Phylogeny and Biogeography of the Stilbonematinae

The Stilbonematinae are marine free-living nematodes that are unique by living in symbiosis with chemoautotrophic sulphur-oxidizing bacteria covering their body surface in species-specific and highly ordered manner. These worms have become a model system for understanding bacterial ectosymbioses, their evolution, their role in the partnership, their specificity, partner recognition and transmission mode, and the cell biology of the symbiotic bacteria. The mechanisms regulating such ectosymbioses are similar to those involved in bacterial infections and therefore of general biological and medical interest. In contrast to these advances in functional biology, the systematics, phylogeny and biogeography have lagged behind. The proposed project aims to provide a solid taxonomic and phylogenetic framework for the group, using modern molecular methods. Furthermore the biogeography of the Stilbonematinae will be established. Samples for both molecular and morphological investigations of both host and symbionts will be taken in all major ocean basins. Morphospecies will be assigned to phylogenetic clades leading to rigorous genus definitions. The resulting data should allow constructing a phylogeny of this unique animal group, indicating possible co- speciation between the partners.

Stilbonematinae are 3 to 10mm long, marine, free-living nematodes living in the interstitial space of coastal sediments. They are remarkable for a symbiosis with chemoautotroph bacteria covering their body surface and are fed upon by the worms. Although the Stilbonematinae are monophyletic (originating from a single ancestral species) according to the possession of peculiar glands and shared marker genes, they are morphologically diverse (structure of the body surface, anatomy of the foregut, shape of the lateral sense organ, presence and form of copulatory accessory organs). Also, the symbiotic bacteria are very diverse in form and arrangement on the host. Phylogenetic analyses within the project including molecular marker genes (16S rRNA, CO1) and also morphometric and anatomic characters showed, that morphologically similar structures, especially of the foregut, have arisen several times independently within the Stilbonematinae and appear corelated with the special geometry of the symbiont coat. The phylogenetic part of the project focused on the merging of classical morphological methods with modern molecular analyses. This led to the successful characterization and distinction of "cryptic species" using multivariate analyses of morphometric data and sequencing of marker genes. The molecular data suggest to separate Stilbonematinae from Desmodoridae as a distinct family. The biogeographic part documented the distribution of Stilbonematinae in all oceans. The genera that had been considered rare proved to be worldwide distributed. Several new genera are described. Tropical/subtropical (Caribbean Sea, Indian Ocean, Philippines) and warm-temperate seas (Mediterranean Sea) revealed exceptional high diversity. Subtidal sands - especially in back-reef areas - are the preferred habitat of this symbiosis. On one hand, there is ample interstitial space for mesopsammic animals, on the other hand there is a several cm distance between the oxygenized surface and deeper sulfidic layers. An association with a moving animal host that bridges this distance is of decisive advantage for sulfur-oxidizing bacteria that need both oxygen and hydrogen sulfide. In samples taken in such sands we found up to a dozen species of almost the same number of genera. The mechanisms of niche differentiation and resource partitioning are still unclear