542-488 million years ago
Tunicate heads possess an eyespot and a gravity sense organ. The genes expressed in these structures are homologs of those expressed in the vertebrate visual system and the vertebrate ear. As in vertebrates, regions of the brain can participate in light detection.
The tunicate CNS only possesses about 100 neurons including an eyespot
which is composed of 3 lens cells, 1 pigment cup cell, and 20 photoreceptors.
There are 3 opsin proteins (the photoreceptor proteins in vertebrate eyes)
in tunicates: Ci-opsin1 is expressed in the photoreceptors, Ci-opsin2
is expressed in a different set of brain cells and Ci-opsin3 (RGR) is
expressed throughout the brain and visceral ganglion (Tsuda, 2003). The
eyespot expresses the gene Ci-opsin1, a homolog of retinal and pineal
opsins in vertebrates. This median eyespot may be homologous to the pineal
eye. Although Ci-opsin1 is expressed in the developing brain at early
embryonic stages, it is later restricted to the ocellus (Kusakebe, 2001).
In tunicates, three visual proteins, RGR (retinal G protein coupled receptor
from which rhodopsin may have been derived), CRALBP (cellular retinaldehyde-binding
protein which is important in the isomerizaiton of photopigments necessary
for the perception of light), and BCO/RPE65 (required for mammalian vision)
are expressed throughout the brain and visceral ganglion, indicating that
the central nervous system in general participates in light detection
in addition to the specialized eyespot (Tsuda, 2003).
Tunicate brains also contain a second sensory organ: the gravity sense
organ (otolith). Genetic evidence suggests that the origins of the vertebrate
ear date to the early chordates. The Pax gene expressed in the tunicate
brain (HrPax-258) is homolog of Pax 2, 5, and 8 in vertebrates. Although
sensory placodes are considered to be a characteristic of the vertebrates,
tunicates do form a sensory organ from an epidermal thickening which expressed
HrPax-258 and appears to be homologous to the vertebrate ear (Wada, 1998).