Published on March 1st, 2019 | by David Marshall Episode 97: Opsins
Opsins are the photosensitive proteins in the eye, responsible for converting a photons of light into an electro-chemical signals. Different opsins react to different wavelengths of light, each corresponding to a different band of colour. In humans, the ‘visible spectrum’ of light (a very anthropocentric term) is covered by three opsins, receptive to red, green and blue wavelengths. Other animals have opsins that are capable of subdividing the ‘visible spectrum’ and responding to a large number of very specific wavelengths of interest. All in all, the ability to detect light and recognise colour is not the same throughout the animal kingdom.
In this episode, we are joined by Dr James Fleming of Keio University, Japan to discuss the evolution of opsins in the ecdysozoa (the group containing arthropods and a fair few worms). We talk about the fundamentals of light detection and how, using phylogenetics, we are able to tell which colours certain extinct animals were capable of detecting.
The ecdysozoa are a Superphylum of animals that contains the arthropods, onycophorans (pictured), tardigrades, loriciferans and nematode worms to name a few. All ecdysozoans grow by moulting: a process called ecdysis.
Vision is incredibly important to many ecdysozoans and whilst their eyes may be viewed as ‘primative’ compared to those of us vertebrates, they can be remarkably well-adapted to incredibly specific tasks. The compound eyes of mantis shrimp (pictured) are fantastically complex and capable of performing a whole range of visual tasks.
Compound eyes are fixed and immobile, so we are able to reconstruct the visual performance of extinct animals using the same simple optical principles that govern the study of modern compound eyes. We have excellent knowledge of the visual ecologies of animals such as trilobites. Image courtesy and copyright of Peter Cameron. All rights reserved.
We know that vision has been incredibly important to the Ecdysozoa throughout their entire geological history. This anomalocaridid eye from the Early Cambrian Emu Bay Shale has incredibly high resolution, featuring a huge number of tiny lenses (see inset), each of which responsible for monitoring a unique area of space around the eye. Image courtesy of Dr John Paterson.
Unfortunately, from the physical eye we can only interpret such visual factors as its resolution and light gathering capabilities. Information such as which colours it was capable of seeing are held only in the opsins, which are lost in the fossilisation process. Compound eye of the Antarctic krill Euphausia superba (Photo by Gerd Alberti and Uwe Kils) (CC BY-SA 3.0).
Through study of the different types of opsins in modern ecdysozoans, along with an understanding of the relationships between its subgroups, it is possible to make predictions as to which groups had which opsins. From the tree above it is possible to assume that the diplopod and chilopods had the opsins represented by the light blue, green and purple circles, although it isn’t possible to say about the black, pink or dark blue circles. Other opsins may have been independently gained or lost. This kind of inference is called phylogenetic bracketing.
The previous tree only shows the modern animals, whereas there were a whole range of historically important, but now extinct, animals that fall between the modern groups. The stem group arthropods for instance (including the charismatic anomolocaridids) would fall between the onychophorans and the chelicerates on the previous tree, so show could we infer which opsins they had?
To do this, the first step is to see how closely all the modern opsins are related. Then, based on the amount of genetic difference between them, you can estimate how long ago each opsin diverged, on the assumption that differences are acquired at a steady rate. This is termed a genetic clock. In this tree, you can see the different opsins and how they are related. Each branch of the tree is given an estimated divergence date; the bigger the difference between two opsins, the longer ago they diverged.
Through a combination of all the trees and data, you can then make estimates as to the likely opsin groups present in these extinct stem arthropods (left tree). Furthermore, the timing of the origin of different opsins can be placed in geological context (right tree). Here you can see the estimated origin time of the full complement of arthropod opsins (519 -540 million years ago) occurred immediately before the onset of the Cambrian explosion (541 million years ago).
Such evidence lends weight to the theory that the Cambrian Explosion was, in part, caused by the evolution of highly visual predators that sparked an evolutionary arms race.