Differential expression of immune receptors in two marine sponges upon exposure to microbial-associated molecular patterns.

The innate immune system helps animals to navigate the microbial world. The response to microbes relies on the specific recognition of microbial-associated molecular patterns (MAMPs) by immune receptors. Sponges (phylum Porifera), as early-diverging animals, provide insights into conserved mechanisms for animal-microbe crosstalk. However, experimental data is limited. We adopted an experimental approach followed by RNA-Seq and differential gene expression analysis in order to characterise the sponge immune response. Two Mediterranean species, Aplysina aerophoba and Dysidea avara, were exposed to a "cocktail" of MAMPs (lipopolysaccharide and peptidoglycan) or to sterile artificial seawater (control) and sampled 1 h, 3 h, and 5 h post-treatment for RNA-Seq. The response involved, first and foremost, a higher number of differentially-expressed genes in A. aerophoba than D. avara. Secondly, while both species constitutively express a diverse repertoire of immune receptors, they differed in their expression profiles upon MAMP challenge. The response in D. avara was mediated by increased expression of two NLR genes, whereas the response in A. aerophoba involved SRCR and GPCR genes. From the set of annotated genes we infer that both species activated apoptosis in response to MAMPs while in A. aerophoba phagocytosis was additionally stimulated. Our study assessed for the first time the transcriptomic responses of sponges to MAMPs and revealed conserved and species-specific features of poriferan immunity as well as genes potentially relevant to animal-microbe interactions.