Molecular and Functional Basis of Phenotypic Convergence in White Lizards at White Sands
Erica B. Rosenblum, Holger Rompler, Torsten, Schoneberg and Hopi E. Hoekstra
PNAS (2010) 107(5): 2113-2117
This paper is part of a series of papers by Rosenblum which attempts to describe the molecular basis of a white color adaptation in different species of lizards. Potentially the most remarkable aspect of this paper is that it identifies separate protein coding changes that result in convergent evolution of a phenotype. In the end, 2 of the 3 species of lizards analyzed here have specific amino acid substitutions which are shown to result in reduced function of the protein. However, the mechanism by which the protein’s function is disrupted is different between the 2 species, as is the dominance of the derived alleles. This is a nice illustration of how different mutations can lead to the same phenotype… at least the same loss of function. This gets to one of our most interesting discussions regarding the paper. In summary, is convergence more common for loss-of-function than for gain-of-function mutations?
Loss-of-function mutations in the melanocortin-1 receptor (Mc1r) gene lead to a white, or “blanched”, lizard phenotype. Multiple lizard species are found as blanched and dark types. This paper focused on three species found in and around White Sands, New Mexico: S. undulatus, A. inornata and H. maculata. Each of these species has a strong correlation between a different amino acid substitution in Mc1r and blanched coloration. However, the presence or absence of these amino acid substitutions in the dark lizards already hints that their dominance is different.
The strength of the paper is what they do with the blanched/derived and dark/wild-type proteins which shows how the different mutations can exert different loss-of-function effects on the Mc1r protein. Mc1r is known to induce an accumulation of cAMP which eventually result in the activation of other genes involved in pigmentation. By putting the different wild-type and derived lizard genes into cultured cells, they were able to show that S. undulatus and A. inornata derived alleles resulted in a reduction of cAMP relative to their wild-type counterparts. Interestingly, this was not the case for H. maculata even though it exhibited the strongest correlation with white habitat and it’s single substitution. What is the reason for this strong correlation without a change in function? Perhaps this hints at an unknown mechanism of Mc1r. Regardless, I think this is an outstanding illustration of what can be done to show functional changes in the absence of a genetically tractable organism. Next, the authors show (again in cell culture) that the substitution in S. undulatus results in reduced protein at the cell surface. This seems to be able to explain the dominance of the S. undulatus blanched allele because Mc1r acts as a dimer and this substitution would effectively create a dominant negative, preventing the wild-type protein from localizing to the cell surface.
Much of our discussion focused on the idea in the paper that convergence through different mutations may be more common for loss-of-function mutations than for gain-of-function mutations. The idea being that there are more ways to break a pathway than to create novel functions. Initially, I think this is a reasonable assumption. However, it is important to keep in mind that even if this is true, not all mutations result in viable organisms. Perhaps partially reflective of this is the observation that the Mc1r gene seems to have been repeatedly mutated to give rise to blanched phenotypes, and not the many other downstream genes involved in pigmentation. That is, there certainly are constraints on where mutations are tolerated, and the relative tolerance of partial gain-of-functions versus partial loss-of-functions to a specific pathway does not seem clear. It will be quite interesting to see what the functional affects of the H. maculata Mc1r substitution is. As is often the case, the assays used in this paper would be unlikely to reveal a gain-of-function. Empirically, we still seem to have a lack of protein coding gain-of-function examples relative to loss-of-functions, though whether this is more a result of ascertainment bias or actually reflective of true biological patterns remains to be seen.