All sensory modalities serve a similar objective, which is to
decode input by making predictions in time and space about
an animal’s surroundings. The evolution of sensory modalities
is driven by the need to shape effective behavioural
outputs, and in turn increase survival. Throughout evolution,
sensory systems have undergone a great deal of specialization;
and even though some modalities are derived from
unique origins within different phyla, they still exhibit many
common design features (Strausfeld and Hildebrand 1999;
Eisthen 2002; Jacobs et al. 2007). We now have detailed
mechanistic data on how sensory systems operate within specific
animals (Buck and Axel 1991; Chalasani et al. 2007;
Sato et al. 2008; Wicher et al. 2008), however it is still not
clear how sensory signalling pathways evolve at the molecular
level, and whether these evolutionary mechanisms are
shared between diverse taxa. Here we set out to investigate
the molecular evolution of signalling pathway members
across olfactory, gustatory, and photosensory modalities
from very divergent phyla in an attempt to develop a model
of molecular evolution for sensory systems. From our pairwise
intraphylum analysis we found that sensory signalling
pathways unusually undergo high levels of functional constraint
that are higher than genomewide global levels of
constraint, and this purifying selection is common within
the very divergent taxa we examined. We also find that
gene duplication events represent a conserved but heterogeneous
driver of evolution within sensory signalling pathways.
Taken together, we propose a ‘sessile’ mechanism of
sensory signalling pathway evolution, which on one side
facilitates bursts of gene duplication and relaxed selection
and on the other side it is unusually anchored by high levels
of selective constraint that preserves core sensory function
