Molecular mechanisms of partner recognition and association maintenance in marine ectosymbiosis

Symbiosis is known to have a dramatic influence on evolution. Ectosymbiosis is especially useful to understand the evolution of mutualistic reletionships between organisms because both partners are similar to their closest non- symbiotic relatives. This allows to reconstruct the scenario in which symbiosis evolved. Although the application of molecular biology techniques to ecological studies promoted the understanding of host-symbiont interactions, the crucial question of how the partners recognize each other and stably associate still has to be addressed. Lectin- sugar interactions are supposed to mediate partners recognition in a series of associations such as the ones between fungi and nematodes, bacteria and root-knot nematodes, bacteria and amoebae, and bacteria and squids. Intriguingly, recent experiments performed in our laboratory point to an involvement of lectin-sugar interactions in the adhesion of ectosymbiotic bacteria to Laxus oneistus, a marine nematode (Desmodoridae, Stilbonematinae) characterized by a mono-specific bacterial coat covering all but the anterior part of its cuticle. The bacteria were identified to be g -Proteobacteria closely related to known sulphur oxidising chemoautotrophic symbionts. We want (1) to prove that lectin-sugar interactions are required for the specific adhesion of g -Proteobacteria to L. oneistus and (2) to understand the degree of conservation of this adhesion mechanism among different stilbonematids which all show symbiosis with bacteria. In order to achieve our first goal, nematode surface coat proteins will be extracted, preliminarly charcterized on Western blots and putative lectins isolated by affinity chromatography on sugar columns. Production of polyclonal antibodies against the latter proteins, together with the parallel construction of a L. oneistus l-cDNA library will enable us to identify corresponding lectin cDNA(s), as well as to perform functional essays and to analyse their expression pattern. Concerning the question to which extent the adhesion mechanisms evolved by the L. oneistus-bacteria association are conserved among different stilbonematid associations, its surface lectin composition will be compared to those of other species which do not present a mono-specific bacterial epigrowth. Finally, if the antibodies raised against L. oneistus surface lectins will cross-react, the comparison of lectins expression pattern of different stilbonematids will help us to understand which of them are responsible for the mono-specificity of the bacterial epigrowth.

As in the early 70s the enormous animal communities associated with deep-sea hydrothermal vents were discovered, it became clear that life on earth might exist even in the complete absence of light. This is because animal communities associated with hot vents exclusively depend on the production of chemoautotrophic, especially sulphur-oxidizing, bacteria. Although thiotrophic symbioses have been subjected to intense study for more than three decades, nothing is known about the molecular mechanisms of symbiont acquisition in such symbioses. A coat of sulphur-oxidizing bacteria covers the cuticle of the roundworm Laxus oneistus and we took advantage of this shallow water association to identify a novel mannose-binding protein, which we named Mermaid. Mermaid is localized on the worm cuticle and appears to mediate symbiont adhesion to the worm, as well as symbiont-symbiont aggregation. This is the first report of a molecule involved in host-bacteria adhesion in a thiotrophic symbiosis and therefore it will help clarifying how other similar symbioses are established. Surprisingly, Mermaid shows significant homology to a class of human immunoreceptors that, in the early stages of the immune response, recognize specific carboydrate structures on the surface of pathogens such as the HIV virus. This similiarity suggests that mammalian immune defence molecule might have evolved from molecules mediating ancient beneficial interactions between invertebrates and bacteria.