Colorful pigmentation in animals is important in social signaling, mating behavior, warning coloration and mimicry, but little is known about its genetic basis and evolution. This study examines the evolution of genes and gene expression involved in the coloration of Anolis lizards. These lizards are an ideal group in which to study the evolution of color because there are nearly 400 species and they show a variety of bright colors, particularly in the reds and oranges. Many species of Anolis have evolved similar colorful pigmentation although they are not close relatives; this means that similar patterns have evolved independently multiple times. This study will identify genes and genetic pathways associated with colorful pigmentation in nine species of Anolis, with an emphasis on genes involved in the production of red and orange pigments which are important components of Anolis color patterns. Because these red and orange pigments are major components of coloration in a wide range of animals, from arthropods to vertebrates, this study will have far-reaching impact on understanding the evolution of morphological diversity.
As part of this research undergraduates will be trained in a variety of field and laboratory research methods. Because color variation provides a striking example of evolution, the results of this study could influence public understanding of evolution. The results of this study will be featured in exhibits on evolutionary biology at the Harvard Museum of Natural History, thereby reaching a broad audience of educators, students, and the general public.
Evolution often produces similar forms in response to similar forces of natural selection. A classic example is the convergent evolution of succulent plants by members of two disparate families of plants, the Cactaceae in the New World and the Euphorbiaceae in Africa. Both have evolved small, waxy leaves (or lost their leaves altogether), have photosynthetic stems, and water storage tissues in the stems and roots, all in response to similar pressures of natural selection in their desert environments. Convergent evolution has long fascinated biologists but only recently have we begun to examine the genetic basis of convergence. The simplest question is this, "Does convergent evolution of the phenotype reflect convergent evolution of the genotype, or can convergent phenotypes be produced by different genetic mechanisms?" With support from Doctoral Dissertation Improvement Grant # 1011544, we examined the convergent evolution of colorful pigmentation in the colorful, extensible dewlaps of anoline lizards. Lizards in the genus Anolis have undergone extensive adaptive radiation on the islands of the Greater Antilles in the Caribbean and, with nearly 400 species of Anolis, there are dozens of examples of convergent evolution of similarly colored dewlaps. Colorful signals, and especially the dewlap, play important roles in species recognition and social interactions, but very little is known about the genetics of colorful pigmentation in vertebrates. Pteridines comprise a large component of the pigments that produce red, orange and yellow coloration in Anolis and the biochemical pathway that produces pteridine pigments is reasonably well known. Using this as a starting point, we used a combination of RNA sequencing and quantitative PCR to understand first how the green, red, and white coloration of A. carolinensis body regions is related to pteridine gene expression. This work was important to identify key genes that contribute to red, orange and yellow pigmentation and determine how various body regions can be differently colored. As part of this work we used the complete genome sequence of the Green Anole, Anolis carolinensis, to identify gene transcripts that we sequenced. We noted immediately that many of the transcripts did not map to the assembled AnoCar genome assembly, but that most of those represented genes that appeared to be homologous to genes in other vertebrate genomes. This suggested that the annotation and assembly of the AnoCar genome was incomplete. Subsequently, a large effort was undertaken to better annotate the AnoCar genome using complete transcriptome sequencing of multiple tissues. The publication describing this effort is in press (Eckalbar et al. 2013 "Genome reannotation of the lizard Anolis carolinensis based on 14 adult and embryonic deep transcriptomes" BMC Genomics). However, prior to the improved annotation of the AnoCar genome, we were able to identify several critical genes in the pteridine synthesis pathway. Most importantly, we identified the homolog of the fruit fly clot gene which is responsible for producing drosopterin, the red pigment in the orange eyes of wild D. melanogaster. Transcript abundance of this gene, TXNDC17, is positively correlated with the abundance of drosopterin in the red dewlaps of Anolis and, in A. carolinensis, is absent from the green dorsal and white ventral skin. This gene and its association with drosopterin pigmentation in vertebrates had not previously been reported and the genetic basis for drosopterin production in vertebrates had not been elucidated. Our results provide strong evidence that TXNDC17 is an important candidate gene associated with red pigmentation in vertebrates. It is likely that this gene may be an important contributor to the orange spots in guppies and red coloration in cichlid fish, both of which are important in sexual selection and speciation, as well as red and orange coloration in birds and other reptiles. Additional studies on the mechanisms and genetic mutations associated with this gene and others in the pteridine biosynthetic pathway will likely lead to a revolution in our understanding of colorful pigmentation - a revolution analogous to that driven by our understanding of the genes in the melanin pigmentation pathway. We have just opened the book on the genetics of colorful pigmentation and ongoing research, on organisms as disparate as butterflies and birds, is beginning to tell the story of how evolution has produced the remarkable diversity of coloration in the animal kingdom.
