylians partially controlled by host-microbe genetic evolution.