Microbiome reduction and endosymbiont gain from a switch in sea urchin life history

Animal gastrointestinal tracts harbor a microbiome that is integral to host function, yet species from diverse phyla have evolved a reduced digestive system or lost it completely. Whether such changes are associated with alterations in the diversity and/or abundance of the microbiome remains an untested hypothesis in evolutionary symbiosis. Here, using the life history transition from planktotrophy (feeding) to lecithotrophy (nonfeeding) in the sea urchin Heliocidaris, we demonstrate that the lack of a functional gut corresponds with a reduction in microbial community diversity and abundance as well as the association with a diet-specific microbiome. We also determine that the lecithotroph vertically transmits a Rickettsiales that may complement host nutrition through amino acid biosynthesis and influence host reproduction. Our results indicate that the evolutionary loss of a functional gut correlates with a reduction in the microbiome and the association with an endosymbiont. Symbiotic transitions can therefore accompany life history transitions in the evolution of developmental strategies.

Automated summary

Animal gastrointestinal tracts harbor a microbiome that is crucial to host function, but some species have evolved a reduced or completely lost digestive system. Research in sea urchins reveals that the lack of a functional gut corresponds to reduced microbial community diversity and abundance, along with a diet-specific microbiome. Additionally, it is found that sea urchins transmit a Rickettsiales that may influence host nutrition and reproduction.