Recent progress in understanding mammalian color vision

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1 Ophthal. Physiol. Opt : The Verriest Lecture 2009 Recent progress in understanding mammalian color vision Gerald H. Jacobs Department of Psychology and Neuroscience Research Institute, University of California, Santa Barbara, CA 93106, USA Abstract There have been significant advances in our understanding of mammalian color vision over the past 15 years. This paper reviews a number of topics that have been central to these recent efforts, including: (1) the extent and nature of ultraviolet vision in mammals, (2) the evolutionary loss of shortwavelength-sensitive cones in some mammals, (3) the possible roles of rod signals in mammalian color vision, (4) the evolution of mammalian color vision, and (5) recent laboratory investigations of animal color vision. Successes in linking opsin genes and photopigments to color vision have been key to the progress made on each of these issues. Keywords: evolution of color vision, mammalian color vision, opsin genes, photopigments, ultraviolet vision Introduction Results obtained from comparative studies of retinas, and inferences drawn from opsin gene phylogenies, show that at an early stage in vertebrate history, almost certainly by the time jawless and jawed vertebrates diverged (540 million years ago (mya)), our ancestors had already evolved four classes of cone photopigment and so possessed the photopigment basis for color vision (Collin et al., 2009). Shortly thereafter, colored oil droplets, another retinal appurtenance usually associated with complex color vision, also appeared (Robinson, 1994). Three or four classes of cone pigment are present in many contemporary representatives from four of the major vertebrate groups (fishes, birds, amphibians and reptiles) while colored oil droplets are found in numerous members of the latter three groups (Bowmaker, 1991, 2008): thus, to varying degrees, many vertebrates probably maintained a capacity for elaborate color vision over the long sweep of their histories. Received: 10 September 2009 Revised form: 18 December 2009 Accepted: 25 December 2009 Correspondence and reprint requests to: Gerald H. Jacobs. Tel.: ; Fax: address: jacobs@psych.ucsb.edu Mammals were traditionally believed to constitute significant exceptions to this picture. In his classic treatise on vertebrate eyes, Gordon Walls encapsulated that idea by noting that, although primates stand as a clear exception, ÔWithin the mammals color vision is by no means widespreadõ (Walls, 1942). Over the next 50 years this idea was echoed on numerous occasions by other writers. WallsÕ explanation for the elaborate color vision detected in most contemporary vertebrate groups, and the simplified nature (or, indeed, complete absence) of a color vision capacity in most mammals, was that it simply reflected the early history of mammals when they had undergone a long period of predominant nocturnality, and as a consequence had largely abandoned the machinery required to support many quintessential daylight visual capacities, including color vision. In an earlier review of the literature on mammalian color vision I concluded that, counter to the conclusions of Walls and his followers, the presence of color vision, at least as it is technically defined, is actually quite widespread among contemporary mammals and that cones, rather than having sometimes been lost and subsequently regained as Walls had surmised, were most likely carried forward over the unbroken sweep of mammalian history (Jacobs, 1993). The years since that review have witnessed considerable further progress towards understanding mammalian color vision. Here I comment on several topics having to do with doi: /j x

2 The Verriest Lecture: G. H. Jacobs 423 mammalian color vision that have come to the fore during this period. Ultraviolet vision in mammals It has long been known that many terrestrial arthropods have high sensitivity to ultraviolet (UV) light (Jacobs, 1992; Goldsmith, 1994). Detection of similar mechanisms in vertebrates is more recent, but even as these were eventually established in various birds, fishes, reptiles and amphibians, it was still supposed that mammals were insensitive to UV, a conclusion principally based on the mistaken idea that the lenses of all mammalian eyes have low transmissivity to shortwavelength light (Goldsmith, 1990). That changed with the discovery that the retinas of several common species of rodent (including mice, rats, gerbils and gophers) in fact contain a separate spectral mechanism with maximal sensitivity in the UV (Jacobs et al., 1991). This claim was contested (Soucy et al., 1998), but subsequent electrophysiological (Lyubarsky et al., 1999), spectrophotometric (Yokoyama et al., 1998), and behavioral measurements (Jacobs et al., 2001), have all verified the presence of cones containing UV pigment in rodent retinas. Although it is not the predominant arrangement among mammals, UV cones have subsequently been detected in a number of other rodent species (Peichl, 2005) as well as in several species of bat (Wang et al., 2004; Muller et al., 2009; Zhao et al., 2009) and some marsupials (Strachan et al., 2004; Arrese et al., 2005; Hunt et al., 2009b). There are almost certainly other mammalian species so far unstudied that also possess UV cones. A better understanding of the origin of UV cones in mammals has emerged from recent molecular genetic studies of photopigment opsin genes. All vertebrate cone pigments having maximum sensitivity (k max ) in the short wavelengths (360 nm to 440 nm) are specified by genes from one (SWS1) of the four cone-opsin gene families (Bowmaker, 2008). Cross-species comparisons of the residues implicated in the spectral tuning of the SWS1 cone opsins suggest that the ancestral mammalian SWS1 pigment was in fact a UV pigment (Hunt et al., 2001). In many mammalian lineages the occurrence of a small number of amino acid substitutions subsequently shifted the k max of the cone pigment from the UV to a variety of locations in the visible spectrum (Yokoyama, 2009). Evolutionary changes of this kind, though common, have not been universal; in particular, the mammalian species noted above have all retained the ancestral mammalian short-wavelength pigment. Why have some mammals retained their UV cones while others have not? Answering that question would be easier if we understood the relative values and costs of UV vision for mammals. In diurnal birds, where UV vision is common and much studied, it appears that UV sensitivity can be useful in both mate choice and foraging, among other activities, although it is unclear if avian UV vision actually evolved to subserve these purposes (Church et al., 2001; Hart and Hunt, 2007). Thus far there is no evidence that mammalian UV vision can be employed to achieve similar goals; indeed, there are some indications to the contrary: for instance, an explicit laboratory test of foraging in house mice found that these rodents were indifferent to the presence or absence of UV-linked cues (Honkavaara et al., 2008). An alternative possibility has come from the observation that the urine of some rodent species has high reflectivity to UV (Viitala et al., 1995), suggesting that in such animals scent marking with urine might utilize a UVsensitive communication channel (Chavez et al., 2003). Whether that is true or not remains to be seen, but it is noteworthy that high reflectivity is not characteristic of the urine of many mammals, even those known to have UV cones, e. g., the house mouse (Kellie et al., 2004). An additional point of concern is that most of the species so far known to have UV cones are nocturnal, thus being predominantly active during a phase of the illumination cycle when UV light is not naturally very abundant (Johnsen et al., 2004). Finally, it has been suggested that natural fluctuations in UV light probably play only a limited role in the entrainment of mammalian circadian systems (Hut et al., 2000). In summary, although laboratory tests of mammals with UV cones show clearly that they are capable of exploiting signals from these receptors to guide behavioral choices under photopic test conditions (Jacobs et al., 2003, 2004) we still have little idea of how this capacity may be employed naturally. Evolutionary loss of SWS1 cones As noted, the photopigments of all mammalian shortwavelength-sensitive cones are specified by opsin genes drawn from the SWS1 family. Some years ago studies involving both opsin immunolabelling (Wikler and Rakic, 1990), and behavioral and electrophysiological measurements (Jacobs et al., 1993; Deegan and Jacobs, 1996), failed to detect functional short-wavelengthsensitive cones in the retinas of two species of nocturnal primates the anthropoid Aotus (owl monkey) and the strepsirrhine Otolemur (bushbaby). A subsequent genetic examination revealed that the absence of S cones in these primates results from mutational changes in the S-cone opsin genes that render them incapable of expressing opsin protein, i. e., they had become pseudogenes (Jacobs et al., 1996b). Since these two primates are only distantly related, and since the structural nature of their SWS1 gene defects differed, it seemed likely that the conversions of the SWS1 genes to pseudogene status

3 424 Ophthal. Physiol. Opt : No. 5 must have occurred independently in the two lineages. Further, because both of these species are nocturnal it seemed plausible to assume there must be other nocturnal mammals in whom gene mutations had also rendered their S-cone pigments nonfunctional (Jacobs et al., 1996b). This latter prediction has been amply borne out. Scattered species from four orders of eutherian mammals (various rodents, primates, cetaceans, and carnivores) similarly lack functional S cones, and in cases where it has been examined, their absence can be traced to corresponding opsin gene defects (Jacobs, 2009). Photic activity classifications are imprecise, but all the mammals so far found to lack S cones are principally nocturnal, as were the primates in whom S-opsin pseudogenes were first detected. Supporting the possibility of a causal link between pigment loss and photic activity cycle, is the observation that while most of the carnivore procyonids are nocturnal, and of these both Procyon (the raccoons) and Potos (kinkajous) lack short-wavelength cones, a closely-related procyonid (Nasua, the coati) is diurnal and retains functional S cones (Jacobs and Deegan, 1992). Although it seems that nocturnality sets the stage for SWS1 cone opsin genes to become pseudogenes, that feature cannot be the sole issue since many nocturnal mammals retain a full complement of functional short-wavelength cones, e.g., rats and mice. There are also the extreme examples offered by some subterranean mammals, animals that lead lives almost completely devoid of light exposure yet still retain fully functional shortwavelength sensitive cones (Peichl et al., 2004; Williams et al., 2005). Finally, if a nocturnal lifestyle promotes the pseudogenization of SWS1 opsin genes it is curious why this did not happen in widespread fashion during the long period in their early history when mammals were principally nocturnal. In recent years pseudogenes associated with receptor operation have been discovered in other sensory systems; for example, within families of olfactory (Gilad et al., 2004), pheromone (Zhang and Webb, 2003), and gustatory (Go et al., 2005) receptor genes. A common suggestion is that the transition of genes to pseudogenes occurs when the function(s) they support become dispensable and, that being the case, this process and its dependence on details of the interaction of organisms with their environments may be particularly easy to observe in sensory systems (Go et al., 2005) One possible example so cited is the large increase in the proportion of anthropoid olfactory receptor genes that are pseudogenes, relative to what is found in rodents. That difference is attributed to the lessened importance of a keen sense of smell among the primates, perhaps occurring in exchange for their increased dependence on vision (Gilad et al., 2004). Can similar functional explanations account for the presence of mammalian S-opsin pseudogenes? In most mammalian retinas SWS1-specified cones are infrequent relative to the numbers of LWS-specified cones, typically making up no more than 10% of the total cone complement. Because of their relative sparsity, as well as limitations imposed by the optics of mammalian eyes, S cones make little contribution to total photon capture and they support significantly lower spatial and temporal resolution than do the more abundant long wavelength cones (Calkins, 2001). Rather, the principal role subserved by mammalian S cones is to generate a signal that can be contrasted to that derived from stimulation of longer wavelength cones, with the combination thus providing the basis for a dimension of color vision. Since most mammals have only a single type of LWS cone, the loss of viable S cones eliminates the possibility of any cone-based color vision, and that is just what has happened in those species in which the SWS1 genes have become pseudogenes. What values and costs might be associated with abandoning a dimension of color vision? If animals are nocturnal, as at least most of these species seem to be, then they would normally be behaviorally active when ambient light levels are insufficient to support conebased vision and thus color vision in such animals would seem at first glance to offer minimal advantage. On the other hand, many contemporary mammals classified as nocturnal are also active at dawn and dusk (Macdonald, 2001), times when illumination conditions could well support some role for cone vision. In addition, even the most resolutely nocturnal species occasionally awaken and become active during daylight hours in order to initiate behaviors for which color vision might prove useful; for instance, to escape predation, to respond to weather contingencies, or to initiate foraging driven by the stress of food scarcity (Bearder et al., 2006). If there seems to be at least some potential value in retaining color vision in a nominally nocturnal species, then perhaps one should look instead to the debit side of maintaining color vision. Energy efficiency has been shown to act as a strong selective force in brain evolution (Niven and Laughlin, 2008) and conceivably that issue is at play here. Although there seems no way as yet of evaluating the possibility directly, the shortwavelength sensitive cones required to support a dimension of color vision are few in number, which would seem to minimize their metabolic expense. In summary, if there are any general adaptive reasons associated with the inactivation of mammalian SWS1 cone opsin genes, they are not yet apparent. Among mammals so far studied, SWS opsin pseudogenes seem sometimes to have emerged near the evolutionary base of the lineage and in other cases only in the distal branches of the family. Among the latter

4 The Verriest Lecture: G. H. Jacobs 425 examples would be the procyonids, described above, where fairly closely-related genera can have either functional or non-functional S cone pigments. Perhaps most striking among the former are the marine mammals. A genetic survey of the SWS1 opsin genes in 16 species of cetaceans identified mutational changes in all of these species that should obviate the production of functional S-cone pigment (Levenson and Dizon, 2003). In fact, in one cetacean sub-order (the odontocetes) all the species share in common a mis-sense mutation in their S-cone opsin genes implying that pseudogenes must have been present prior to the time these animals began to diverge in the Oligocene (25 38 mya). Further, in support of earlier evidence derived from opsin immunolabelling (Peichl and Moutairou, 1998), a genetic survey found that all the pinnipeds (seals, sea lions, walrus) also have a gene-linked loss of S-cone function (Levenson et al., 2006). The complete absence of SWS1 cones in both of these two distinct mammalian orders raises the possibility that such a loss may have yielded some adaptive advantages. What those might be is unclear, although some suggestions have been offered (Peichl et al., 2001). Particularly puzzling in this regard is that present day cetaceans and pinnipeds occupy distinctively different photic environments: the former strictly aquatic, often active in environments where photons are a scarce commodity; whereas pinnipeds are amphibious inhabiting both aquatic and terrestrial habitats, the latter often characterized by high photopic light loads. Observations made on the owl monkey (Aotus) may argue against expecting any simple relationships between photic environments and S-cone absence. Aotus is a nocturnal monkey, but is believed to have evolved from diurnal ancestors some mya (Setoguchi and Rosenberger, 1987). Several contemporary species of Aotus share in common a mis-sense mutation which renders their S-cone opsin gene nonfunctional and this implies that the pseudogene appeared early in the history of the genus, perhaps not long after the transition to nocturnality (Levenson et al., 2007). Although most of the animals comprising modern Aotus have remained stringently nocturnal, one species, A. azarae, is cathemeral, i.e., it is frequently behaviorally active during daylight hours as well as at night (Fernandez-Duque, 2003). Despite the absence of functional S cones, and thus any possibility of a conventional color vision capacity, this monkey forages quite successfully on colored fruits and tree flowers under lighting conditions where its vision must be based on signals from only a single type of cone pigment. If nothing else, this example underlines the fact that we are only at the beginning of understanding the extent and practical implications of the gene-driven losses of S-cone that characterizes some mammals. Rods and mammalian color vision Because rods and cones overlap in their operating ranges (in human vision by some 4 log units of intensity), and because the signals from these receptor types share neural pathways into the central visual system, it has long been apparent that rod signals can potentially influence cone-based vision. Among those demonstrated influences are cases where rod signals cause complex alterations in color appearance (Volbrecht et al., 1995; Buck, 2004) and cases involving viewing conditions (mesopic light levels, large test fields) where rod signals can be contrasted to signals derived from a single class of cones to yield novel color vision (Smith and Pokorny, 1977). The following examples illustrate that similar influences from rod signals on color vision also operate in non-human mammals. An early behavioral experiment conducted on a strepsirrhine primate, the ring-tailed lemur (Lemur catta), included tests of spectral sensitivity and color discrimination (Blakeslee and Jacobs, 1985). The latter provided evidence for the presence of some (relatively feeble) color discrimination in the red-green portion of the spectrum; specifically, these animals were able to make unique dichromatic color matches (540 nm nm = 570 nm) with the match proportions significantly displaced in the protan direction relative to those made by normal human trichromats. Since subsequent results derived from both electrophysiological measurements (Jacobs and Deegan, 1993, 2003), and from an analysis of cone opsin genes (Tan and Li, 1999), show that this species expresses only a single cone photopigment active in the middle to long-wavelength portion of the spectrum (with k max of 545 nm), the color discriminations found in the earlier study must perforce have derived from the ability of these animals to jointly utilize rod and cone signals. That case is not unique; for example, genetic examination reveals that the pinniped California sea lion (Zalophus californaus) has only a single cone type (Levenson et al., 2006) yet it too seems capable of making color discriminations that would be technically impossible without the exploitation of rod signals (Griebel and Schmid, 1992). One important point to be derived from these examples is that deductions about color vision based solely on knowledge of the cone complement, as for instance is commonly done following examination of cone opsin genes, will miss possible influences from rod contributions. Such influences may be particularly relevant for those many mammals that have heavily rod-dominated retinas because, as noted above, such animals often display photic rhythms that render them behaviorally active under illumination conditions favorable for supporting joint rod and cone contributions.

5 426 Ophthal. Physiol. Opt : No. 5 Among the most noteworthy features of mammalian retinas are the large species variations in rod/cone ratios and in the pattern of distribution of cones in the retinal mosaic (Ahnelt and Kolb, 2000). These variations will significantly impact the thresholds and dynamic ranges for rod and cone vision and they will influence the limits of color discrimination. Both of these facts were earlier taken to suggest that the relative rod/cone mix and their spatial distributions could be targets for selection in the evolution of color vision (Jacobs, 1993). That idea begins to seem more plausible in the face of recent research that compared structural features of the visual cortex and the retina in a variety of nocturnal and diurnal mammals and showed that, indeed, relative rod and cone complements are very sensitive to niche-specific selection pressures and that plasticity stands in striking contrast to the much greater conservatism of the size of central visual structures (Kaskan et al., 2005). The relative numbers of retinal cell types can be readily altered through nothing more elaborate than changes in the schedule of retinal neurogenesis, and such schedule alterations could thus provide a proximate mechanism through which selection might impact the relative influences of rods on mammalian color vision (Finlay, 2008). Evolution of color vision in mammals At the time of the 1993 review, a renewed interest in the evolution of color vision was just beginning to manifest itself. Triggered by a substantial accrual of information about opsin genes, as well as by new examinations of the ecology of color vision, much more has now been learned about this. A number of reviews dealing with various aspects of this topic have appeared in recent years (for the most recent of these see Osorio and Vorobyev, 2008; Collin et al., 2009; Hunt et al., 2009a; Jacobs, 2009; Yokoyama, 2009) so I provide here only a brief summary of the relevant findings. The schematic of Figure 2 suggests the evolutionary fate of these four cone opsin gene families in mammals. As for amphibians, cone opsin genes from the Rh2 family are not found in any contemporary mammal suggesting it was lost prior to the onset of mammalian divergence. Contemporary monotremes (platypus and echidna) have photopigments from the SWS2 and LWS families and their genomes also contain a pseudogene from the SWS1 family (Davies et al., 2007). Genes drawn from the SWS2 family are not present in either marsupial (Strachan et al., 2004; Cowing et al., 2008) or eutherian mammals and must, therefore, also have been lost prior to the divergence of these two lineages (Figure 2). Both of these lines retain viable representatives from the SWS1 and LWS families. These various gene losses are usually suggested to have occurred during the long period of early mammalian nocturnality, but exactly how that exposure may have fostered such a loss is not known. Whatever the reason, the outcome has been to limit most animals in these groups to only two types of cone pigment although, as noted below, significant exceptions to this rule occur among primates. Sequence comparisons of cone opsin genes from contemporary eutherian mammals suggest that the SWS1 and LWS gene families provided ancestral eutherian mammals with cone pigments having k max values of 360 nm and 560 nm (Hunt et al., 2001; Yokoyama Evolution of opsin genes Phylogenetic analysis shows that all vertebrate photopigments are specified by opsin genes belonging to five families four for the cone opsins, the other for rod opsins (Yokoyama, 2000). Each of these gene families produce opsins structured to yield photopigments that cover the range of spectral peaks indicated in Figure 1 (top). As a result of prior gene duplications, these cone opsin gene families are believed to have already emerged at a point early in vertebrate history. Pigments drawn from each of the four cone opsin gene families are found in various present-day birds, fishes, and reptiles. Representation from only three of these families (Rh2 is missing) has so far been detected among contemporary amphibians (Bowmaker, 2008). Figure 1. Spectral range of vertebrate photopigments. All vertebrate photopigment opsins are specified by members of the five opsin gene families listed at the top. When combined with an 11-cis-retinal chromophore, variations in the gene sequences yield photopigments whose k max values cover the spectral ranges indicated by the horizontal lines. Cone photopigments in eutherian mammals come exclusively from the SWS1 and LWS families. The two ancestral cone pigments found in these animals are believed to have had the spectral absorption curves sketched at the bottom. (Modified from Jacobs, 2009).

6 The Verriest Lecture: G. H. Jacobs 427 Figure 2. Suggested fate of the four cone opsin gene families during mammalian evolution. The range of photopigment absorption properties of pigments derived from the four families is shown in Figure 1. All four gene families are believed to have arisen early in vertebrate evolution. The Rh2 gene family is not present in any contemporary mammals and so is presumed to have been lost during the early evolution of mammals. The distribution of the extant gene families among the three groups of contemporary mammals is given at the top; SWS1 is a pseudogene in present-day monotremes. Representation of the SWS2 gene family was lost prior to the divergence of marsupial and eutherian mammals. (Modified from Jacobs, 2009). et al., 2008). These cone pigments (bottom of Figure 1) represent the shortest and longest spectral positions that can be generated from cone opsins linked to a retinal-1 chromophore and would have provided the photopigment potential for dichromatic color vision. Spectral positioning of mammalian cone pigments Since all mammalian photopigments are constructed from the same chromophore, retinal-1, variations in their spectral absorption properties must be due to opsin variations. Molecular genetic studies show that variations at a limited number of positions in the opsin molecule are largely responsible for all the variations in the spectral positioning of photopigments. In the case of mammalian LWS pigments, for example, dimorphic variations at only five amino acid sites cause variations in pigment spectral positioning, with combinations of changes occurring at these critical sites allowing for the production of pigments occupying quite a number of possible spectral positions (Yokoyama and Radlwimmer, 2001). Mutagenesis studies show that four amino acid positions can potentially influence the spectral tuning of the primate LWS pigments (Asenjo et al., 1994), with only three of these accounting pretty well for all the naturally observed variations (Neitz et al., 1991; Carroll and Jacobs, 2008). A similarly small number of amino acid substitutions are linked to variations in the mammalian photopigments specified by the SWS1 genes (Hunt et al., 2004). Although interactions between environmental signals and sensory capacities impacting evolution can be complex (Endler, 1992), one common assumption is that the spectral positioning and number of cone pigment types that evolve reflect those best adapted to support the visual tasks requisite for survival (Lythgoe and Partridge, 1989). In contemporary mammals pigments from the LWS family span a range of spectral positions having k max values from 500 to 560 nm. If, as believed (above), the spectral location of the ancestral LWS pigment was close to the latter location, there must have been numerous shifts in the spectral position of this pigment toward the shorter wavelengths. Most eutherian mammals also have an SWS1 cone pigment. Through an analysis of a collection of natural images viewed in conjunction with a popular model of color discrimination Chiao et al. (2000) examined how photopigment spectral positioning might influence color discrimination. For pigment combinations involving short-wavelength pigments with k max >400 nm, variations in the positioning of the LWS pigment from its longest to its shortest position had only very modest effects on predicted discriminability. From their computations these authors additionally inferred that color discrimination in such dichromats could be maximized by increasing the spectral separation between the two pigments, irrespective of the nature of the visual environment. Modeling analyses such as this one thus provide no obvious explanation for the significant variations in the position of the LWS pigment across these dichromatic mammals. One possibility is that in such cases the spectral tuning of the LWS pigment has been more impacted by the demands of those capacities supported by achromatic vision (Chiao et al., 2000; Osorio and Vorobyev, 2005). Another is that, within some fairly broad limits, pigment positioning is not critically important for supporting visual needs; that rather the observed variations seen among mammals better reflect events that occurred in the earlier history of the various animal groups, than it does in matching current visual demands. This latter scenario merits attention because it at least seems to provide the best explanation for variations in photopigment positioning in many insects (Briscoe and Chittka, 2001). The primate story Primates have long been known to constitute a special case, but it is only in recent years that a fuller appreciation of the diversity of primate color vision

7 428 Ophthal. Physiol. Opt : No. 5 emerged, and along with it a more detailed understanding of its evolution. In large part these changes were fostered by studies of the cone opsin genes and cone photopigments in many different primates. Several recent reviews may be consulted for access to what is now an extensive literature on this topic (Regan et al., 2001; Osorio et al., 2004; Jacobs, 2007, 2008). Although the idea is not without its critics (e. g., Tan et al., 2005), it is usually believed that the earliest primates were nocturnal, and thus like most eutherian mammals probably had two types of cone pigment drawn, respectively, from the SWS1 and LWS opsin gene families. In mammals the LWS cone opsin genes are located on the X-chromosome, but, unlike other mammals, catarrhine primates (Old World monkeys, apes and humans) have two different LWS genes that specify cone photopigments with peaks at about 530 nm and 560 nm (commonly called M and L respectively). Since these two are effectively conserved across all the catarrhines they apparently emerged as a consequence of a gene duplication that occurred close to the base of the catarrhine radiation, some mya (Nathans et al., 1986). In conjunction with the pigment product of an autosomal SWS1 gene, all of the species of this group express three classes of cone photopigment and have trichromatic color vision. Thus catarrhine primates, alone among eutherian mammals, have been able to add a second version of an LWS gene and exploit its pigment product to acquire a new dimension of color vision. The other large group of anthropoid primates, the New World platyrrhine monkeys, has highly diverse color vision and, as a consequence, has been much studied in recent years (Jacobs, 2007). With only two apparent exceptions, this entire group features X-chromosome opsin gene polymorphisms, the most common arrangement featuring three alternate forms of the LWS gene with each allele specifying a photopigment with k max in the nm range. As a consequence of early X-chromosome inactivation, heterozygous females express two types of M/L pigment and derive trichromatic color vision while homozygous females and all males have a single M/L pigment and are dichromatic. This arrangement yields a total of six distinct color vision phenotypes. The two exceptions are Aotus, which has only a single LWS pigment and thus lacks conventional color vision (above), and the howler monkey Alouatta which resembles the catarrhine norm in having two populations of M/L cone pigments (Jacobs et al., 1996a) and being uniformly trichromatic (Araujo et al., 2008). Evidence suggests that the addition of a second X-chromosome opsin gene in the howler monkey occurred independently from the gene addition that occurred in the catarrhine primates; in the case of howler monkeys probably emerging against a background of earlier polymorphisms similar to that seen in most of the other contemporary platyrrhines (Kainz et al., 1998; Dulai et al., 1999). The third group of primates, the strepsirrhines, is usually described as more primitive. Animals of this group feature afoveate, more rod-dominated, retinas and their eyes often contain a tapetum. To date these primates have been less well studied, but they too show significant variations in their cone photopigment complements. Three principal variants have been identified. Two of these have been described above some are like the bushbaby (Otolemur) in having only a single type of cone pigment and thus lacking color vision; while others resemble the ring-tailed lemur (Lemur catta), and many other mammals, in having two types of cone pigment and dichromatic color vision (Kawamura and Kubotera, 2004). In a third variant, some species from this group have polymorphic X-chromosome opsin genes and thus, similar to the platyrrhines, have the photopigment basis to support a mixture of dichromatic and trichromatic phenotypes (Tan and Li, 1999; Jacobs et al., 2002; Velleux and Bolnick, 2009). An understanding of the evolution of opsin genes and color vision in this group of primates remains very much a goal for future studies. The production of color vision requires, as a minimum, multiple types of receptor containing different photopigments and a nervous system capable of contrasting the pattern of photon absorption in the different types of photoreceptor. Two such neural arrangements are generally believed to characterize mammalian retinas (Lee, 2004; Wa ssle, 2004). One involves a dedicated class of bipolar cells (the S-cone bipolars) that selectively contact short-wavelength cones. Signals from these cones are fed via S-cone bipolar cells to a class of small bi-stratified ganglion cells that also receive antagonistic inputs from a group of bipolar cells that contact M/L cones. The combination of these inputs provides the basis for a spectrally-opponent pathway that can support a dimension of color vision. Although the comparative evidence is still somewhat scanty, it seems likely that this neural pathway is characteristic of the retinas of all eutherian mammals and thus has been conserved throughout the history of this group. The other circuit for extracting color information is unique to primate retinas. It originates from the M or L cone inputs to midget bipolar cells which in turn synapse on midget ganglion cells where that signal is combined in opponent fashion with signals originating from neighboring M or L cones. These form the substrate for the second spectrally-opponent channel, setting the stage for an additional dimension of color vision (Martin, 1998). The midget cell pathway has been identified as being present in retinas of a number of disparate primate species, even those lacking trichromatic color vision, and so is believed to have appeared early in primate evolution (Silveira et al., 2005). There remains lively debate as to

8 The Verriest Lecture: G. H. Jacobs 429 the nature of spatial arrangements of L and M cone signals to this second pathway (Solomon and Lennie, 2007) and of the function(s) that this pathway may have subserved in primates prior to the points at which a second type of M/L cone appeared (Mollon, 1989; Lee, 2004), Recent years have seen a marked increase in the number of studies asking how well suited the various forms of primate color vision are for various lifesupporting visual behaviors and, by extension, perhaps thereby shedding some light on the circumstances that led to the evolution of the mechanisms underlying color vision. Such investigations typically start with detailed measurements of natural spectral environments and then use one or other of the computational models of visual processing to predict discriminative performance. These exercises show consistently that the discrimination capacities inherent in primate trichromacy are well suited to support the demands of foraging, whether the targets are edible fruits or foliage viewed in their natural surrounds (Osorio and Vorobyev, 1996; Sumner and Mollon, 2000; Dominy and Lucas, 2001; Regan et al., 2001; Parraga et al., 2002). With their dramatic individual variations in color vision platyrrhine monkeys provide a rich resource for examining the linkages between color vision capacity and behavior. Since there is evidence that the M/L cone pigments of the platyrrhines have been under selection for a considerable period of time (Surridge et al., 2003), one might confidently expect to find among these monkeys individual differences in behavior that correlate with individual differences in color vision. Experiments conducted in laboratory settings have in fact detected some differences in foraging efficiency for monkeys of different phenotypes (Caine and Mundy, 2000; Smith et al., 2003); however, studies of several different platyrrhine species in their natural habitats have so far proven singularly unsuccessful in detecting individual variations in behavior that can be compellingly traced to individual variations in color vision (Dominy et al., 2003; Smith et al., 2003; Vogel et al., 2007; Hiramatsu et al., 2008; Bunce, 2009). Why this Table 1. Recent laboratory investigations of mammalian color vision Order Exemplars (Genus, common name) Goal of study* Reference Marsupalia Macropus (wallaby) Dichromacy Hemmi, 1999 Sminthopsis (dunnart) Trichromacy Arrese et al., 2006 Rodentia Mus (mouse) Dichromacy Jacobs et al., 2004 Rattus (rat) Dichromacy Jacobs et al., 2001 Cavia (guinea pig) Dichromacy Jacobs and Deegan,1994b Meriones (gerbil) Dichromacy Jacobs and Deegan, 1994a Spermophilus (ground squirrel) Color thresholds van Arsdel and Loop, 2004 Primate Alouatta (howler monkey) Trichromacy Araujo et al., 2008 Callithrix (marmoset) Distinctiveness of color Derrington et al., 2002 Callithrix (marmoset) Polymorphism Pessoa et al., 2005a Cebus (capuchin monkey) Stimulus size and color vision Gomes et al., 2005 Eulemur (black lemur) Presence of color vision Gosset and Roeder, 2000 Leontopithecus (golden lion) Polymorphism Pessoa et al., 2005b Pan (chimpanzee) Color classification Matsuno et al., 2004 Papio (baboon) Color categorization Fagot et al., 2006 Saguinus (tamarin) Polymorphism Pessoa et al., 2003 Scandentia Tupaia (tree shrew) Color thresholds van Arsdel and Loop, 2004 Cetacea Tursiops (dolphin) Rod contributions to color Griebel and Schmid, 2002 Artiodactyla Bos (cow) Dichromacy Phillips and Lomas, 2001 Dama (fallow deer) Presence of color vision Birgersson et al., 2001 Perissodactyla Equus (horse) Dichromacy Pick et al., 1994 Dichromacy Macuda and Timney, 1999 Dichromacy Smith and Goldman, 1999 Dichromacy Geisbauer et al., 2004 Dichromacy Hanggi et al., 2007 Dichromacy Ahmadinejad et al., 2008 Color Thresholds Roth et al., 2008 Carnivora Felis (cat) Presence of color vision Tritsch, 1993 Color thresholds Tritsch, 1995 Sirenia Trichechus (manatee) Dichromacy Griebel and Schmid, 1996 *The meanings of the comments are explained in the text.

9 430 Ophthal. Physiol. Opt : No. 5 should be so is puzzling and remains under active investigation. Laboratory investigations of mammalian color vision A large majority of the publications on the topic of mammalian color vision produced over the past 15 years, including most of those referenced above, deal not with color vision but with various biological mechanisms linked to that capacity. There are probably at least two factors that have contributed to this imbalance. For one thing, behavioral studies of color vision in non-human species are especially challenging and time consuming relative to studies of mechanisms, often taking months, even years, to complete. A second impediment is that in current times funding agencies have show only modest inclination to support such ventures. Despite these challenges, there have nevertheless been a number of investigations that posed direct questions about color vision in various mammals. Reports from such studies that have come to my attention are listed in Table 1. Space does not permit extended discussion of these investigations of mammalian color vision. Instead, a summary comment is offered for each in Table 1. A number of these studies sought to establish the dimensionality of color vision in some target species. These (indicated as ÔDichromacyÕ or ÔTrichromacyÕ depending on the results claimed) involved tests using either spectral lights or calibrated colored papers as test stimuli. The trichromatic color vision found in the marsupial Sminthopsis (the dunnart) is particularly noteworthy because that species expresses only two different cone opsins, the third pigment required to support its trichromacy being, possibly, a rod pigment expressed in a cone (Cowing et al., 2008). Similar kinds of color vision tests were conducted on several species of platyrrhine monkeys and these had the general goal of documenting individual differences in color vision for correlation with L/M cone photopigment variations (ÔPolymorphismÕ in Table 1). Other experimenters either sought to establish the presence of color vision or to examine a more complex feature of color perception (color categorization, color classification or the distinctiveness of color). Finally, thresholds for color vision were determined for representatives of four different taxa. The horse was the hands-down winner as the most popular subject during this recent period, attracting attention from seven different groups of investigators. Based on earlier measurements of the cone pigments in this species (Carroll et al., 2001) these animals were predicted to have dichromatic color vision and the experiments listed in Table 1 all compellingly establish that fact, while also demonstrating the close linkage that exists between cone pigments and color vision in this species. Conclusion Recent progress toward gaining a more complete picture of mammalian color vision can be largely attributed to technical advances in molecular genetics, cell biology, and electrophysiology, each of which has significantly expanded our understanding of the current picture of the distribution of cone pigments across extant mammals, of what these pigments predict about color vision, and of how these arrangements may have evolved. 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