Genetics, Development and Evolution of Adaptive Pigmentation in Vertebrates
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Heredity (2006) 97, 222–234 & 2006 Nature Publishing Group All rights reserved 0018-067X/06 $30.00 www.nature.com/hdy SHORT REVIEW Genetics, development and evolution of adaptive pigmentation in vertebrates HE Hoekstra Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093-0116, USA The study of pigmentation has played an important role in biology. I then discuss recent empirically based studies in the intersection of evolution, genetics, and developmental vertebrates, which rely on these historical foundations to biology. Pigmentation’s utility as a visible phenotypic marker make connections between genotype and phenotype for has resulted in over 100 years of intense study of coat color ecologically important pigmentation traits. These studies mutations in laboratory mice, thereby creating an impressive provide insight into the evolutionary process by uncovering list of candidate genes and an understanding of the the genetic basis of adaptive traits and addressing such long- developmental mechanisms responsible for the phenotypic standing questions in evolutionary biology as (1) are adaptive effects. Variation in color and pigment patterning has also changes predominantly caused by mutations in regulatory served as the focus of many classic studies of naturally regions or coding regions? (2) is adaptation driven by the occurring phenotypic variation in a wide variety of verte- fixation of dominant mutations? and (3) to what extent are brates, providing some of the most compelling cases for parallel phenotypic changes caused by similar genetic parallel and convergent evolution. Thus, the pigmentation changes? It is clear that coloration has much to teach us model system holds much promise for understanding the about the molecular basis of organismal diversity, adaptation nature of adaptation by linking genetic changes to variation in and the evolutionary process. fitness-related traits. Here, I first discuss the historical role of Heredity (2006) 97, 222–234. doi:10.1038/sj.hdy.6800861; pigmentation in genetics, development and evolutionary published online 5 July 2006 Keywords: adaptation; Agouti; candidate gene; color; gene regulation; Mc1r; Peromyscus Introduction variety of biological processes (Silvers, 1979), leading to a wealth of information about genes involved in pigmen- Understanding the generation and maintenance of tation and their developmental interactions. Because phenotypic diversity requires the integration of genetics, melanin-based pigmentation biology is highly conserved development and evolutionary biology in an ecological across vertebrates, a deep understanding of mouse coat context. Historically, biologists have used two parallel color genetics translates easily and directly into testable approaches to study evolutionary change, one working hypotheses for studying the molecular basis of pigment- at the level of genotype and a second working at the level ation variation in natural vertebrate populations (Bennett of phenotype (Lewontin, 1974). For example, population and Lamoreux, 2003). Most important, color quality and/ geneticists have focused on temporal and spatial changes or color patterns frequently exhibit dramatic variation in allele and genotype frequencies, whereas organismal both within and between species in a way that can be biologists have studied how individuals differ in quantified (Endler, 1990) and is conspicuously affected phenotypic traits across natural environments. However, by natural selection (Caro, 2005). In particular, selective research linking genotype and phenotype (ie, identifying forces such as crypsis, aposematism, thermoregulation, the molecular changes responsible for phenotypic and sexual signaling drive variation in both pigmenta- adaptation and the developmental mechanisms by tion and color pattern (Thayer, 1909; Cott, 1940). Thus, which genotypes encode phenotypic traits (eg, Carroll pigmentation phenotypes in natural populations present et al, 2001; Brakefield et al, 2003)) is necessary to truly an ideal opportunity for studying the genetic basis of understand the processes responsible for generating both phenotypic diversity and evolutionary change. genetic and organismal diversity. Here, I first discuss how the rich history of pigmenta- The pigmentation system is a particularly promising tion biology in the fields of genetics, development and phenotype in which to explore connections between evolution provide the essential background information genotype and phenotype for ecologically important to address fundamental questions in evolutionary traits. Coat color mutations in laboratory mice have developmental biology. Then I provide empirical exam- served as a premier model for studying gene action in a ples in which the link between genotype and phenotype has been successfully made for adaptive pigmentation in natural populations. Throughout, I focus primarily on Correspondence: HE Hoekstra, Division of Biological Sciences, 9500 studies of mice and melanin-based pigmentation, from Gilman Drive, MC-0116, University of California, San Diego, La Jolla, CA 92093-0116, USA. E-mail: [email protected] which the most data are available, but draw on studies of Received 30 January 2006; accepted 1 June 2006; published online other vertebrates and pigment types whenever possible. 5 July 2006 Together, these studies provide exciting insight into how Evolution and development of vertebrate pigmentation HE Hoekstra 223 adaptation proceeds at the molecular level and also shed demonstrate epistasis and pleiotropy using coat color light on the evolutionary process in general. variation in hooded rats – animals that were mostly white except for a pigmented area on the head and neck. History of pigmentation biology In these experiments, the size of the pigmented area or ‘hood’ was selected to be small in some lines and large in The study of pigmentation has played a critical role in others. Castle initially thought these size differences the fields of genetics, development and evolution. reflected different alleles of the major gene responsible Beginning in 18th century China and Japan, so-called for hooding; however, Wright showed that so-called mouse fanciers collected, maintained and bred together modifier genes were responsible for variation in hood unusual morphs of wild mice (Morse, 1978); in doing so, size, providing the first experimental demonstration of these novice geneticists generated mice with a large epistasis (Little, 1917). Wright’s work with pigmentation diversity of color variation, much of which is represented in guinea pigs also revealed the importance of inter- today in laboratory mice. With the development of actions within and among gene systems. Wright diverse mouse strains, pigmentation phenotypes were demonstrated that apparently unrelated or pleiotropic readily available for study, and much of our knowledge phenotypes can have a single underlying developmental of the pigmentation process has subsequently come from mechanism, suggesting how adaptive change in one trait studies of these laboratory mice. Correspondingly, might easily dictate nonadaptive change in other traits. observations of coat color variation, first in the laboratory These classic experiments using coat color phenotypes and later in the field, have played an essential role clearly influenced the fields of quantitative and popula- in the understanding of many fundamental biological tion genetics. processes (Table 1). In the 1950s and 1960s, mouse coat color genetics became increasingly used as a test system for studying Genetics: the pigmentation loci induced and spontaneous mutation. Arguably, the ear- liest and most comprehensive estimates of mammalian In the early 1900s as Mendel’s principles were redis- mutation rates relied directly on visible phenotypes, covered, coat color differences were used to test several including a number of coat color loci (eg, agouti (a), brown fundamental theories of genetics, and 50 years later, (b), albino (c), dilute (d), leaden (ln), pink-eyed dilution (p) mutations of the same genes provided the first estimates and piebald-spotting (s)). First, a student of Wright, of mammalian mutation rates. As early as 1903 (before William Russell created a T-stock mouse packed with the term ‘genetics’ was even coined), independent seven recessive, viable, radiation-induced mutations, six experiments by Lucien Cue´not (1904) and William Castle of which were coat color mutations (Russell, 1951). By and Allen (1903) first demonstrated Mendelian inheri- examining large numbers of progeny (485 000) from tance in mammals by documenting segregation patterns crosses of T-stock mice to mutagenized animals, Russell of albino phenotypes in genetic crosses. Then, in 1915, was able to estimate the rate of heritable gene mutations, Haldane et al (1915) published the first genetic linkage an approach later termed ‘the specific locus test’. These study in vertebrates, establishing linkage between the experiments revealed a 10 times higher mutation rate in pink-eyed dilution locus and the albino locus in the mouse. mammals relative to earlier estimates in Drosophila as Parallel studies of pigmentation genetics in other well as a high degree of variation in mutability among vertebrate taxa provided additional early examples of loci (reviewed in Davis and Justice, 1998). This work was genetic linkage, perhaps most notably in guppies (Winge, followed by a series of studies of spontaneous mutation 1927). rates in the 1960s, when large production colonies were