Color Vision Evolution

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Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components. Researchers studying the genes responsible for color vision pigments—opsin genes—have long known that there exist four photopigment opsins in birds, reptiles and teleost fish[1]. This indicates that the common ancestor of tetrapods and amniotes had tetrachormatic vision, or the ability to discern four different types of color [2]. Today most mammals possess dichromatic vision, and are only able to discern between short and long wavelengths. Primates are unique as they possess trichromatic color vision, and are able to discern between violet [short wave (SW)], green [medium wave (MW)], and yellow-green [long wave (LW)] [3]. Within the primate sub-order, the catarrhines (Old World monkeys, humans, and apes) are routinely trichromatic, meaning that both males and females possess three opsins sensitive to SW, MW, and LW [4]. Platyrrhines (New World monkeys), on the other hand, are non-routinely trichromatic; only a small fraction of platyrrhines are trichromats [5].

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[edit] Genetic Mechanisms Behind Trichromacy in Primates

In catarrhines, the SW opsin is encoded by autosomal gene on chromosome 7 and the MW and LW pigments by adjacent genes on the X chromosome. Separate genes for each opsin insures that routine trichromatic vision is the rule, and not the exception. Platyrrhines, on the other hand, have only one X-chromosome gene locus for MW and LW opsins; this leaves male platyrrhines dichromatic, because they have only one X-chromosome. Heterozygous females, however, can have non-routine trichromacy if one X-chromosome gene locus encodes a MW opsin and the other chromosome codes for a LW opsin. This form of trichromacy is non—routine [6].

[edit] Two Conflicting Hypotheses for Color Vision Evolution in Catarrhines and Platyrrhines

Evolutionary biologists are now trying to answer why and how the catarrhines and platyrrhines evolved different forms of trichromacy. Molecular studies demonstrate that the spectral tuning (response of a photopigment to a specific wavelength of light) of the three pigments in both sub-orders is the same. This leads some experts in the field to believe that the LW and MW photopigments had a common evolutionary origin [7]. There are two popular hypotheses that explain the evolution of catarrhine and platyrrhine vision differences from this common origin point.

Hypothesis 1: Polymorphism as the Ancestral Mode of Color Vision

The first hypothesis dictates that the two-gene system of the catarrhines evolved from a crossing over mechanism. Unequal crossing over between the chromosomes carrying alleles for LW and MW variants could have resulted in separate LW and MW genes at different loci of the X-chromosome [8]. This hypothesis requires that the catarrhine/platyrrhine divergence after polymorphic system of platyrrhines was already in place.

This hypothesis proposes that soon after the platyrrhine/catarrhine divergence, a heterozygous female led to male and female offspring with separate M and L opsins on the X-chromosome [9]. A genetic phenomenon known as X-inactivation permits each cone cell to express only an M or an L opsin (not both), which endowed the catarrhines with routine trichromacy.

Hypothesis 2: Gene Duplication as the Ancestral Mode for Color Vision

Another possibility is that routine trichromacy arose early in primate evolution and was subsequently lost in some primate taxa. Use of the “molecular clocks” ideology, which is a technique geneticists use to determine an evolutionary sequence of events. It deduces elapsed time from a number of minor differences in DNA sequences [10]. Nucleotide sequencing of opsin genes suggests that the genetic divergence between New World primate opsin alleles (2.6%) is considerably smaller than the divergence between Old World primate genes (6.1%) [11]. Based on this data, some researchers believe that gene duplication preceded the polymorphism seen in extant New World primates. These scientists argue that New World primates could have lost or gained alleles based on selective pressures of the environment. They also propose that the polymorphism in the opsin gene might have arisen independently through point mutation on one or more occasions [12], and that the spectral tuning similarities are due to convergent evolution.

[edit] Natural Selection and Color Vision

The simplest explanation for the maintenance of the polymorphism is through a consistent advantage for heterozygous trichromatic females [13]. However, if this hypothesis was correct, then gene duplication, followed by routine trichromacy, would have been strongly selected for in New World primates. This has not been the case; the previously mentioned experiment by Boissinot, et al. (1998) shows that the polymorphism itself is being selected for, not just the alleles for trichromacy. Within the tri-allelic system of New World primates, heterozygous females have not demonstrated a statistically significant increase in fitness compared to homozygotes; this could help explain the stability of allelic trichromacy [14].

Another strong explanation is frequency dependence [15]. This possibility arises when there are some situations in which dichromats have an advantage over trichromats. The foraging of different phenotypes in visually distinct niches will then maintain both trichromats and dichromats in a population through frequency dependence [16].

Non-routine trichromacy offers an advantage to platyrrhines because of their unique diet and behaviors. Platyrrhines forage in groups, and are insectivorous as well as of frugivorous and foliverous [17]. Group foraging could explain the importance of maintaining such a high number of dichromats in the New World primate sub-order. The trichromatic heterozygous females can forage for ripe fruit and leaves while the dichromatic members have advantages in finding camouflaged vegetation and insects. The dichromats would also be able to better fend off against camouflaged predators, which would offer a strong fitness advantage to platyrrhine species [18].

Exceptions to the Rule: How Vision Changes Under the Influence of Natural Selection

One notable case is the Howler monkey (Alouatta). The Howler monkey was recently discovered to share the same of routine trichromacy that is characteristic of Old World primates. Gene divergence studies place the development of routine trichromacy in Howler monkeys after that of Catarrhines [19]. It is widely believed that the Howler monkey developed routine trichromacy by an unequal crossing over mechanism of opsin alleles, which led to a duplication for the MW/LW gene [20]. Though Howler monkeys developed routine trichromacy after catarrhines, both groups are assumed to have developed their trichromacy through similar genetic processes [21].

The environment of Howler monkeys is different from that of most other New World primates; this may account for the selection of routine trichromacy in this species. Howler monkeys, for example, are perhaps the most foliverous of the New World monkeys, and they rarely consume fruits or other foods. They also forage alone, unlike most other platyrrhine species. A study performed by Lucas, et al. (2003) outlined the advantages of routine trichromacy; a study of these advantages suggests that routine trichromacy allows Howler monkeys to better exploit their unique environment than non-routine trichromacy.

The night monkey (Aotus), a nocturnal primate, is also an exception among the platyrrhine sub-order. This species lacks color vision altogether. The Night monkeys have lost their S photopigments as well as their polymorphic M/L opsin genes. The loss of color vision in the Night monkey is more so an adaptation than a loss of a vestige because color vision is actually counterproductive in low levels of light. Based on previously mentioned research by Lucas, et al (2003), color vision leads to “chromatic noise”, which makes it difficult to detect outlines of shapes. The cones responsible for color vision therefore hinder an organism’s ability to see in low levels of light, and it is advantageous for the Night monkey to be achromatic instead of dichromatic or trichromatic [22].

[edit] References

  1. ^ Yokoyama, S., and B. F. Radlwimmer. 2001. The molecular genetics and evolution of red and green color vision in vertebrates. Genetics Society of America. 158: 1697-1710.
  2. ^ Bowmaker, J. K. 1998. Evolution of colour vision in vertebrates. Eye. 12: 541-547.
  3. ^ Dulai, K. S., and M. von Dornum. 1999. The evolution of trichromatic color vision by opsin gene duplication of new world and Old World primates. Genome Research. 9: 629-638.
  4. ^ Bowmaker, J. K. 1998. Evolution of colour vision in vertebrates. Eye. 12: 541-547.
  5. ^ Surridge, A. K., and D. Osorio. 2003. Evolution and selection of trichromatic vision in primates. Trends in Ecol. and Evol. 18: 198-205.
  6. ^ Lucas, P. W., and N. J. Dominy. 2003. Evolution and function of routine trichromatic vision in primates. Evolution. 57: 2636-2643.
  7. ^ Neitz, et al. 1991
  8. ^ Nathans, J., and D Thomas. 1986. Molecular genetics of human color vision: the genes encoding blue, green and red pigments. Science. 232: 193–203
  9. ^ Surridge, A. K., and D. Osorio. 2003. Evolution and selection of trichromatic vision in primates. Trends in Ecol. and Evol. 18: 198-205.
  10. ^ Hillis, D. M. 1996. Inferring complex phytogenies. Nature. 383: 130-131.
  11. ^ Hunt, D. M., and K. S. Dulai. 1998. Molecular evolution of trichromacy in primates. Vision Research. 38: 3299-3306.
  12. ^ Surridge, A. K., and D. Osorio. 2003. Evolution and selection of trichromatic vision in primates. Trends in Ecol. and Evol. 18: 198-205.
  13. ^ Lucas, P. W., and N. J. Dominy. 2003. Evolution and function of routine trichromatic vision in primates. Evolution. 57: 2636-2643.
  14. ^ Cropp, S., Boinski, S., and W. H. Li. 2002. Allelic Variation in the Squirrel Monkey X-Linked Color Vision Gene. Journal of Mol. Evo. 54: 734-745.
  15. ^ Morgan, M. J., Adam, A., and J. D. Mollon. 1992. Dichromats detect colour-camouflaged objects that are not detected by trichromats. Proceedings: Biological Sciences. 1323: 291-295.
  16. ^ Surridge, A. K., and D. Osorio. 2003. Evolution and selection of trichromatic vision in primates. Trends in Ecol. and Evol. 18: 198-205.
  17. ^ Surridge, A. K., and D. Osorio. 2003. Evolution and selection of trichromatic vision in primates. Trends in Ecol. and Evol. 18: 198-205
  18. ^ Tovee, M. J., and J. K. Bowmaker. 1991. The relationship between cone pigments and behavioral sensitivity in a New World monkey (Callithrix jacchus jacchus). Vision Res. 32: 867-878.
  19. ^ Dulai, K. S., and M. von Dornum. 1999. The evolution of trichromatic color vision by opsin gene duplication of new world and Old World primates. Genome Research. 9: 629-638.
  20. ^ Lucas, P. W., and N. J. Dominy. 2003. Evolution and function of routine trichromatic vision in primates. Evolution. 57: 2636-2643.
  21. ^ Dulai, K. S., and M. von Dornum. 1999. The evolution of trichromatic color vision by opsin gene duplication of new world and Old World primates. Genome Research. 9: 629-638.
  22. ^ Morgan, M. J., Adam, A., and J. D. Mollon. 1992. Dichromats detect colour-camouflaged objects that are not detected by trichromats. Proceedings: Biological Sciences. 1323: 291-295.
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