We sequenced 28 transcriptomes from 20 invertebrate taxa that lack genomic resources, but are well-suited for answering questions about the function, development, and evolution of eyes and other light-interacting structures (Table 1). For example, we generated transcriptomes from RNA expressed by the eyes and skin of certain cephalopod mollusks (squid and octopus). These animals may have the most complex light-influenced behaviors of any invertebrate [51 ,52 (link)], but it appears that the eyes of cephalopods tend to contain only a single spectral class of photoreceptor ([53 (link)]; though see [54 (link)] as an exception). Additional physiological complexity may be suggested by the results of high throughput sequencing. It is also possible that certain visually-influenced behaviors in cephalopods – such as dynamic camouflage – may be influenced by molecular components that are expressed outside of their eyes. For example, past work suggests that certain cephalopods express LIT genes in their light-producing photophores [55 (link)] and in certain dermal cells [56 (link)].
We also sequenced transcriptomes for a range of arthropods. We chose to study stomatopods (mantis shrimp) because they have an unsurpassed ability to distinguish different aspects of light. Certain species are maximally sensitive to twelve distinct wavelength peaks and some species can identify both linearly and circularly polarized light [57 (link)-60 (link)]. Similarly, we chose to study odonates (damselflies and dragonflies) because they have physiologically complex eyes [61 (link)] and display a diversity of visually-influenced behaviors [62 -64 (link)]. To study the degeneration of eyes in arthropods from subterranean environments, we examined certain species of isopods and crayfish in which closely related species or populations live either above or below ground. Specifically, we sequenced tissues from the eye-bearing, surface-dwelling isopod Caecidotea forbesi and its eyeless, cave-dwelling congeneric C. bricrenata. We also sequenced transcriptomes for different populations of the isopod Asellus aquaticus, which has a surface-dwelling form and multiple cave-dwelling populations with typical cave morphologies like degenerated eyes [65 (link),66 (link)]. Likewise, we generated transcriptome data from a pair of surface (Procambarus alleni) and cave (P. franzi) freshwater crayfish. Crayfish have previously been the focus of molecular evolutionary studies of opsin in cave/surface comparisons [67 (link)]. To study the evolution of sexually dimorphic eyes, we generated a transcriptome for the RNA expressed by developing eyes from the ostracodEuphilomedes carcharodonta, a species in which males have compound eyes, but females do not [68 ,69 (link)]. Other species in this family of ostracods exhibit a similar, but independently evolved eye dimorphism, suggesting that these ostracods may be a promising system for the study of sex-specific convergent phenotypic evolution [70 (link)].
Lastly, we sequenced transcriptomes for Tripedalia cystophora, a cubozoan cnidarian (box jellyfish). Cubozoans are the only cnidarians with camera-type eyes and, for that reason, have been the subject of numerous studies of visual neurobiology [71 (link)-74 (link)], morphology [75 (link),76 (link)], and behavior [77 (link),78 (link)]. Transcriptomic resources will aid these efforts. Further, as cnidarians, cubozoans may help us understand the evolutionary origins of the metazoan phototransduction cascade [79 (link)-81 (link)].
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