Non-coincidental pleiotropy of pigmentation, brain, and behavior and its evolution
Published in neuro.philica.com
Since the beginning of modern genetics a century ago, mutants causing changes in cuticle, fur, or skin pigmentation have been associated with alterations in animal behavior. The connection has commonly been considered coincidence. However, the more recent discoveries of neural pigmentation gene expression (e.g. Drapeau et al. 2003) suggest that these genes have genuine functions in the brain. This classical genetic pattern â€“ a non-coincidental pleiotropy of pigmentation, brain, and behavior â€“ is common. Why?
I hypothesize that the evolution of a pigmentation gene product to a neural function may have occurred as follows. First, assuming that primitive organismal pigmentation predated the evolution of a complex central nervous system (CNS), the pleiotropic genes under consideration most likely first had a pigmentation function. Second, during CNS evolution along more complex lineages, some external pigmented cells were utilized internally. (Pigmented nerves containing melanin may be a consequence of this.) Three, after pigmentation genes were functioning (unnecessarily) in the CNS, their protein products were relatively unconstrained and free to evolve new functions while retaining an ancient pigmentation function. These novel brain functions are observed today as being required for normal adult animal behavior.
Pigmentation genes may have a tendency to be required for sex-limited behaviors. Because animal coloration is very often sex-specific (e.g. ducks, butterflies, peacocks), pigmentation genes are under sex-specific control at a transcriptional level. (There is some empirical evidence for this in model organisms.) Hence, since these particular genes would not have to evolve sex-specific regulation in the brain ‘de novo’, we may expect them to play sex-specific roles in the brain and behavior. One example is the requirement of the ‘yellow’ gene for the male courtship ritual of the fruit fly Drosophila melanogaster (e.g. Drapeau et al. 2003, 2006).
This observation stems from my Ph.D. dissertation research on a gene required for both pigmentation and behavior in fruit flies, early informal talks with B. Fuller at UC-Irvine, and many discussions with additional scientists who have done extensive work on genes required for pigmentation, behavior, or both.
Drapeau, M. D., Radovic, A., Wittkopp, P. J., & Long, A. D. (2003). A gene necessary for normal male courtship, yellow, acts downstream of fruitless in the Drosophila melanogaster larval brain. Journal of Neurobiology, 55, 53-72.
Drapeau, M. D., Cyran, S. A., Viering, M. M., Geyer, P. K., & Long, A. D. (2006). A cis-regulatory sequence within the yellow locus of Drosophila melanogaster required for normal male mating success. Genetics, 172, 1009-1030.
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Published on Wednesday 26th July, 2006 at 14:32:26.
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The full citation for this Observation is:|
Drapeau, M. (2006). Non-coincidental pleiotropy of pigmentation, brain, and behavior and its evolution. PHILICA.COM Observation number 18.