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More and more, evolutionary biologists are discovering examples where evolution has squeezed out a tremendous amount of variation by tinkering with the timing of development. Snakes, for example, have hundreds of vertebrae, while mice have just 50 and humans, just 33. Each vertebra is formed from an embryonic structure called a somite and there’s an internal clock that governs how quickly these somites are formed. In snake embryos, this clock ticks into overdrive so that it develops a hefty collection of smaller somites. That leads to more vertebrae. The owl monkey’s super-sensitive retinas are just another example of this phenomenon.

Dyer and Martins, from the St Jude Children’s Research Hospital in Memphis, studied the RPCs of owl monkey and capuchin foetuses, as their retinas developed. They kept an eye on these cells using a mildly radioactive chemical called bromodeoxyuridine, which gets incorporated into the DNA of dividing cells. This marker revealed that an owl monkey’s RPCs start producing the various cells of its retina much later (and over a shorter period of time) than those of a capuchin’s. This delay means that the owl monkey produces fewer of the earliest cells in the sequence and more of the later types, including the rods that are so vital for night-vision.

The shift in timing could well explain another feature of the owl monkey eye that sets it apart from other monkeys and apes. It lacks a fovea, a spot at the centre of human retinas where our vision is sharpest, where cone cells are abundant and where rods are virtually absent. But as the owl monkey’s retina develops, its dearth of cones and its surplus of rods mean that this focal point never forms. Indeed, the owl monkey has little need for a fovea – acuity is a minor issue when one has to worry about sensing any light at all.

The owl monkey evolved from diurnal ancestors around 15 million years ago. During this relatively short time, it has developed most of the adaptations that other nocturnal mammals have. Its eyes have become bigger and it has lost the third cone cell that other monkeys and apes use to detect light at the red end of the spectrum. But all of its other adaptations – more rods, fewer cones, no fovea – could well be the result of a single change to its development.

It’s a wonderfully elegant system because it means that the proportions of all types of cells in the retina can be adjusted in a coordinated way. Dyer and Martin point out that the alternative explanation – that the fate of each type of cell is controlled individually – isn’t ruled out by their work, but is “an unlikely and rather cumbersome possibility”.

But why should the retina’s development go through such an orderly sequence, where diurnal and nocturnal features are so conveniently grouped? Some will no doubt choose to read too much into this, but Dyer and Martins think that the advantages of such organisation are “not hard to discern”.

All primates, owl monkeys included, come from a long line of animals that have crossed over from diurnal to nocturnal living and back again, many times over.  This ecological flip-flopping would have filtered out programmes of development that make it easier to cross between the two ways of life. Such programme would produce cells that are relevant to each one at different times, and ensure that boosting one group downplays the other – exactly the system found in mammalian retinas.

Reference: Dyer, M., Martins, R., da Silva Filho, M., Muniz, J., Silveira, L., Cepko, C., & Finlay, B. (2009). Developmental sources of conservation and variation in the evolution of the primate eye Proceedings of the National Academy of Sciences DOI:10.1073/pnas.0901484106

5 thoughts on “From day to night – a lesson in eye evolution with the owl monkey

  1. I presume bromodeoxyuridine isn’t necessarily radioactive, but that a radioactive version was created for the experiment.

  2. My guess is that the bromodeoxyuridine (BrDU) was not radiocative, as the whole point of using BrDU is that it can be detected by commercially available antibodies, and thus allows for the non-radioactive detection of DNA synthesis and, hence, mitosis.

  3. Sorry, yes, that sentence was missing a couple of words. They used a combination of radioactive thymidine and non-radioactive BrDU to study the cell cycle in the RPCs. Well spotted.

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