Question

Short answer Table 1. Comparisons between eyes and gespots of estant chordate groups Hagfish tamprey lawed vertebrate Amphious Sea squirt Camera-type eye Lens Transparent cornea Extraocular muscles Retinal cell layers Photoreceptor class Cillary Cones/Rods Ribbon synapse RPE65 Central projection ? Presence or absence of features is indicated by Ciliary Cillary 5 Cones? Ciliary 4 Cones. Rod 12 17 Hypothalamus Tectum Tectum/thalamus present, X, absent2 unknoww/uscertain. Compare the structure, photoreceptors and development of the eyes of two groups selected from the table above.

Answer 1

Sea squirts (urochordate)

These have a simple eye, an ocellus, and a balancing organ, a statocyst.

Amphioxus (cephalochordate)

• has both ciliary and rhabdomeric photoreceptors, located in four separate non-imaging structures: the frontal ‘eye’ and lamellar body (ciliary), and the Joseph cells and dorsal ocelli (rhabdomeric).
• Amphioxus has six distinct opsins some of which are melanopsin-like opsin, encephalopsin and peropsin homologues.
• one possibility is that rhabdomeric photoreceptor in an ancestral chordate similar to amphioxus might have received synaptic input from ciliary photoreceptors and subsequently evolved in craniates to become retinal projection neurons.

Vertebrates

The Hagfish Eye

• Hagfish eyes are small, conical, completely lacking in any sign of a lens, an iris, a cornea or intra- or extraocular muscles, and buried beneath unpigmented translucent skin.
• The retina contains only two main nuclear layers, with no obvious bipolar or amacrine cells, and the photoreceptors connect directly to the output neurons (ganglion cells).
• These ganglion cells project predominantly to the hypothalamus.
• Hagfish photoreceptors have a simple structure with rather poorly organized outer-segment membranes.
• In each of these respects, the hagfish eye resembles the pineal organ of non-mammalian vertebrates, with the exception that the hagfish eye is arranged bilaterally. Behaviourally, the hagfish seems to be almost blind, and its weak response to light is unaffected by the removal of its eyes.
• Thus, the hagfish ‘eye’ seems not to subserve vision; instead, it seems more likely to function as a circadian organ, similar to the gnathostome pineal complex (which hagfish lack)

Lamprey eyes

• The eye of the adult lamprey is remarkably similar to our own, and it possesses numerous features (including the expression of opsin genes) that are very similar to those of the eyes of jawed vertebrates.

• The lamprey’s camera-like eye has a lens, an iris and extra-ocular muscles (five of them, unlike the eyes of jawed vertebrates, which have six), although it lacks intra-ocular muscles.

• Its retina also has a structure very similar to that of the retinas of other vertebrates, with three nuclear layers comprised of the cell bodies of photoreceptors and bipolar, horizontal, amacrine and ganglion cells.

• The southern hemisphere lamprey, Geotria australis, possesses five morphological classes of retinal photoreceptor and five classes of opsin, each of which is closely related to the opsins of jawed vertebrates.

Jawed vertebrates

• Remarkably, one finds that all jawed vertebrates possess eyes so closely similar to our own that it is indisputable that the last common ancestor of jawed vertebrates (that lived around 430 Mya) was equipped with an eye fundamentally the same as ours.