Seeing things clearly
Matthew - Birds are exceptionally visual organisms and their visual systems are much more complex than the human visual system at least at the level of the eye. We humans, our colour vision system relies on three different cone photoreceptors. So, these are light sensitive cells that are sensitive to red wavelengths light, green wavelengths of light, and blue wavelengths of light. But birds in comparison use 4 different cone photoreceptors sensitive to red, green, and blue just like humans, but they also have a violet or an ultraviolet sensitive cone photoreceptor that they use for colour vision. Those photoreceptors are also more complex than human photoreceptors. So not only do they have a light sensitive part of the cell. They also, within the cell, have a light filter and they combine that light filter and light sensor together to really finely tune what wavelengths of light the photoreceptor is going to respond to.
Chris - Now, when you say they’ve got a filter in that, why do they need to do that?
Matthew - So, that filter serves as a long pass colour filter so it’s absorbing a lot of the shorter wavelengths of light passing them onto the sensitive part of the cell. That narrows the range of wavelengths that a given cell will respond to. It will reduce overlap in sensitivity with the other colour-sensitive cells.
Chris - So it gives them the ability to discriminate colours more acutely than we can. Has it always been like that?
Matthew - So, one of the major divisions among bird species or biggest differences in their visual system is that some bird species are sensitive to violet light – that fourth cone is sensitive to violet light – and in other species, that cone is sensitive to ultraviolet light. This seems to be a switch that’s happened several times throughout the evolution of birds. What we also see in these species as they switch from violet to ultraviolet sensitivity is that they change the sensitivity of the accompanying photoreceptors. So the blue sensitive cell in particular kind of moves over to track that violet/ultraviolet shift. What we were trying to get at with our work or trying to better understand is, how they made that shift in the filtering pigment within the blue.
Chris - Do you think that those changes occurred almost synchronously or did the birds have to suffer not such good colour discrimination for a while while they then re-evolve the ability to retune the other photoreceptors to give them that better discrimination?
Matthew - That’s a good question. We don’t actually know the sequence of events at this point, but the switch between violet and ultraviolet sensitivity is relatively easy to make. It’s just one small change in the gene for that light sensitive part of the cell whereas to shift the filtering of the oil droplet, it’s a change in a pathway that metabolises the pigments that are the filter in the blue cone so it’s a bit more complicated to shift the blue cone. So, it may be the case that first, we get a shift in the violet to ultraviolet and then the accompanying changes happen in the other cell type.
Chris - Now excuse this terrible pun, but how did you then shed light on how this was achieved?
Matthew - What we did was make very detailed measurements of the light absorbent properties of those filtering pigments in the blue cone and accompany that by chemical characterisation of the pigments that are in that cone. We determined that the shift in the filtering of the cone is the result of a change in the chemical composition of that filter. So, they're actually just eliminating a single double bond in the pigment molecule that’s in that filtering pigment. So, it’s a very small change in chemical structure, yields a rather dramatic shift in the light absorbing properties of that filter.
Chris - Now you’ve sussed out how this happens, what are the implications of this? How does this sort of change our thinking of bird vision?
Matthew - What our study have shown is that evolution of ultraviolet vision in birds involves not only a change in that one photoreceptor – violet cone shifting to ultraviolet sensitivity but also, a whole suite of other adaptations are occurring within the eye to optimise vision for this new expanded sensitivity. That’s what I think is really exciting is these very different mechanisms in different cell types, different tissue types, with different biochemical properties and physiological properties are all coordinated towards a common outcome that we can understand in terms of facilitating the discrimination of colour.