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Ecological Symbiomics

Lead: Monika Bright (Uni Vienna)

Rationale

Transmission has been identified as key factor in the evolution of symbiotic associations. Two fundamentally different modes of transmission are recognized: horizontal from a free-living source of symbionts and vertical with maternal (and more rarely biparental) symbiont transfer. Even mixed types exist, involving both vertical and horizontal transfers from the environment and intraspecific or interspecific host switching. Horizontally transmitted symbioses show little evidence for phylogenetic congruence and cospeciation, reflecting cotransmission over evolutionary time scales. In exclusively vertically transmitted symbionts, phylogenetic trees are congruent, and in mixed types congruence may be only partial. It has been hypothesized that transmission modes affect the virulence or benevolence of symbionts, but a complex mix of selection pressures may play a more important role than transmission alone. Although mathematical models have been developed to predict under which conditions vertical and horizontal transmission can occur, rigorous testing of these evolutionary models and computational simulations has not yet been possible due to the lack of detailed information, so that our understanding of the implications of transmission pathways does not yet extend to their causes.

Objectives

We propose to study key ecological aspects in a wide range of symbiotic systems to cover the variety of characteristics related to transmission: intracellular endosymbionts in the vestimentiferan trophosome (Project_11), extracellular endosymbionts in the body wall of gutless oligochaetes (Project_12), gill intracellular endo- and ectosymbionts in lucinid, mytilid and thyasirid bivalves (Projects_13 and 14), and extracellular endosymbionts in the nephridia of earthworms (Project_15). Although in some of our selected systems the principal mechanisms of symbiont transmission are known, important ecological aspects necessary to test existing evolutionary models (Project_1) need to be better understood. These include (i) the diversity, habitat and biogeographical distribution of partners in symbiosis (Projects_11, 13 and 15) and free-living counterpart of symbiont (Projects_11, 12 and 13) (ii) dispersal, colonization, succession of partners (Project_11) (iii) density of partners in the environments and contact rates (Projects_11 - 15) (iv) connectivity between symbiotic and free-living population in direction towards the host via transmission (Projects_12, 13 and 14) and in direction towards the free-living population via release mechanisms (Projects_11 and 14).

Key methods

In this work package a wide arrow of innovative in situ and in vivo experiments combined with state-of-the-art molecular techniques such as PCR-based analyses of key genes of host and symbiont and localization with in situ hybridization combined light and electron microscopy (Projects 11 - 15), substractive mRNA analyses and immunohistochemistry and western blot analyses for protein detection (Project 14), assessment of target genes with siRNA approach (Project 14), and NMR and mass spectrometry (Project 15) are proposed. In situ experiments will be carried out at deep-sea hydrothermal vent, cold seeps and wood falls and shallow-water sediments and include the use of successfully tested artificial devices and quantitative collections of natural substrates (Projects 11, 12 and 13). In vivo cross infection experiments (Projects 14 and 15) as well as artificial curing of symbionts experiments (Project 14) and high pressure symbiont release experiments (Project 11) will be conducted.