The Genetic Basis of Color Polymorphism in the Orb-Web Spider Gasteracantha cancriformis

Animal coloration is a complex trait useful for studying adaptive evolution. In consequence, a major goal in evolutionary biology is to understand the genetic and developmental basis underlying coloration in animals and thus establish direct links between genetic and phenotype variability. Genomics tools have contributed the identification the specific genes and variants responsible for color variation in nonmodel animals, but the evidence is biased toward some taxa, while groups such as arachnids have been neglected. In this study, we aimed to identify genes that underlie coloration in spiders and generate genomic resources for these organisms. By deep sequencing RNA-seq libraries from yellow, orange, white, and black female morphs of the color polymorphic spider Gasteracantha cancriformis, we assembled a transcriptome for this species and identified genes that were differentially expressed between color morphs. Among these, we detected genes with known roles in pigmentation pathways for carotenoids, melanin, ommochromes, and pteridines, suggesting that both pigmentary and structural coloration are involved in abdominal coloration in spiders. Signatures of positive selection on these genes suggest that abdominal coloration in G. cancriformis plays an adaptive role, although the specific selective pressures remain unknown. However, the fact that we identified more venom gland genes expressed in the most conspicuous morphs we tested (i.e., yellow and orange) suggests that abdominal coloration likely serves a defensive function against predators in these spiders. Overall, our results provide evidence that several pathways that control pigmentation in insects and vertebrates also play a role in arachnids.