Transcriptomic and proteomic insights into innate immunity and adaptations to a symbiotic lifestyle in the gutless marine worm Olavius algarvensis

Abstract

Background: The gutless marine worm Olavius algarvensis has a completely reduced digestive and excretory system, and lives in an obligate nutritional symbiosis with bacterial symbionts. While considerable knowledge has been gained of the symbionts, the host has remained largely unstudied. Here, we generated transcriptomes and proteomes of O. algarvensis to better understand how this annelid worm gains nutrition from its symbionts, how it adapted physiologically to a symbiotic lifestyle, and how its innate immune system recognizes and responds to its symbiotic microbiota.

Results: Key adaptations to the symbiosis include (i) the expression of gut-specific digestive enzymes despite the absence of a gut, most likely for the digestion of symbionts in the host's epidermal cells; (ii) a modified hemoglobin that may bind hydrogen sulfide produced by two of the worm's symbionts; and (iii) the expression of a very abundant protein for oxygen storage, hemerythrin, that could provide oxygen to the symbionts and the host under anoxic conditions. Additionally, we identified a large repertoire of proteins involved in interactions between the worm's innate immune system and its symbiotic microbiota, such as peptidoglycan recognition proteins, lectins, fibrinogen-related proteins, Toll and scavenger receptors, and antimicrobial proteins.

Conclusions: We show how this worm, over the course of evolutionary time, has modified widely-used proteins and changed their expression patterns in adaptation to its symbiotic lifestyle and describe expressed components of the innate immune system in a marine oligochaete. Our results provide further support for the recent realization that animals have evolved within the context of their associations with microbes and that their adaptive responses to symbiotic microbiota have led to biological innovations.

Keywords: Annelida; Carbon monoxide; Chemosynthetic symbiosis; FREP; Immunology; Oligochaeta; PGRP; Phallodrilinae; RNA-Seq; Respiratory pigment; SRCR.

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

Fig. 2

Domain structures of peptidoglycan recognition proteins. Structure of conserved functional domains in Olavius algarvensis peptidoglycan recognition proteins; OalgPGRP1: comp330541_c4; OalgPGRP2: comp250229_c0; OalgPGRP3: comp335695_c10; OalgPGRP4: comp314994_c0; OalgPGRP5: comp332570_c2; OalgPGRP6: comp1100768_c0

Fig. 3

Protein alignment of hemoglobin A2 chains. Protein alignment of hemoglobin A2 chains from marine and terrestrial annelids: Riftia pachyptila (GenBank accession number: CAD29155), Tevnia jerichonana (GenBank accession number: AAP04530), Lamellibrachia satsuma (GenBank accession number: BAN58231), Lamellibrachia sp. XB-2003 (GenBank accession number: AAP04528), Oasisia alvinae (GenBank accession number: AAP04531), Oligobrachia mashikoi (GenBank accession number: Q7M413), Arenicola marina (GenBank accession numbers: A2a, CAI56308; A2b, CAJ32740; A2c, CAJ32741), Lumbricus rubellus (GenBank accession number: BF422675.2), Lumbricus terrestris (GenBank accession number: P02218), Tylorrhynchus heterochaetus (GenBank accession number: P09966) and Olavius algarvensis (comp287449_c0_seq1)

Fig. 4

Protein alignment of peptidoglycan recognition proteins. Protein alignment of PGRP domain sequences from different model organisms and Olavius algarvensis; Dmel Drosophila melanogaster (GenBank accession numbers: PGRP-SA, Q9VYX7; PGRP-LA, Q95T64; PGRP-LB, Q8INK6; PGRP-LC, Q9GNK5), Mmus Mus musculus (GenBank accession numbers: PGRP1, O88593; PGRP2, Q8VCS0; PGRP3, A1A547; PGRP4, Q0VB07), Hsap Homo sapiens (GenBank accession numbers: PGRP-S, O75594; PGRP-L, Q96PD5), Oalg Olavius algarvensis (OalgPGRP1, comp330541_c4; OalgPGRP2, comp250229_c0; OalgPGRP3, comp335695_c10; OalgPGRP4, comp314994_c0; OalgPGRP5, comp332570_c2; OalgPGRP6, comp1100768_c0). Conserved active-site residues that confer amidase activity are shown in red; mutation of at least one active-site residue (pink) removes amidase activity