Fig. 1.

Schematic overview of proposed molecular host-symbiont interactions. a) Light micrograph of an Olavius algarvensis worm. Scale bar 5 mm. b) Light micrograph of a longitudinal section through O. algarvensis, tissue stained with toluidine blue. Scale bar 50 μm. C) Transmission electron micrograph of the symbiotic region, longitudinal section. Red asterisks, symbiont cells; black arrow, cuticle; white arrow, epidermal cell extensions. Scale bar 5 μm. The boxes in a), b) and c) frame regions corresponding to the image to their right (in a) and b)) or below (in c). Images a, b, and c do not show the same worm specimen. d) Schematic overview of the main groups of expressed pattern recognition molecules, components of the Toll immune signaling pathway and proposed interactions between the host and its symbionts. Ig, immunoglobulin domain proteins; PGRP, peptidoglycan recognition proteins; SRCR scavenger receptor-like cysteine rich proteins; TLR, Toll-like receptors; FREP, fibrinogen-related proteins; AMPs, antimicrobial proteins