The Evolution Of Colouration And Opsins In Tarantulas

Abstract

Tarantulas paradoxically exhibit a diverse palette of vivid colouration despite their crepuscular to nocturnal habits. The evolutionary origin and maintenance of these colours remains a mystery. In this study, we reconstructed the ancestral states of both blue and green colouration in tarantula setae, and tested how these colours correlate with the presence of stridulation, urtication, and arboreality. Green colouration has likely evolved at least eight times, and blue colouration is likely an ancestral condition that appears to be lost more frequently than gained. While our results indicate that neither colour correlates with the presence of stridulation or urtication, the evolution of green colouration appears to depend upon the presence of arboreality, suggesting that it likely originated for, and functions in, crypsis through substrate matching among leaves. We also constructed a network of opsin homologs across tarantula transcriptomes. Despite their crepuscular tendencies, tarantulas express a full suite of opsin genes - a finding that contradicts current consensus that tarantulas have poor colour vision on the basis of low opsin diversity. Overall, our results support the intriguing hypotheses that blue colouration may have ultimately evolved via sexual selection and perhaps proximately be used in mate choice or predation avoidance due to possible sex differences in mate-searching.