Evolutionary Mechanisms Behind Regressive Evolution in Ogilbia galapagosensis

Why Did the Galapagos Cuskeel Lose Its Eyes?

Nick Wenner

Darwin, Evolution, and Galapagos

Professor Durham

October 9, 2008

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Evolutionary Mechanisms Behind Regressive Evolution in Ogilbia galapagosensis

Why Did the Galapagos Cuskeel Lose Its Eyes?

Known commonly as the Galapagos Cuskeel, Ogilbia galapagosensis is a small

cave-dwelling fish endemic to the Galapagos Islands of Ecuador. Like many other cave-

dwelling creatures, O. galapagosensis has evolved eyes that are greatly reduced in size

and function. Explaining this regressive evolution is more complicated than one might

first assume. The most obvious answer – that the species lost its eyes simply because it

did not need them – does not hold up under close scrutiny. In an evolutionary context,

such a teleological explanation fails because while natural selection is capable of

preserving useful traits and eliminating detrimental ones, it does not have the ability to

eliminate traits that are simply useless (like eyes in a dark cave). What, then, is the

negative selective pressure causing eyes to disappear in cave organisms? Even Charles

Darwin had no answer to this question: “As it is difficult to imagine that eyes, although

useless, could be in any way injurious to animals living in darkness, I attribute their loss

wholly to disuse” (Darwin 1859: 137).

To this day, scientists are still debating the question, concluding only “the

evolutionary mechanisms responsible for eye degeneration in cave-adapted animals have

not been resolved” (Jeffery 2005: 1). While a consensus on the problem has yet to be met,

there are currently four main hypotheses, one pointing to the absence of selective

pressure and genetic drift as the driving force and three others exploring the existence of

selective pressures for eye degeneration. This paper will investigate these four hypotheses

and ultimately identify and qualify the most likely and powerful of them in the regressive

evolution and loss of eyes in O. galapagosensis.

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Background:

O. galapagosensis is identified under the order Ophidiiformes and the family

Bythitidae. Members of the Bythitidae are commonly known as brotulas and

characterized by their cryptic nature and preference for low-light marine and freshwater

environments. The Galapagos Cuskeel’s closest relative is another Galapagos endemic,

O. deroyi, an eyed marine form from which O. galapagosensis is thought to have

diverged. O. deroyi inhabits the cracks and gaps in rocks near the lower limit of the

intertidal zone of St. Cruz and St. Fe islands of the Galapagos Archipelago. It cannot

tolerate the low salinity environment in which its cave relative thrives, and Illife (2004:

224) suggests this provided the isolation mechanism necessary for speciation to occur.

O. galapagosensis is found only in the fresh to brackish water caves of Santa

Cruz Island. The fish has a smoothly tapered body 40-60 mm in length with variable

reduction in both its eyes and pigmentation. Its eyes range from reduced but functional to

completely blind and coloration ranges from white to pink or beige. Numerous sensory

papillae line O. galapagosensis’ head and help the fish maintain position as the top

Ogilbia galapagosensis

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A grieta on Santa Cruz Island, Galapagos

predator in its environment, where it preys on the small 2 – 3 mm cave shrimp Typhlatya

galapagensis.

The fish is stygobitic, meaning it is an obligate cave dweller so completely

adapted to caves it is restricted to this environment (Iliffe: 2007). (With respect to

terminology, the current trend is to use the prefixes ‘stygo’ when referring to aquatic

forms and ‘troglo’ when referring to terrestrial forms.) It inhabits shallow water in or

around cracks or gaps between rocks in colonies of four to twenty individuals, each with

its own rock cavity into which it flees when disturbed. While its cave environment is

open to light at the surface, O. galapagosensis prefers staying close to rocks or root

masses in darker sections of the pools (Iliffe 2007).

There are three main types of cave systems in

the Galapagos Islands. They are found in tectonic

faults, lava tubes, and lava rock pools. All are

anchialine, or “bodies of haline waters, usually with

restricted exposure to open air, always with more or

less extensive subterranean connections to the sea,

and showing noticeable marine as well as terrestrial

influences” (Iliffe 1991: 2). The most extensive

aquatic caves are found between the high cliffs and

deep fissures of tectonic faults. They are known

locally as ‘grietas.’ Typically, the fissures consist of

opposite-facing sheer rock walls separated by about 10 m and extending as far as 30 m

high. Grietas exist most notably on the south coast of Santa Cruz Island with faults

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running generally in an east-west direction and extending in some cases as far as 20 km.

Ogilbia galapagosensis is found only in these Santa Cruz grietas (Iliffe 1991: 223). The

other cave systems found in lava tubes and lava rock pools are less extensive yet still

contain an array of unique creatures.

In all, the anchialine cave fauna of Galapagos are not particularly diverse and four

families of stygobitic shrimp-like organisms and O. galapagosensis constitute the entire

cast. In almost all cases, the creatures exhibit extreme to complete reduction in both

pigmentation and eyes, and due to the isolated nature of the environment, there is high

endemism among the species.

The cave systems of Galapagos are significant in their isolation, which is a key

factor in the process of evolution. Galapagos anchialine habitats represent the only

known subterranean habitat of their type in the Eastern Pacific and they are located

midway between cave areas of the South Pacific and the Caribbean. By their nature,

caves are extremely isolated environments and their placement on oceanic islands as

remote as the Galapagos – 600 miles from Ecuador and the nearest land – makes them

essentially islands within islands and prime locations for the independent development of

unique creatures and traits. Unique environment, ecology, and behavior are all factors

that through some mechanism or mechanisms led to the loss of O. galapagosensis’ eyes.

The next sections will explore exactly what such mechanisms may be.

Current Hypotheses:

As earlier stated, the four main hypotheses regarding regressive evolution and eye

loss fall into two broad categories: those proposing the absence of selective pressure as

the driving force and those focusing on the influence of selective pressure and natural

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The eyeless Ogilbia pearsei of the Yucatan Peninsula, Quintana Roo, Mexico.

selection. In the absence of selective pressure, the neutral mutation hypothesis invokes

random genetic drift as the evolutionary

mechanism. The other three hypotheses focus on

1) the energetic demands of eye development, 2)

the energetic demands of eye maintenance, and

3) the idea that eye degeneration is a byproduct

of some entirely different yet genetically

connected adaptation.

Hypothesis One: The Neutral Mutation Hypothesis

The neutral mutation hypothesis argues that in the absence of selective pressures

for maintaining a functional eye, random and natural genetic mutations will accumulate

over time and result in eye degeneration. Parallel regressive evolution in many disparate

organisms tends to discount this process. Parallel evolution is defined as the independent

evolution of similar traits, starting from similar ancestral condition.

A strong example of parallel regressive evolution exists between O.

galapagosensis and its Mexican congener, O. pearsei. Also known as the Dama Ciega

Blanca and the

Mexican Blind

Brotula, O.

pearsei lives in

the anchialine

limestone caves

of the Yucatan

Eyes Loss in Cave Fishes

Neutral Mutation Hypothesis

Absence of Selective Pressure

Development Maintenance

Energy Economy Pleiotropy"correlation of growth"

Selective Pressure and Natural Selection

Hypotheses

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Peninsula near freshwater aquifers where groundwater seeps from the porous substrate. It

is the only other stygobitic member of the genus Ogilbia. Similar to the Galapagos

Cuskeel, the Mexican Blind Brotula – while small in size at 90 mm – is the top predator

in the Quintana Roo anchialine community where it eats various crustaceans. Most

significantly, O. pearsei is thought to have descended from an eyed marine form and

subsequently lost its eyes in the cave environment, just as with O. galapagosensis (Iliffe

2007).

The eyes of the Mexican Blind Brotula have disappeared entirely. While it

responds to the slightest vibrations, the fish does not react in any way to lights from

divers (Iliffe 2007). Given that the two eyeless species evolved in parallel under a

separation of more than 1500 miles of impassable land and saline ocean, it would be

remarkable if random mutations were the culprit in producing their similar forms and

lack of eyes.

W.R. Jeffery (2005) has documented a similar case of parallel regression in a

freshwater cavefish of Texas and Mexico, Astyanax mexicanus. Both surface and cave

populations of Astyanax mexicanus exist, with the surface populations having eyes and

the cave populations having lost them entirely. Remarkably, the eyed and eyeless forms

of A. mexicanus, being members of the same species, are closely related and can

interbreed. Based on distribution and phylogenetic analysis, Jeffery concludes it is likely

that eye degeneration occurred in parallel in at least five of the thirty distinct populations

of eyeless forms (193). This further example of parallel regression strengthens the

argument against the neutral mutation hypothesis.

There is additional genetic and developmental evidence that strongly discounts

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Lens replacement experiment, A. mexicanus. (Jeffery 2005: 189)

neutral mutation. In a 2005 experiment,

Jeffery swapped the embryonic lenses

on one side of the head between

developing members of the cave and

surface populations of A. mexicanus.

Astonishingly, the initially eyeless

specimen developed a complete if not

functional eye on one side of its head,

and, conversely, the surface fish was left

with one eye on one side and a non-

functioning pit on the other. In addition

to developing an eye on one side, the cavefish also developed all the craniofacial bone

structure necessary to accommodate that eye. This experiment illustrates that cave

populations of A. mexicanus have and are capable of using all the necessary genetic

factors for eye development. This, according to Jeffery, is “perhaps the strongest single

piece of evidence against the neutral mutation hypothesis” (Jeffery 2005: 194).

Hypothesis Two: The Energetic Demands of Eye Development

Perhaps the most common-sense hypothesis for eye degeneration focuses on the

energetic demands of developing an eye, arguing that the raw materials and demands of

constructing an eye are strong enough negative selective pressures to preclude eye

development. While the argument seems quite plausible, studies on the development of

A. mexicanus tend to discount the theory (Protas 2007, Jeffery 2005). The most

convincing fact is that during the course of development, the embryonic eye structures of

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the fish – particularly the lenses – undergo a process of dynamic apoptosis up until a

remarkably late stage of development. This means that shortly after an embryonic cell

undergoes division and begins to develop into a functional eye structure it undergoes cell

death. Eventually, the rate of apoptosis overpowers the rate of division, and the result is

that the eye develops into a remarkably late stage of development before it degenerates

and disappears. Because the cavefish eye continues to experience the high costs of cell

growth and development, it is unlikely the process of eye degeneration is linked to

developmental energy conservation (Jeffery 2005: 188-189).

Hypothesis Three: The Energetic Demands of Eye Maintenance

Some researchers believe the key to eye degeneration is in energetic demands, but

that the development hypothesis addresses the wrong costs. They argue instead that it is

the energetic demands of maintaining the eye that provides the negative selective

pressure. Protas et al. (2007) supports this hypothesis well and shows that a functioning

vertebrate eye is a costly energy sink, relating that the retina surpasses brain tissue in

metabolic expense and that 10% of the photoreceptor discs are replaced daily – meaning

that the photoreceptor is entirely replaced over thirty-five times per year (454). It is also

shown that the vertebrate eye requires more energy when functioning in the dark, where

the photoreceptor is retained in a hyperpolarized state until depolarized with exposure to

light, and the retinal oxygen consumption is 50% greater (454). The conclusion is that

while the energetic costs of developing an eye may be trivial, the costs of maintaining the

eye likely are strong enough to preclude eye development.

Hypothesis Four: Pleiotropy

The pleiotropy hypothesis argues that where genes have more than one mode of

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Induced apoptosis in A. mexicanus due to Hh mRNA injection. (Jeffery 2005: 192)

expression in the phenotype, the loss of eyes can be seen as a byproduct of some

completely different yet genetically connected adaptation. Seemingly far-fetched, the

hypothesis has been strongly supported by W.R. Jeffery (2007) in experiments dealing

with the injection of hedgehog mRNA, the basis of an important developmental signaling

pathway. A signaling pathway is best explained in metaphor. Imagine every cell in an

organism is like a room in a house. Each cell is capable of producing the entire body’s

array of proteins, as if there was a

blueprint for the entire house in each

room. Not every cell needs to make every

protein, however, and a signaling

pathway essentially serves to tell the cell

which page of the blueprint to turn to

using a special protein to trigger a

cascade of reactions.

W.R. Jeffery noticed changes in

gene expression patterns in A. mexicanus

that suggested changes in signaling

pathways and designed an experiment in

which he unilaterally injected hedgehog

mRNA into a developing surface fish embryo. Sonic hedgehog expression was

consequently expanded, and Pax6 was down-regulated along with changes in pax2 and

tiggy winkle hedgehog (192). While these words may seem silly, they represent serious

genes that are strongly tied to eye development. The physical result was reduced eye

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primordia, lens apoptosis, and arrested eye growth in the embryo, resulting ultimately in a

surface fish missing an eye on one side of its head. This experiments shows that eye

degeneration results from a change in gene expression rather than gene function. Most

significantly, it shows that regression is controlled by signals emanating from outside the

eye itself (193). In this frame, changes in cavefish phenotype result in modified Hh

expression, and eye degeneration comes about as a byproduct. The specific trait

controlling Hh expression is still unknown, but Jeffery suggests features related to more

efficient feeding structures and behaviors may hold the key. This is supported by the fact

that Shh is closely tied with the development of teeth, taste buds, and craniofacial

structures, all of which are enhanced in many cavefish (194).

Conclusions:

It is unlikely that neutral mutation and the energetic demands of eye development

are strong factors in the regressive evolution of eyes seen broadly in many cave creatures

and in particular in Ogilbia galapagosensis. Due to the high cost of maintaining the

vertebrate eye and to the results of W.R. Jeffery’s hedgehog mRNA injection experiment,

it is most likely instead that maintenance and pleiotropic effects most strongly influence

degeneration. The ultimate force could be based only on maintenance, only on pleiotropy,

or on some superposition of the two.

Further Study:

Preliminary further studies suggest that pleiotropy may be the stronger of the two

forces. This conclusion results from the fact that both males and females exhibit the same

amount of eye degeneration despite the greater energetic costs required by the female for

egg production (Jeffery 2005: 194). The expected result of greater eye reduction in

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females has actually been recorded in a species of cave beetle (194). Similarly, certain

cavefish colonies underneath bat colonies display marked eye degeneration despite an

apparent abundance of nutrients (194). This further discounts the development hypothesis

and at least throws maintenance into question. Only further formal study can determine

the exact proportions between the effects of pleiotropy and maintenance. Most

importantly, we must specifically identify the phenotypical variation regulating Hh

signaling and the consequent eye degeneration. Because Shh expression was expanded

asymmetrically in the rostrum as well as in the eye in Jeffery’s injection experiment

(192), perhaps the developed sensory papillae and other haptic senses on the heads of O.

galapagosensis, O. pearsei, and many other cavefish hold more insight into this

perplexing question.

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Works Cited

Darwin, C. R. (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray.

Iliffe, T. M. (1991). Anchialine Fauna of the Galapagos Islands. In M.J. James, (ed.),

Galapagos Marine Invertebrates, (pp. 209-231.) New York: Plenum Press. Iliffe, T.M. (2007, December). Anchialine Cave and Cave Fauna of the World.

Retrieved from http://www.tamug.edu/cavebiology/fauna/bonyfish/T_pearsei.html

Jeffery, W.R. (2005). Adaptive Evolution of Eye Degeneration in the Mexican Blind

Cavefish. Journal of Heredity, 96(3), 185-196. Nielsen, J.G., Cohen, D.M., Markle, D.F, Robins, C.R. (1999). Ophidiiform Fishes of

the World. FAO Fisheries Synopsis Number 125, 18, 135. Protas, M., Conrad, M., Gross, J.B., Tabin, C., & Borowsky, R. (2007). Regressive

Evolution in the Mexican Cave Tetra, Astyanax mexicanus. Current Biology, 177, 452-454.

Photos: Ogilbia galapagosensis

http://www.fishbase.org/identification/specieslist.cfm?famcode=472&areacode=

Ogilbia pearsei

http://www.aquariofilia.net/forum/index.php?act=Print&client=printer&f=167&t=35093

Santa Cruz Grieta:

http://picasaweb.google.com/gonzaloalvarezb/FotosGalapagos#5222902936522569986