"trichromat" redirects here. For the chemical ion species, see trichromate.
Trichromacy or trichromaticism is the possessing of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats.
The normal explanation of trichromacy is that the organism's retina contains three types of color receptors (called cone cells in vertebrates) with different absorption spectra. In actuality the number of such receptor types may be greater than three, since different types may be active at different light intensities. In vertebrates with three types of cone cells, at low light intensities the rod cells may contribute to color vision.
Humans and other animals that are trichromats
Humans and some other mammals have evolved trichromacy based partly on pigments inherited from early vertebrates. In fish and birds, for example, four pigments are used for vision. These extra cone receptor visual pigments detect energy of other wavelengths, including sometimes ultraviolet. Eventually two of these pigments were lost (in placental mammals) and another was gained, resulting in trichromacy among some primates.Humans and closely related primates are usually trichromats, as are some of the females of most species of New World monkeys, and both male and female howler monkeys.
Recent research suggests that trichromacy may also be quite general among marsupials. A study conducted regarding trichromacy in Australian marsupials suggests the medium wavelength sensitivity, MWS, cones of the honey possum (Tarsipes rostratus) and the fat-tailed dunnart (Sminthopsis crassicaudata) are features coming from the inheritedreptilianretinal arrangement. The possibility of trichromacy in marsupials potentially has another evolutionary basis than that of primates. Further biological and behavioural tests may verify if trichromacy is a common characteristic of marsupials.
Most other mammals are currently thought to be dichromats, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active). Most studies of carnivores, as of other mammals, reveal dichromacy, examples including the domestic dog, the ferret, and the spotted hyena. Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet, blue and green instead of blue, green and red.
Research indicates that trichromacy allows animals to distinguish red fruit and young leaves from other vegetation that is not beneficial to their survival. Another theory is that detecting skin flushing and thereby mood may have influenced the development of primate trichromate vision. The color red also has other effects on primate and human behavior as discussed in the color psychology article.
Types of cones specifically found in primates
Primates are the only known placental mammalian trichromats.[not in citation given] Their eyes include three different kinds of cones, each containing a different photopigment (opsin). Their peak sensitivities lie in the blue (short-wavelength S cones), green (medium-wavelength M cones) and yellow-green (long-wavelength L cones) regions of the color spectrum. (Schnapf et al, 1987). S cones make up 5–10% of the cones and form a regular mosaic. Special bipolar and ganglion cells pass those signals from S cones and there is evidence that they have a separate signal pathway through the thalamus to the visual cortex as well. On the other hand, the L and M cones are hard to distinguish by their shapes or other anatomical means – their opsins differ in only 15 out of 363 amino acids, so nobody has yet succeeded in producing specific antibodies to them. But Mollon and Bowmaker did find that L cones and M cones are randomly distributed and are in equal numbers.
Mechanism of trichromatic color vision
Trichromatic color vision is the ability of humans and some other animals to see different colors, mediated by interactions among three types of color-sensing cone cells. The trichromatic color theory began in the 18th century, when Thomas Young proposed that color vision was a result of three different photoreceptor cells. Hermann von Helmholtz later expanded on Young's ideas using color-matching experiments which showed that people with normal vision needed three wavelengths to create the normal range of colors. Physiological evidence for trichromatic theory was later given by Gunnar Svaetichin (1956).
Each of the three types of cones in the retina of the eye contains a different type of photosensitive pigment, which is composed of a transmembrane protein called opsin and a light-sensitive molecule called 11-cis retinal. Each different pigment is especially sensitive to a certain wavelength of light (that is, the pigment is most likely to produce a cellular response when it is hit by a photon with the specific wavelength to which that pigment is most sensitive). The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) wavelengths respectively.
Since the likelihood of response of a given cone varies not only with the wavelength of the light that hits it but also with its intensity, the brain would not be able to discriminate different colors if it had input from only one type of cone. Thus, interaction between at least two types of cone is necessary to produce the ability to perceive color. With at least two types of cones, the brain can compare the signals from each type and determine both the intensity and color of the light. For example, moderate stimulation of a medium-wavelength cone cell could mean that it is being stimulated by very bright red (long-wavelength) light, or by not very intense yellowish-green light. But very bright red light would produce a stronger response from L cones than from M cones, while not very intense yellowish light would produce a stronger response from M cones than from other cones. Thus trichromatic color vision is accomplished by using combinations of cell responses.
It is estimated that the average human can distinguish up to seven million different colors.
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The evolution of color vision in primates is unique compared to most eutherianmammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs. Most teleost fish, reptiles and birds are therefore tetrachromatic while all mammals, with the exception of some primates and marsupials, are strictly dichromats.
Primates achieve trichromacy through color photoreceptors (cone cells), with spectral peaks in the violet (short wave, S), green (middle wave, M), and yellow-green (long wave, L) wavelengths. Opsin is the primary photopigment in primate eyes, and the sequence of an organism's opsin proteins determines the spectral sensitivity of its cone cells. Not all primates, however, are capable of trichromacy. The catarrhines (Old World monkeys and apes) are routine trichromats, meaning both males and females possess three opsins (pigments) sensitive to short-, medium-, and long wavelengths. 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 (consistent trichromats) and Aotus (consistent monochromats).
Mechanism of color vision
Genetically, there are two ways for a primate to be a trichromat. All primates share an S opsin encoded by an autosomal gene on chromosome 7. Catarrhine primates have two adjacent opsin genes on the X chromosome which code for L and M opsin pigments.
In contrast, platyrrhines have only a single, polymorphic X chromosome M/L opsin gene locus. Therefore, every male platyrrhine is dichromatic because it can only receive either the M or L photopigment on its single X chromosome in addition to its S photopigment. However, the X chromosome gene locus is polymorphic for M and L alleles, rendering heterozygous platyrrhine females with trichromatic vision, and homozygous females with dichromatic vision.
Proximate Causation Hypotheses
Some evolutionary biologists believe that the L and M photopigments of New World and Old World primates had a common evolutionary origin; 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. There are two popular hypotheses that explain the evolution of the primate vision differences from this common origin.
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 requires that the evolution of the polymorphic system of the platyrrhine pre-dates the separation of the Old World and New World monkeys.
This hypothesis proposes that this crossing-over event occurred in a heterozygous catarrhine female sometime after the platyrrhine/catarrhine divergence. Following the crossing-over, any male and female progeny receiving at least one X chromosome with both M and L genes would be trichromats. Single M or L gene X chromosomes would subsequently be lost from the catarrhine gene pool, assuring routine trichromacy.
The alternate hypothesis is that opsin polymorphism arose in platyrrhines after they diverged from catarrhines. By this hypothesis, a single X-opsin allele was duplicated in catarrhines and catarrhine M and L opsins diverged later by mutations affecting one gene duplicate but not the other. Platyrrhine M and L opsins would have evolved by a parallel process, acting on the single opsin gene present to create multiple alleles. Geneticists use the "molecular clocks" technique to determine an evolutionary sequence of events. It deduces elapsed time from a number of minor differences in DNA sequences. 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%). Hence, the New World primate color vision alleles are likely to have arisen after Old World gene duplication. It is also proposed that the polymorphism in the opsin gene might have arisen independently through point mutation on one or more occasions, and that the spectral tuning similarities are due to convergent evolution. Despite the homogenization of genes in the New World monkeys, there has been a preservation of trichromacy in the heterozygous females suggesting that the critical amino acid that define these alleles have been maintained.
Ultimate Causation Hypotheses
This theory encompasses the idea that this trait became favorable in the increased ability to find ripe fruit against a mature leaf background. Research has found that the spectral separation between the L and the M cones is closely proportional to the optimal detection of fruit against foliage. 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, recent research has gone on to disprove this theory. Studies have suggested that the cone pigments found in dichromats can actually distinguish the color differences between fruit and the foliage surrounding it. Furthermore, the survival benefits favoring the selection for this trait are not completely necessary as many trichromatic primates live in environments where fruits have similar color tones in relation to the surrounding foliage and yet studies have shown that these primates obtain the same proportion of fruit compared to their dichromatic relatives. However, starting from an initial environment where all fruits were colored like their surrounding, the first mutations slightly shifting the duplicated opsin sensitivity toward red need only to have been selectively neutral for some of the generated new alleles to remain in the gene pool at low frequency. Then following an environmental change favoring ripe fruits slightly shifted toward red, the new alleles might have provided a survival advantage to their bearer by favoring discrimination of these fruits from their surrounding beyond the capability of completely dichromatic individuals. If these new environmental conditions remained during sufficient time, even a small net survival benefit in favor of the more trichromatic individuals would end up to fixation of these new alleles in the gene pool. Afterwhile a subset of the ancestral Catarrhine species population might have moved in an environment where fruits were still colored like their surrounding and the selective pressures on the duplicated opsin would have vanished, allowing the diversification of numerous new opsin alleles without them being wiped out by selective constraints. When these primates moved back to the initial environment, mixing back with their ancient relatives, they might have found the tree to propose ripe fruits even more shifted toward red. But then their enhanced opsin diversity would have favored their evolvability toward even better trichromatic vision compared to the weaker diversity that stucked their relatives in a suboptimal adaptative solution concerning colour vision.
Young Leaf Theory
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.
Long-Distance Foliage Hypothesis
This hypothesis suggests that trichromacy has evolved to adapt to distinguishing objects from the background foliage in long distance viewing. This hypothesis is based upon the fact that there is a larger variety of background S/(L+M) and luminance values under long-distance viewing.
Short-Distance Foliage Hypothesis
This hypothesis suggests that trichromacy has evolved to show higher sensitivity to low spatial frequencies. Spatiochromatic properties of the red-green system of color vision may be optimized for detecting any red objects against a background of leaves at relatively small viewing distances equal to that of a typical “grasping distance."
Evolution of olfactory systems
The sense of smell may have been a contributing factor in selection of color vision. Studies suggest that the loss of olfactory receptor genes may coincide with the evolved trait of full trichromatic vision. In other words, as sense of smell deteriorated, and thus the ability to identify the most vital nutritional sources, the necessity for advancement in other senses increased and the likelihood for trichromatic color vision mutations to remain selected for became higher. 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 shown that the concentration of olfactory receptors is directly related to color vision acquisition. Interestingly, 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.
Health of offspring
Trichromacy may also be evolutionarily favorable in offspring health (and therefore increasing fitness) through mate choice. M and L cone pigments maximize sensitivities for discriminating blood oxygen saturation through skin reflectance. Therefore, the formation of trichromatic color vision in certain primate species may have been beneficial in modulating health of others, thus increasing the likelihood for trichromatic color vision to dominate a specie’s phenotypes as the fitness of offspring increases with parental health.
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. For example, the night monkeys (Aotus) have lost their S photopigments and polymorphic M/L opsin gene. Because these anthropoids are and were nocturnal, operating most often in a world where color is less important, selection pressure on color vision relaxed. On the opposite side of the spectrum, diurnal howler monkeys (Alouatta) have reinvented routine trichromacy through a relatively recent gene duplication of the M/L gene. This duplication has allowed trichromacy for both sexes; its X chromosome gained two loci to house both the green allele and the red allele. The recurrence and spread of routine trichromacy in howler monkeys suggests that it provides them with an evolutionary advantage.
Howler monkeys are perhaps the most folivorous of the New World monkeys. Fruits make up a relatively small portion of their diet, 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.
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