Molecular Basis of Evolution Gaining New Insight

1. Molecular Basis of EvolutionGaining New Insight

2. Molecular Basis of Evolution Every step in evolution is caused by a change in molecules But biologists have rarely traced adaptive changes to their molecular roots

3. Yokoyama, S. H. Zhang, F.B. Radlwimmer and N.S. Blow. 1999. Adaptive evolution of color vision of the Comoran coelacanth (Latimeria chalumnae) Proceedings of the National Academy of Sciences 96:6279-6284.

4. Yokoyama and his colleagues have described how they pinpointed the changes in visual pigment genes that enabled the coelacanth to see in the dim light of the deep ocean, 200 m below sea level

6. Coelacanths Coelacanths receive only a narrow range of color, at about 480 nm (400-700 nm – blue to red) Retina consists mostly of rods, with a small number of cones to see in this photic environment

7. Coelacanths Rods are extremely sensitive to light - allowing them to see in dim light Rods produce colorless, poorly defined images Cones are specialized for color vision & allow for fine detailed vision

8. Visual Pigments or Opsins Rod pigment rhodopsin RH1 Cone pigments RH1-like RH2 short wavelength sensitive SWS1 SWS1-like SWS2 long wavelength sensitive LWS or middle wavelength sensitive MWS

9. Assessed 3 things in this study Assessed which pigment genes were present How the genes were altered during the course of evolution How each change affected protein function and the ability of the organism to see

10. Determining the Molecular Mechanisms Cloning and characterization of the opsin genes Identification of amino acid changes that are potentially important in shifting the wavelength of maximal absorption of the pigments using phylogenetic inferences Determination of the actual effects of these mutations using in vitro assays

11. Southern Blot Analysis The Southern analysis revealed strongly hybridizing bands to RH1 and SWS1 probes but not LWS Thus the coelacanth has RH1 and SWS1-like genes, but no LWS/MWS genes

12. Sequence Data Showed two functional genes - RH1 and RH2 (70% homology) The SWS1 gene was a pseudogene

13. Amino Acid Changes Ancestral rhodopsins are generally more sensitive to green light of 500 nm - a longer wavelength than that penetrating down to where coelacanths live To I.D. which amino acids changes had nudged their peak absorbencies to the shorter wavelength - they compared the sequences to those of other fish that live nearer the surface They found two amino acid changes in each of the coelacanth rhodopsins that seemed likely to underlie the shift in wavelength absorbencies

14. Ancestral

15. RH1

16. RH2

17. A Test of Predictions Introduced mutations into the rhodopsin genes to change those amino acids to those found in the typical fish rhodopsin Measurement of the wavelength sensitivities of the normal and altered coelacanth opsins showed that their predictions were correct

18. Predictions Each mutation contributed additively to shifting the coelacanth opsins from their natural sensitivity peaks - a wavelength of 485 + 3 nm for RH1 and 478 + 1 nm for RH2 - to longer wavelengths

20. Mechanism? The amino acid change apparently alters the fit between the chromophore, which starts to vibrate when hit by light, and therefore affects the chromophores responsiveness to particular wavelengths - allowing the coelacanth to see the limited range of color available in that environment

21. Conclusions The approach taken by Yokoyama is certainly spreading, because tracing adaptation to molecular changes allows evolutionary hypotheses to be tested far more rigorously than we could previously imagine

22. Chen, L., A.L. DeVries, and C.C. Cheng. 1997. Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish Proceedings of the National Academy of Sciences 94:3811-3816