Abstract:
New World monkeys exhibit prominent colour vision variation due to allelic polymor-
phism of the long-to-middle wavelength (L/M) opsin gene. The known spectral varia-
tion of L/M opsins in primates is broadly determined by amino acid composition at
three sites: 180, 277 and 285 (the ‘three-sites’ rule). However, two L/M opsin alleles
found in the black-handed spider monkeys (Ateles geoffroyi) are known exceptions,
presumably due to novel mutations. The spectral separation of the two L/M photopig-
ments is 1.5 times greater than expected based on the ‘three-sites’ rule. Yet the conse-
quence of this for the visual ecology of the species is unknown, as is the evolutionary
mechanism by which spectral shift was achieved. In this study, we first examine L/M
opsins of two other Atelinae species, the long-haired spider monkeys (A. belzebuth)
and the common woolly monkeys (Lagothrix lagotricha). By a series of site-directed
mutagenesis, we show that a mutation Y213D (tyrosine to aspartic acid at site 213) in
the ancestral opsin of the two alleles enabled N294K, which occurred in one allele of
the ateline ancestor and increased the spectral separation between the two alleles. Sec-
ond, by modelling the chromaticity of dietary fruits and background leaves in a natu-
ral habitat of spider monkeys, we demonstrate that chromatic discrimination of fruit
from leaves is significantly enhanced by these mutations. This evolutionary renovation
of L/M opsin polymorphism in atelines illustrates a previously unappreciated dyna-
mism of opsin genes in shaping primate colour vision.
