Evolution of color vision in primates

While color vision is dependent on many factors, discussion of the evolution of color vision is typically simplified to two factors: • the breadth of the visible spectrum (which wavelengths of light can be detected), and • the dimensionality of the color gamut (e.g. dichromacy vs. tetrachromacy). In vertebrates, both of these are almost perfectly correlated to an individual's cone complement. The retina comprises several different classes of photoreceptors, including cone cells and rod cells. Rods usually do not contribute to color vision (except in mesopic conditions) and have not evolved significantly in the era of primates, so they will not be discussed here. It is the cone cells, which are used for photopic vision, that facilitate color vision. Each type - or class - of cones is defined by its opsin, a protein fundamental to the visual cycle that tunes the cell to certain wavelengths of light. The opsins present in cone cells are specifically called photopsin. The spectral sensitivities of the opsins are dependent on their genetic sequence. The most important (and often only important for discussions of opsin evolution) parameter of the spectral sensitivity is the peak wavelength, i.e. the wavelength of light to which they are most sensitive. For example, a typical human L-opsin has a peak wavelength of 560 nm. The cone complement defines an individual's set of cones in their retina - usually consistent with the set of opsins in their genome. The breadth of an individual's visual spectrum is equal to the minimum and maximum wavelengths to which at least one of their cones is sensitive. In vertebrates, the dimensionality of the color gamut is usually equal to the number of cones/opsins, though this simple equivalence breaks down for invertebrates. Primate cone complement The cone complements exhibited by primates can be monochromatic, dichromatic or trichromatic. The catarrhines (Old World monkeys and apes) are routine trichromats, meaning both males and females possess three opsins classes. In nearly all species of platyrrhines (New World monkeys) males and homozygous females are dichromats, while heterozygous females are trichromats, a condition known as allelic or polymorphic trichromacy. Among platyrrhines, the exceptions are Alouatta (routine trichromats) and Aotus (routine monochromats). All primates with the exception of Aotus exhibit an S-opsin (short wave sensitive) in the cone most sensitive to blue light (S-cone). This opsin is encoded by an autosomal gene on chromosome 7. The other cones differ between primates. Catarrhine cone complement The taxa Catarrhini includes old world monkeys (e.g. baboons) and apes (e.g. humans). In addition to the S-opsin, catarrhine primates have two adjacent opsin genes on the X chromosome: • M-opsin (middle wave sensitive, encoded by OPN1MW gene), the cone most sensitive to green light • L-opsin (long wave sensitive, encoded by OPN1LW gene), the cone most sensitive to red light Platyrrhine cone complement The taxa Platyrhini includes new world monkeys (e.g. Squirrel monkeys). In addition to the S-opsin, trichromatic platyrrhine primates generally have only a single opsin gene locus, but it is polymorphic, with different alleles encoding opsins of different peak wavelength. Individuals homozygous for the gene will have only two opsin classes and therefore exhibit dichromacy. However, heterozygous individuals will have three opsin classes and therefore be trichromats. Since the gene is on the X-chromosome, ==Phylogenetics==

Mammalian Ancestors The common vertebrate ancestor (ca. 540MYA) had 4 photopsins in their complement (SWS1, SWS2, Rh2, LWS) and likely had tetrachromatic vision. Today, most other vertebrate classes have retained their 4 cones and exhibit tetrachromacy, including birds, reptiles, teleosts (fish) and amphibians. However, mammalian ancestors lost 2 of the 4 opsins due to the nocturnal bottleneck and most modern mammals are therefore dichromats, retaining only the SWS1 (UV–sensitive) and LWS (red–sensitive) opsins. Approximately 35 MYA the LWS class of opsins in catarrhine ancestors split into OPN1MW and OPN1LW. Evolutionary pathway of SWS1 Mutagenesis experiments involving the Boreoeutherian ancestor to humans have shown that seven genetic mutations are linked to losing UV vision and gaining the blue light vision that most humans have today over the course of millions of years. There are two popular hypotheses that explain the evolution of the primate vision differences from this common origin. Polymorphism The first hypothesis is that the two-gene (M and L) system of the catarrhine primates evolved from a crossing-over mechanism. Unequal crossing over between the chromosomes carrying alleles for L and M variants could have resulted in a separate L and M gene located on a single X chromosome. This hypothesis proposes that this crossing-over event occurred in a heterozygous catarrhine female sometime after the platyrrhine/catarrhine divergence. 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%). == Ultimate causation hypotheses ==

There exist several theories for the main evolutionary pressure that caused primates to evolve trichromatic color vision, namely the red-green opponent channel. Fruit theory This theory postulates that trichromacy became favorable due to the increased ability to find ripe fruit against a foliage background. Research has found that the spectral separation between the L and the M cones is closely proportional to the optimal for detection of many colors of fruit (red) against foliage (green). The reflectance spectra of fruits and leaves naturally eaten by the Alouatta seniculus were analyzed and found that the sensitivity in the L and M cone pigments is optimal for detecting fruit among leaves. While the "fruit theory" holds much data to support its reasoning, some recent research has criticized this theory. One study shows that the difference in the fruit-spotting task between trichromats and dichromats is largest when the tree is far away (~12m), inferring that the evolutionary pressure may have been on spotting fruit trees from a distance, rather than picking fruit. Those findings were based upon the fact that there is a larger variety of background S/(L+M) and luminance values under long-distance viewing. Young leaf hypothesis This theory is centered around the idea that the benefit for possessing the different M and L cone pigments are so that during times of fruit shortages, an animal's ability to identify the younger and more reddish leaves, which contain higher amounts of protein, will lead to a higher rate of survival. This theory supports the evidence showing that trichromatic color vision originated in Africa, as figs and palms are scarce in this environment thus increasing the need for this color vision selection. However, this theory does not explain the selection for trichromacy polymorphisms seen in dichromatic species that are not from Africa. this study has been challenged, and two of the authors retracted it. The theory is that as sense of smell deteriorated, selective pressures increased for the evolution of trichromacy for foraging. In addition, the mutation of trichromacy could have made the need for pheremone communication redundant and thus prompted the loss of this function. Overall, research has not shown that the concentration of olfactory receptors is directly related to color vision acquisition. Research suggests that the species Alouatta does not share the same characteristics of pheromone transduction pathway pseudogenes that humans and Old World monkeys possess and leading howler monkeys to maintain both pheromone communication systems and full trichromatic vision. Therefore, trichromacy alone does not lead to the loss of pheromone communication but rather a combination of environmental factors. Nonetheless research shows a significant negative correlation between the two traits in the majority of trichromatic species. Skin tone Trichromacy may also be evolutionarily favorable in recognizing changes in skin tone. The spectral sensitivity of M- and L-opsins maximize sensitivity to changes in skin color that correspond to blood oxygen levels. Recognizing changes in skin tone that indicate states of health would be one advantage. Dichromatic humans report trouble with recognizing sunburn, rash, pallor and jaundice. Recognizing when offspring are sick allows parents to care for or provide treatment to them. Likewise, mate choice that excludes sick individuals increases the viability of offspring. Similarly, other causes of skin tone change such as blushing or rump-reddening convey important information between potential sexual partners. Therefore, the formation of trichromatic color vision in certain primate species may have been beneficial in recognizing the state of health/fertility of others. ==Anomalies in New World monkeys==

Aotus and Alouatta There are two noteworthy genera within the New World monkeys that exhibit how different environments with different selective pressures can affect the type of vision in a population. and the type of leaves they consume (young, nutritive, digestible, often reddish in color), are best detected by a red-green signal. Field work exploring the dietary preferences of howler monkeys suggest that routine trichromacy was environmentally selected for as a benefit to folivore foraging. == See also ==