Abstract
Primate colour vision depends on a matrix of photoreceptors, a neuronal post receptoral structure and a combination of genes that culminate in different sensitivity through the visual spectrum. Along with a common cone opsin gene for short wavelengths (sws1), Neotropical primates (Platyrrhini) have only one cone opsin gene for medium-long wavelengths (mws/lws) per X chromosome while Paleotropical primates (Catarrhini), including humans, have two active genes. Therefore, while female platyrrhines may be trichromats, males are always dichromats. The genus Alouatta is inferred to be an exception to this rule, as electrophysiological, behavioural and molecular analyses indicated a potential for male trichromacy in this genus. However, it is very important to ascertain by a combination of genetic and behavioural analyses whether this potential translates in terms of colour discrimination capability. We evaluated two howler monkeys (Alouatta spp.), one male A. caraya and one female A. seniculus, using a combination of genetic analysis of the opsin gene sequences and a behavioral colour discrimination test not previously used in this genus. Both individuals completed the behavioural test with performances typical of trichromatic colour vision and the genetic analysis of the sws1, mws, and lws opsin genes revealed three different opsin sequences in both subjects. These results are consistent with uniform trichromacy in both male and female, with presumed spectral sensitivity peaks similar to Catarrhini, at ~ 430 nm, 532 nm, and 563 nm for S-, M- and L-cones, respectively.
Highlights
Colour vision is thought to play an important role in the survival of primates
These polymorphisms may lead to trichromacy in heterozygous females, with two distinct lws opsin genes in each X chromosome that are translated into opsins that can be spectrally tuned toward the green or the red regions of the light spectrum [5]
Our results strongly support the hypothesis of uniform trichromacy in A. caraya and A. seniculus
Summary
Colour vision is thought to play an important role in the survival of primates. The ability to visually discriminate a target from the background by hue differences may improve the capacity to find food sources and to detect predators [1, 2]. Neotropical primates (Parvorder Platyrrhini), like most other placental mammals, have one copy of the lws opsin gene in the X chromosome, meaning that both, males, with a single X chromosome, and homozygous females, are dichromats. Allelic polymorphisms of the LWS is observed in many species of Platyrrhini, leading to a variety of colour vision phenotypes among different species and among individuals of a same species [4] These polymorphisms may lead to trichromacy in heterozygous females, with two distinct lws opsin genes in each X chromosome that are translated into opsins that can be spectrally tuned toward the green (middle wavelengths) or the red (long wavelengths) regions of the light spectrum [5]
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