The evolution of immunity: a low-life perspective.

Area of Science:

• Evolutionary immunology
• Invertebrate genomics

Background:

• Vertebrate immune genes and pathways are thought to have evolved relatively recently.
• The evolutionary origins of innate immunity remain incompletely understood, particularly in basal animal lineages.

Purpose of the Study:

• To investigate the evolutionary origins of key immune pathways in basal animals.
• To determine the presence of canonical immune pathways in cnidarians.

Main Methods:

• Analysis of whole-genome sequences and expressed sequence tag (EST) datasets from cnidarians (corals, sea anemones, Hydra).
• Comparative genomics to identify genes related to Toll/Toll-like receptor (TLR) and complement pathways.

Main Results:

• The Toll/Toll-like receptor (TLR) pathway and a complement-effector pathway (C3, MAC-PF) are present in corals and sea anemones.
• These pathways appear degenerated in Hydra.
• A protein related to deuterostome RAG1 (recombination activation gene 1) was identified in cnidarian genomes.

Conclusions:

• Innate immune pathways have much earlier origins than previously assumed, dating back to basal animals.
• Cnidarians possess key components of innate immunity, suggesting deep evolutionary roots.
• The RAG1 gene may have originated from a Transib transposase, opening avenues for in silico identification in simpler organisms.