The Evolution Of Colouration And Opsins In Tarantulas

Abstract

14Tarantulas paradoxically exhibit a diverse palette of vivid colouration despite their crepuscular 15 to nocturnal habits. The evolutionary origin and maintenance of these colours remains a 16 mystery. In this study, we reconstructed the ancestral states of both blue and green colouration 17 in tarantula setae, and tested how these colours correlate with the presence of stridulation, 18 urtication, and arboreality. Green colouration has likely evolved at least eight times, and blue 19 colouration is likely an ancestral condition that appears to be lost more frequently than gained. 20 While our results indicate that neither colour correlates with the presence of stridulation or 21 urtication, the evolution of green colouration appears to depend upon the presence of 22 arboreality, suggesting that it likely originated for, and functions in, crypsis through substrate 23 matching among leaves. We also constructed a network of opsin homologs across tarantula 24 transcriptomes. Despite their crepuscular tendencies, tarantulas express a full suite of opsin 25 genesa finding that contradicts current consensus that tarantulas have poor colour vision on 26 the basis of low opsin diversity. Overall, our results support the intriguing hypotheses that blue 27 colouration may have ultimately evolved via sexual selection and perhaps proximately be used 28 in mate choice or predation avoidance due to possible sex differences in mate-searching. 29 30 52for each genus in this study, and conducted a series of correlative tests to determine whether 53 any of these traits might inform the function of blue or green colouration in tarantulas. We 54 hypothesize that blue colour should be associated with stridulating / urticating setae, or both, 55 if it either originally evolved or has been exapted (sensu 8 ) for an aposematic function. 56However, we posit that green colouration could aid in crypsis and should show an association 57 with arboreality. 58It is generally thought that tarantulas possess little or no colour vision, with only a weak 59 colour spectrum discrimination 2,9 , with Hsiung, et al. 2 noting that sexual selection as a driving 60 force behind the evolution of tarantula colouration is unlikely. Morehouse et al. 10 argue that 61 tarantulas are colour-blind on the basis of finding a low opsin diversity in Aphonopelma 10 . We 62 investigate whether this claim holds true for other tarantulas by scoring the presence and 63 relative expression of opsins from transcriptomic data in 25 tarantula genera. Opsins are 64 specialized, transmembrane proteins belonging to the superfamily of G-protein coupled 65 receptors that convert light photons to electrochemical signals through a signalling cascade 11 . 66 While the expression of multiple opsin species does not necessarily imply greater colour 67 discrimination, the absence of opsin orthologs could provide support to the hypothesis that 68 tarantulas, broadly speaking, cannot perceive colours. 69 70 RESULTS 71 72 Phylogenetic Signal of Colour Traits 73Our phylogeny con...