Co-option of lysosomal machinery shapes the evolution of the intracellular photosymbiosis supporting coral reefs.

Abstract

Endosymbiosis has spurred the evolution of new organelles across life. Corals and other cnidarians have repeatedly evolved an organelle, called the symbiosome, which houses intracellular algal symbionts. However, the molecular mechanisms enabling this repeated evolution remain unclear. Using the sea anemone Aiptasia, we generated a high-quality proteome of the symbiosome, revealing protein trafficking mechanisms and the types of biomolecules exchanged during symbiosis. Symbiosomal enrichment of lysosomal proteins, visualization of lysosomal fusion, and reduced symbiosis following knockdown of lysosomal genes indicate that the symbiosome functions through extensive co-option of lysosomal proteins. We identified a symbiosomal bicarbonate/sulfate transporter, SLC26A11, and showed through CRISPR/Cas9 mutagenesis that this lysosomal transporter is required for symbiosis in Aiptasia and a reef-building coral. Together, these findings reveal that corals and anemones have repeatedly co-opted lysosomal proteins to concentrate carbon and shuttle metabolites to support photosymbiosis, providing a relatively simple path for the repeated evolution of new photosymbioses.