Pharyngula

Evolution of vertebrate eyes

Category: Development • Evolution • Science
Posted on: December 21, 2007 1:14 PM, by PZ Myers

A while back, I summarized a review of the evolution of eyes across the whole of the metazoa — it doesn't matter whether we're looking at flies or jellyfish or salmon or shrimp, when you get right down to the biochemistry and cell biology of photoreception, the common ancestry of the visual system is apparent. Vision evolved in the pre-Cambrian, and we have all inherited the same basic machinery — since then, we've mainly been elaborating, refining, and randomly varying the structures that add functionality to the eye.

Now there's a new and wonderfully comprehensive review of the evolution of eyes in one specific lineage, the vertebrates. The message is that, once again, all the heavy lifting, the evolution of a muscled eyeball with a lens and retinal circuitry, was accomplished early, between 550 and 500 million years ago. Most of what biology has been doing since is tweaking — significant tweaking, I'm sure, but the differences between a lamprey eye and our eyes are in the details, not the overall structure.

From those early pre-Cambrian days on, there have been two (well, three, but let's not get into that right now) basic kinds of photoreceptor: ciliary and rhabdomeric. The differences between the two are in cellular organization—rhabdomeric receptors have an apical elaboration of the cell membrane, while ciliary receptors modify a protrusion called the cilium to do the same thing—and in the cellular pathway they use to trigger changes in current flow across the membrane. Different lineages have appropriated these two kinds of photoreceptors in different ways. We vertebrates use ciliary photoreceptors in the image-forming part of our eyes; we have rhabdomeric receptors, too, but they're used in a more general way to sense light and dark, and play a role in circadian rhythms. Most invertebrates instead use rhabdomeric receptors for vision — the eye of the octopus, for instance, which superficially resembles ours, contains rhabdomeric photoreceptors instead of the ciliary rods and cones of our eyes. These ciliary receptors are found in all chordates, even in cephalochordates which lack true eyes, but do have simple light sensors.

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The structure of ciliary photoreceptors at various stages of chordate/vertebrate evolution. The middle row shows schematic diagrams of the entire photoreceptor; the top and bottom rows show electron micrographs of the outer segment and the synaptic terminal, respectively. Note the gradual transition towards a highly organized laminar structure in the outer segment and the appearance of ribbons in the synaptic terminal.

The proteins used in vertebrate photoreceptors are also ancient. The oldest distinction, between the proteins and pathways of rhabdomeric and ciliary receptors, can be traced back to pre-Cambrian animal ancestors, but later refinements, such as the evolution of separate rod and cone photoreceptors, occurred within the vertebrate line. The data suggest that cone opsins (the ones used for color vision) evolved and diversified first, and that the rods evolved later, as a specialized and novel kind of photoreceptor with greater sensitivity and dynamic range. The idea that black-and-white vision had to come first is a cultural artifact; the history of televisions does not repeat the history of vision. Lampreys are an interesting transitional form. They have a rod opsin, but it's intermediate in structure between the basal cone opsin and gnathostome rod opsin, and it's also found in receptors with a cone-like morphology.

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The evolution of vertebrate opsins. On the left of the main figure is a dendrogram of the major opsin classes that are relevant to the evolution of the vertebrate eye. Before the separation of protostomes and deuterostomes, the primordial opsin had already diverged into three main classes: rhabdomeric opsins, which are characteristic of protostome rhabdomeric photoreceptors (see upper photoreceptor schematic) but are also found in melanopsin- containing vertebrate retinal ganglion cells; 'photoisomerase' opsins, such as retinal G-protein-coupled receptor (RGR) opsin and peropsin, which may in fact be G-protein-coupled receptors; and ciliary opsins (see lower photoreceptor schematic), which are characteristic of those photoreceptors in which the pigment-containing region is an expansion of the membrane of a cilium. Vertebrate retinal opsins are represented by the lowermost six rows in the diagram. The primordial retinal opsin of vertebrates diverged into long-wavelength sensitive (LWS) and short-wavelength-sensitive (SWS) branches, and then the latter split into several sub-groups: SWS1, SWS2 and Rh2/RhB, each of which is associated with cone-like photoreceptors. The Rh1 pigment of jawed vertebrates (bottom line) seems to represent the most recent development among these classes, and is expressed in vertebrate rod photoreceptors. A separate class of rod, the 'green rod' of non-mammalian vertebrates, uses the SWS2 pigment that is also present in the blue-sensitive cones of these species. On the right of the main figure are presumed classes of G-protein coupling mechanism, residues at four important locations (in the numbering system for bovine rhodopsin; blue and green shading highlights residue similarity; pink shading highlights a chloride-binding site), and the regional expression of the opsins in vertebrate tissues. AC, amacrine cell; GC, ganglion cell; HC, horizontal cell; RPE, retinal pigment epithelium; VA, vertebrate ancient.

An eye, to us, is much more than a patch of light sensitive cells, so while it's clear the molecular and cellular basis of photoreception is an old, old capability, there's the matter of building the elaborate light collecting structure of the eye, which is relatively more recent. How do vertebrates build an optical instrument of such sophistication? We can get the answer from development, and it also suggests an evolutionary explanation.

The diagram below summarizes the steps in vertebrate eye development, and do check out the flash animation of eye development.

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Development of the vertebrate eye cup. a | The neural plate is the starting point for the development of the vertebrate eye cup. b | The neural plate folds upwards and inwards. c | The optic grooves evaginate. d | The lips of the neural folds approach each other and the optic vesicles bulge outwards. e | After the lips have sealed the neural tube is pinched off. At this stage the forebrain grows upwards and the optic vesicles continue to balloon outwards: they contact the surface ectoderm and induce the lens placode. f | The optic vesicle now invaginates, so that the future retina is apposed to the future retinal pigment epithelium (RPE), and the ventricular space that was between them disappears. Developing retinal ganglion cells send axons out across the retinal surface. The surface ectoderm at the lens placode begins to form the lens pit. This section is midline in the right eye, through the choroid fissure, so only the upper region of the retina and the RPE are visible. g | The eye cup grows circumferentially, eventually sealing over the choroidal fissure and enclosing the axons of the optic nerve (as well as the hyaloid/ retinal vessels; not shown). The ectodermal tissue continues to differentiate and eventually forms the lens. (There is an excellent animation of this illustration online.)

Vertebrate eyes are outpocketings of the brain. The neural tube bulges out two lateral bubbles of tissue that have the potential for light sensitivity; in the ancestral metazoan, this is probably all they had, paired patches of ciliary photoreceptors in the brain. Initially, light collection wouldn't have been a problem in a small, transparent organism, but as they grew larger and more opaque, there would have been selection for animals that had windows of transparency to allow light in to strike the photoreceptors. These could have been localized to just adjacent to the eyes by the acquisition of inductive interactions between the photoreceptive patch of the brain and the overlying epidermis; the eye spot instructs the skin over it to maintain transparency. Thickening of this transparent region into a lens would have gradually improved the image-forming capability of the patch, leading to the formation of the true vertebrate eye. The extended and folded over region of the brain would become the retina, retaining its connection to the rest of the brain through a thin stalk, the optic nerve.

This developmental process of progressive improvements in the structure of the eye reflects the evolutionary history of the eye cup as well. Each step in the process gradually adds more visual functionality, and exactly parallels the sequence of selective events hypothesized by Charles Darwin; every step adds a little more to the image-forming ability of the eye. The emerging functionality of the sequence is also apparent in the development of the lamprey, which carries it out with remarkable slowness. The larva of the lamprey, the ammocoete, is essentially blind, with light-sensitive cells that can do little more than discriminate between night and day. Over the course of about 5 years, it carries out the slow, steady construction of an eyeball with a differentiated retina, a lens, ocular muscles, etc., and then erupts onto the surface of the adult head as a recognizably full-featured vertebrate eye.

The structure of that retina is also important. It is more than just an array of photoreceptors—it's a complex image processing engine, with an array of repeating elements in addition to the photoreceptors that manipulates visual information before passing it on to the brain. Photoreceptors feed into bipolar cells that connect to ganglion cells, which are the actual neurons that send an axon back into the optic nerve. In addition, there are horizontal and amacrine cells that connect laterally, between the converging columns of receptor-bipolars-ganglion cells. The whole is a very precisely layered structure that again develops gradually.

The order of development of the retinal circuitry has some suggestive features that imply an evolutionary sequence.

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The development of retinal neurons and circuitry. a | The cell cycle in the vertebrate retina. The soma of a replicating cell migrates between the outer (ventricular) surface, where mitosis (M) occurs, and the inner (vitread) surface. b | The sequential birth of cell classes in the vertebrate retina, with timings indicated for the ferret in both post-natal weeks and caecal time (that is, the time relative to eye opening), which is probably a better comparator for other species. c-e | The maturation of neural connectivity in the retina (again, timings are for the ferret). c | Initially photoreceptors (which exhibit few adult morphological characteristics) send transient processes to the inner plexiform layer (IPL), where they make synaptic contacts with the two sub-laminae. d | Subsequently these processes retract, and developing bipolar cells insert themselves into the pathway between the photoreceptors and the inner nuclear layer (INL). e | At a later stage, the rod and cone photoreceptors develop inner segments (IS) and outer segments (OS). A, amacrine cell; B, bipolar cell; C, cone photoreceptor cell; G, ganglion cell; H, horizontal cell; ILM, inner limiting membrane; OLM, outer limiting membrane; OPL, outer plexiform layer; R, rod photoreceptor cell.

In short, the ganglion cells, which connect to the brain, and the horizontal and amacrine cells, which form cross-connections within the retina, are the first cells to develop. The photoreceptors form next, but remember, the photoreceptors connect to the deeper elements of the retina through the bipolar cells…and the bipolar cells form last. This suggests that the bipolar cells are a relatively recent addition. Bipolar cells share morphological and gene expression similarities to the photoreceptors, which in turn suggests that they are modified photoreceptors themselves.

"Relatively recent addition," though, is still old. Lampreys have the same retinal circuitry and exhibit the same late-in-development addition of the bipolar cells, which means that the layered retina had to have evolved before the lamprey line split from the line that led to us — and that occurred about 500 million years ago.

This makes lampreys important as markers of an upper bound of the period of evolution of vertebrate image forming eyes. They've got all the major morphological characters of our eyes (and many deep differences as well—let's not get the impression that eye evolution stopped with the lamprey), and that means these features were assembled over 500 million years ago. What we need to do is look at species representing earlier splits from the vertebrate lineage to try and establish a lower bound…and interest focuses on the hagfish. What kind of eyes do hagfish have?

Hagfish turn out to have very poorly developed eyes that don't seem capable of forming an image at all. They are small conical masses buried beneath the skin, and they do not form a lens, they do not have extraocular muscles, they are barely a step up from a photosensitive eye patch. The retina has ganglion and photoreceptor cells, but no amacrine or bipolar cells. It's a very primitive structure.

Now comes the difficult part, though: interpretation. We can say the upper bound of the period of evolution of the eye is 500 million years ago, and where the lower bound falls depends very much on the phylogeny of hagfish, illustrated below.

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The origin of vertebrates. The evolution of jawed vertebrates is illustrated against an approximate time-scale of millions of years ago (Mya). The taxa considered in this Review are indicated with an asterisk and are accompanied by schematics and diagrams of the 'eye' region. The earliest chordates, represented by extant cephalochordates and tunicates, are thought to have appeared around 550 Mya. Jawless craniates (agnathans) were present in the early Cambrian, by 525 Mya, and a time of 530 Mya has been indicated for their presumed first appearance. There is considerable controversy as to whether myxiniformes (solely represented by extant hagfish) diverged before or after the separation of lampreys from jawed vertebrates (shown as dashed black and grey lines). Numerous lines of jawless fish evolved between 500 and 430 Mya ago, although none have survived to the present day. The first jawed vertebrate arose around 430 Mya, and this line is represented today by cartilagenous fish, bony fish and tetrapods. Six 'stages of interest' in vertebrate eye evolution correspond to the time intervals between the divergence of important surviving taxa. This diagram does not include the evolutionary changes that have occurred in the last 400 million years. The presented timeline is based primarily on evidence from the fossil record.

(The mermaid to represent all gnathostomes, including fishes and tetrapods, is a cute touch, but let me assure you that this paper does not endorse the existence of mermaids or other mythical chimeras.)

One hypothesis for the evolution of hagfish is that they are a branch of the chordate lineage that split off approximately 530 million years ago, and are representative of the organization of the basal proto-vertebrate. That would tell us the the eye evolved after that branch point, placing all the major innovations in the morphology of the eye between 530 and 500 million years ago, within populations of lamprey-like ancestors. This is the hypothesis that Lamb and others prefer.

The alternative hypothesis is that hagfish aren't representative at all; they are degenerate forms that split from the lamprey lineage at some time after 500 million years ago. Their eyes are actually neotenous larval lamprey eyes, and don't tell us anything about the primitive state. What this would imply is that the lower bound of this window in time when the eye evolved is unknown; it could be pushed back to 550 million years ago, when the tunicate line split off…but they may have secondarily lost their eyes, too, which leaves that lower bound possibly dangling back a few more tens of millions of years.

The authors do make an argument that hagfish are plesiomorphic craniates that I won't get into. No matter which way the matter is settled, though, hagfish remain an important and interesting group to study in order to decipher the evolution of the eye, and working out the details of the differences between hagfish and lampreys are going to be central to understanding how our eyes arose.

Which brings me to mention two things I very much liked about this review: one is that it exemplifies good science, in that it makes clear, detailed predictions and proposes tests to evaluate those hypotheses. As listed below, these revolve around examining the relationships of tunicates, hagfish, and lampreys, and in identifying further shared and derived properties of the lamprey eye with other vertebrate eyes. This is the kind of specific checklist of ideas from which research scientists work.

Predictions and tests of our hypotheses of vertebrate eye evolution

Prediction 1: the phototransduction cascade components of tunicate ocelli should be homologous with those of hagfish and lamprey photoreceptors.

• Identify the G protein of Ciona intestinalis photoreceptors and compare it with those of hagfish, lampreys and jawed vertebrates.
• Determine whether other homologous cascade components (for example, phosphodiesterase and cyclic-nucleotide- gated channels) are present in tunicate photoreceptors.
• Determine the genomic organization of these cascade components and compare it with that of jawed vertebrates.

Prediction 2: at an early stage of eye evolution there was synaptic contact from ciliary photoreceptors onto rhabdomeric photoreceptors.

• Examine whether synaptic contacts occur between ciliary and rhabdomeric photoreceptors in extant protochordates, such as Amphioxus and C. intestinalis.
• Examine whether microvillar opsin-containing membranes are retained in the retinal projection neurons (ganglion cells) of any extant organism.

Prediction 3: hagfish photoreceptors should exhibit close homology to cones.

• Identify the ciliary opsin (or opsins) of hagfish and determine its (or their) phylogenetic relationship to other ciliary opsins.
• Identify the principal phototransduction proteins (the G protein, the phosphodiesterase and the cyclic-nucleotide-gated channels) in hagfish, and determine their phylogenetic relationship to vertebrate cone and rod isoforms.
• Measure the electrical light responses and light adaptation of hagfish photoreceptors, and compare these with cone and rod responses.

Prediction 4: the hagfish retina should not contain bipolar cells, and its photoreceptors should synapse directly onto the projection neurons (ganglion cells).

• Use retrograde labelling of hagfish ganglion cells to examine their synaptic inputs.
• Use Golgi labelling of hagfish retina to investigate the connectivity of different cell classes.
• Examine the synaptic contacts between cell classes at the ultrastructural level.

Prediction 5: if hagfish are monophyletic with lampreys, then they might represent a form with arrested development, rather than a degenerate form.

• Examine the phylogenetic relationship between cyclostome genes; in particular, examine the relationship between the opsin genes to estimate the stage at which hagfish diverged.

Prediction 6: lampreys ought not to possess true rods.

• Further characterize the photoreceptors of other extant species of lamprey to ascertain whether the morphological and electrophysiological features of true rods are present.
• Further characterize the opsins of lampreys, to ascertain whether the RhA/Rh1 pigment can be considered equivalent to a rhodopsin.

Prediction 7: lampreys should not possess rod bipolar cells.

• Carry out an immunohistochemical characterization of bipolar cell classes in the lamprey retina.
• Carry out an electrophysiological characterization of lamprey bipolar cells.

Prediction 8: if vertebrate bipolar cells are descended from photoreceptors they will share numerous molecular components or have very close homologues.

• Compare the molecular components of cone and rod bipolar cells with those of cones and rods.

And another excellent feature of this paper is that it provides a detailed, step-by-step sequence for the evolution of the vertebrate eye. This isn't just a vague hypothesis, or a guess at unnamed, unspecified forces intervening at poorly understood points, but a description of known changes at the level of molecules, cells, morphology, and taxa. Here it is, a summary of eye evolution over 150 million years:

Proposed sequence of events involved in the evolution of the vertebrate eye

Stage 1: bilateral ancestor (>580 million years ago (Mya))

• Animals with bilateral symmetry exist.
• Numerous families of genes exist.
• A range of G-protein-coupled signalling cascades exist.
• A primordial opsin has evolved into three major classes: rhabdomeric opsins, photoisomerase-like opsins and ciliary opsins.
• A rhabdomeric-type photoreceptor has evolved, using a G_q-based signalling cascade with a rhabdomeric opsin.
• A ciliary-type photoreceptor has evolved, using a variant opsin (the stem ciliary opsin) that probably coupled to a G_o-based signalling cascade.

~580 Mya

• Protostomes separate from our line (deuterostomes).

Stage 2: protochordates (580-550 Mya)

• The ciliary photoreceptor and ciliary opsin continue to evolve, becoming similar to those in extant amphioxus and ascidian larvae.
• A primordial RPE65-like isomerase evolves.
  These protochordates had ciliary photoreceptors with a ciliary opsin and a hyperpolarizing response, and were able to regenerate 11-cis retinal in darkness.

~550 Mya

• Cephalochordates and tunicates separate from our line (chordates).

Stage 3: ancestral craniates (~550-530 Mya)

• A ciliary photoreceptor evolves that has well organized outer-segment membranes, an output synapse close to the soma and a synaptic specialization appropriate for graded signal transmission.
• Ciliary photoreceptors make synaptic contact onto projection neurons that might have been descendants of rhabdomeric photoreceptors.
• The eye-field region of the diencephalon bulges to form lateral 'eye vesicles'.
• These lateral vesicles invaginate, bringing the proto-retina into apposition with the proto-retinal pigment epithelium.
• A primordial lens placode develops, preventing pigmentation of the overlying skin.
  The resulting paired lateral photoreceptive organs would have resembled the 'eyes' of extant hagfish, lacking any image-forming apparatus and subserving non-visual functions.

~530 Mya

• Myxiniformes (hagfish) separate from our line (vertebrates).

Stage 4: lamprey-like ancestors (~530-500 Mya)

• Photoreceptors develop cone-like features:
  • Highly-ordered sac/disc membranes evolve.
  • Mitochondria become concentrated within the ellipsoid region of the inner segment1.
  • Coloured filter material is incorporated into the inner segment for spectral tuning.
  • Ribbon synapses evolve in the synaptic terminal.
  • Genome duplications give rise to multiple copies of the phototransduction genes.
  • Cell classes diverge to give five separate cone-like photoreceptors, each with its own ciliary opsin and with isoforms of transduction proteins.
• Retinal computing power increases:
  • Cone bipolar cells evolve, either from proto-neurons or from photoreceptors.
  • The bipolar cells insert into the pathway from photoreceptors to ganglion cells, through the retraction of photoreceptor processes and the incorporation of new contacts.
  • Bi-plexiform ganglion cells develop.
  • A highly organized three-layered neuronal structure with two intervening plexiform layers develops.
• Ganglion-cell axons project to the thalamus.
• The optics evolve (the lens, accommodation and eye movement):
  • The lens placode invaginates and develops to form a lens.
  • The iris develops and a degree of pupillary constriction becomes possible.
  • Innervated extra-ocular muscles evolve.
  The resulting eye and visual system would have resembled that in extant lampreys and would have provided spatial vision at photopic intensities and over a broad wavelength range.

~500 Mya

• Petromyzoniformes (lampreys) separate from our line.

Stage 5: jawless fish (~500-430 Mya)

• Myelin evolves and is incorporated throughout the nervous system.
• Rod photoreceptors evolve:
  • Rhodopsin evolves from cone opsin.
  • Rod isoforms of most transduction cascade proteins arise.
  • Free-floating discs pinch off within the plasma membrane.
• Rod bipolar cells evolve, possibly from rod photoreceptors.
• The scotopic rod pathway evolves, with a new subset of amacrine cells (AII) providing input into the pre-existing cone pathway.
• A highly contractile iris evolves that can adjust light levels.
• Intrinsic eye muscles develop that permit accommodation of the lens.
  This eye possessed a duplex retina that contained both rods and cones, together with retinal wiring that closely resembled that of jawed vertebrates, with colour-coded photopic pathways and a dedicated scotopic pathway; it was probably similar to that found in many extant fish.

~430 Mya

• The last jawless fish separate from our own line (gnathostomes).

Stage 6: gnathostomes (<430 Mya).

• In the case of tetrapods:
  • The lens develops an elliptical shape to compensate for the added refractive power that is provided by the cornea in air.
  • The dermal component of the split cornea is lost and the eyelids evolve.
  • Certain opsin classes are lost, for example, SWS2 and Rh2 in mammals, under extended nocturnal conditions.

That's a beautiful illustration of the power of theory: it creates a framework of observations in which to work, and is going to help us place new observations and experiments (and there will always be new discoveries) in a context. I'll be filing this away and using is a reference in further reading about eye evolution.

Lamb TD, Collin SP, Pugh EN Jr. (2007) Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nat Rev Neurosci 8(12):960-76.