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Abstract

Virtually all animals have beneficial symbioses with bacteria. The bacterial
symbionts can have a major impact on their hosts, by influencing
their development, defending them against potential natural enemies, or
providing them with nutrition. Several symbioses are known to be highly
specific, even in cases in which the symbionts are not transmitted directly
to the offspring but are acquired from the environment. However, the
molecular mechanisms for host-symbiont recognition are largely
unknown.
One striking example of a highly specific nutritional symbiosis is found
in Bathymodiolus mussels. These mussels are among the most successful
fauna at cold seeps and hydrothermal vents of the deep sea, thanks to
their association with intracellular sulfur-oxidizing bacteria (SOX),
methane-oxidizing bacteria, or both. The SOX symbionts are acquired
from the environment with every new generation of mussels, and also
throughout the lifespan of the mussel, yet the symbiont species is always
specific to the respective host species. This transmission mode gives the
mussel the potential to acquire symbiont strains adapted to the local
environment, with the associated risks of being infected by opportunistic
strains that provide less nutrition, or of not encountering the symbiont at
all. Three key questions arise from this mode of transmission: 1) How do
symbionts and host recognize each other to maintain a long-term
association? 2) Does the metabolism of newly acquired symbionts differ
from the established population? 3) Is there strain variation in the symbiont
population?