Blue light restores functional circadian clocks in eyeless cave spiders

Evolution in profound darkness often leads to predictable, convergent traits, such as the loss of vision. Yet, the consequences of such repeated evolutionary experiments remain obscure, especially regarding fundamental regulatory behaviors like circadian rhythms. We studied circadian clocks of blind cave spiders and their sighted relatives. In the field, cave spiders exhibit low per expression and maintain constant activity levels. Curiously, their clocks are not permanently lost; exposure to monochromatic blue light restores both circadian gene expression and behavioral rhythms. Conversely, blocking blue light in sighted relatives induces an arrhythmic "cave phenotype." Our RNA interference experiments suggest that clock genes regulate the rhythmicity of the huddle response, establishing a link between circadian gene networks and this behavioral rhythm. We demonstrate that circadian regulation is readily toggled and may play a latent role, even in constant darkness. Overall, our study expands understanding of circadian clock variations and paves the way for future research on the maintenance of silent phenotypes.

Reduced adult stem cell fate specification led to eye reduction in cave planarians

Eye loss occurs convergently in numerous animal phyla as an adaptation to dark environments. We investigate the cave planarian Girardia multidiverticulata (Gm), a representative species of the Spiralian clade, to study mechanisms of eye loss. We found that Gm, which was previously described as an eyeless species, retains rudimentary and functional eyes. Eyes are maintained in homeostasis and regenerated in adult planarians by stem cells, called neoblasts, through their fate specification to eye progenitors. The reduced number of eye cells in cave planarians is associated with a decreased rate of stem cell fate specification to eye progenitors during homeostasis and regeneration. Conversely, the homeostatic formation of new cells from stem cell-derived progenitors for other tissues, including for neurons, pharynx, and epidermis, is comparable between cave and surface species. These findings reveal a mode of evolutionary trait loss, with change in rate of fate specification in adult stem cells leading to tissue size reduction.

Molecular Insights Into the Sensory Adaption of the Cave-Dwelling Leech Sinospelaeobdella wulingensis to the Karst Cave Environment

Karst caves are a unique environment significantly different from the external environment; adaptation of cave-dwelling animals to the cave environment is often accompanied by shifts in the sensory systems. Aquatic and terrestrial leeches have been found in the karst caves. In this study, we conducted a transcriptome analysis on the cave-dwelling leech Sinospelaeobdella wulingensis. A total of 29,286 unigenes were obtained by assembling the clean reads, and only 395 genes are differentially expressed in winter and summer samples. Two piezo-type mechanosensitive ion channels (Piezos), eight transient receptor potential channels (TRPs), and six ionotropic glutamate receptors (iGluRs) were identified in the transcriptome. These channels/receptors are transmembrane proteins sharing conserved structural features in the respective protein families. SwPiezo1 shares high identity with Piezos in non-caving leeches. SwiGluRs are conserved in protein sequence and share high identities with homologous proteins in other leeches. In contrast, SwTRPs belong to different subfamilies and share diverse identities with TRPs in other species. Gene expression analysis showed that two SwPiezos, five SwTRPs, and one SwiGluR are abundantly expressed in both winter and summer samples. These results suggest that SwPiezos, SwTRPs, and SwiGluRs are candidate sensory channels/receptors that may have roles in mechanosensory and chemosensory systems. High expression levels of Piezo and TRP genes imply a mechanosensory adaptation of S. wulingensis to the hanging living style in caves. Furthermore, enrichment of sensory genes in the oral sucker indicates the important role of this tissue in response to environmental stimuli. Similar gene expression profiles in winter and summer samples imply a stable physiological status of S. wulingensis in the cave environment.

Transcriptomic Characterization of Phototransduction Genes of the Asian Citrus Psyllid Diaphorina citri Kuwayama

Opsin plays a regulatory role in phototaxis of Diaphorina citri, functioning as the initial station in the phototransduction cascade. Our study aimed to explore the D. citri phototransduction pathway to identify elicitors that may enhance D. citri phototaxis in the future. The RNAi technique was employed to inhibit LW-opsin gene expression, followed by RNA-Seq analysis to identify phototransduction genes. Finally, RT-qPCR was performed to validate whether genes in the phototransduction pathway were affected by the inhibition of LW-opsin expression. A total of 87 genes were identified within the transcriptome as involved in phototransduction based on KEGG functional annotation. Of these, 71 genes were identified as enriched in the phototransduction-fly pathway. These genes encode key proteins in this process, including Gqα, Gqβ, Gqγ, phospholipase C β (PLCβ), the cation channel transient receptor potential (TRP), and TRP-like (TRPL), among others. Moreover, the LOC103513214 (Gqβ) and LOC103518375 (ninaC) genes exhibited reduced expression when LW-opsin gene expression was suppressed. Our results provide a basis for further investigation of phototransduction in D. citri.

Divergent patterns of locomotor activity in cave isopods (Oniscidea: Styloniscidae) in Neotropics

In cave environments, stable conditions devoid of light-dark cycles and temperature fluctuations sustain circadian clock mechanisms across various species. However, species adapted to these conditions may exhibit disruption of circadian rhythm in locomotor activity. This study examines potential rhythm loss due to convergent evolution in five semi-aquatic troglobitic isopod species (Crustacea: Styloniscidae), focusing on its impact on locomotor activity. The hypothesis posits that these species display aperiodic locomotor activity patterns. Isopods were subjected to three treatments: constant red light (DD), constant light (LL), and light-dark cycles (LD 12:12), totaling 1656 h. Circadian rhythm analysis employed the Sokolove and Bushell periodogram chi-square test, Hurst coefficient calculation, intermediate stability (IS), and activity differences for each species. Predominantly, all species exhibited an infradian rhythm under DD and LL. There was synchronization of the locomotor rhythm in LD, likely as a result of masking. Three species displayed diurnal activity, while two exhibited nocturnal activity. The Hurst coefficient indicated rhythmic persistence, with LD showing higher variability. LD conditions demonstrated higher IS values, suggesting synchronized rhythms across species. Significant individual variations were observed within species across the three conditions. Contrary to the hypothesis, all species exhibited synchronization under light-dark conditions. Analyzing circadian activity provides insights into organism adaptation to non-cyclical environments, emphasizing the importance of exploring underlying mechanisms.

The ticking clock in the dark: Review of biological rhythms in cave invertebrates

Circadian clocks, internal mechanisms that generate 24-hour rhythms, play a crucial role in coordinating biological events with day-night cycles. In light-deprived environments such as caves, species, particularly isolated obligatory troglobites, may exhibit evolutionary adaptations in biological rhythms due to light exposure. To explore rhythm expression in these settings, we conducted a comprehensive literature review on invertebrate chronobiology in global subterranean ecosystems, analyzing 44 selected studies out of over 480 identified as of September 2023. These studies revealed significant taxonomic diversity, primarily among terrestrial species like Coleoptera, with research concentrated in the United States, Italy, France, Australia, and Brazil, and a notable gap in African records. Troglobite species displayed a higher incidence of aperiodic behavior, while troglophiles showed a robust association with rhythm expression. Locomotor activity was the most studied aspect (>60%). However, approximately 4% of studies lacked information on periodicity or rhythm asynchrony, and limited research under constant light conditions hindered definitive conclusions. This review underscores the need to expand chronobiological research globally, encompassing diverse geographical regions and taxa, to deepen our understanding of biological rhythms in subterranean species. Such insights are crucial for preserving the resilience of subsurface ecosystems facing threats like climate change and habitat loss.

Identification, expression analysis, and functional verification of three opsin genes related to the phototactic behaviour of Ostrinia furnacalis

Ostrinia furnacalis is a disreputable herbivorous pest that poses a serious threat to corn crops. Phototaxis in nocturnal moths plays a crucial role in pest prediction and control. Insect opsins are the main component of insect visual system. However, the inherent molecular relationship between phototactic behaviour and vision of insects remains a mystery. Herein, three opsin genes were identified and cloned from O. furnacalis (OfLW, OfBL, and OfUV). Bioinformatics analysis revealed that all opsin genes had visual pigment (opsin) retinal binding sites and seven transmembrane domains. Opsin genes were distributed across different developmental stages and tissues, with the highest expression in adults and compound eyes. The photoperiod-induced assay elucidated that the expression of three opsin genes in females were higher during daytime, while their expression in males tended to increase at night. Under the sustained darkness, the expression of opsin genes increased circularly, although the increasing amplitude in males was lower when compared with females. Furthermore, the expression of OfLW, OfBL, and OfUV was upregulated under green, blue, and ultraviolet light, respectively. The results of RNA interference showed that the knockout of opsin genes decreased the phototaxis efficiency of female and male moths to green, blue, and ultraviolet light. Our results reveal that opsin genes are involved in the phototactic behaviour of moths, providing a potential target gene for pest control and a basis for further investigation on the phototactic behaviour of Lepidoptera insects.

Eyeless cave-dwelling Leptonetela spiders still rely on light

Subterranean animals living in perpetual darkness may maintain photoresponse. However, the evolutionary processes behind the conflict between eye loss and maintenance of the photoresponse remain largely unknown. We used Leptonetela spiders to investigate the driving forces behind the maintenance of the photoresponse in cave-dwelling spiders. Our behavioral experiments showed that all eyeless/reduced-eyed cave-dwelling species retained photophobic response and that they had substantially decreased survival at cave entrances due to weak drought resistance. The transcriptomic analysis demonstrated that nearly all phototransduction pathway genes were present and that all tested phototransduction pathway genes were subjected to strong functional constraints in cave-dwelling species. Our results suggest that cave-dwelling eyeless spiders still use light and that light detection likely plays a role in avoiding the cave entrance habitat. This study confirms that some eyeless subterranean animals have retained their photosensitivity due to natural selection and provides a case of mismatch between phenotype and genotype or physiological function in a long-term evolutionary process.

Parallel and convergent genomic changes underlie independent subterranean colonization across beetles

Adaptation to life in caves is often accompanied by dramatically convergent changes across distantly related taxa, epitomized by the loss or reduction of eyes and pigmentation. Nevertheless, the genomic underpinnings underlying cave-related phenotypes are largely unexplored from a macroevolutionary perspective. Here we investigate genome-wide gene evolutionary dynamics in three distantly related beetle tribes with at least six instances of independent colonization of subterranean habitats, inhabiting both aquatic and terrestrial underground systems. Our results indicate that remarkable gene repertoire changes mainly driven by gene family expansions occurred prior to underground colonization in the three tribes, suggesting that genomic exaptation may have facilitated a strict subterranean lifestyle parallelly across beetle lineages. The three tribes experienced both parallel and convergent changes in the evolutionary dynamics of their gene repertoires. These findings pave the way towards a deeper understanding of the evolution of the genomic toolkit in hypogean fauna.

Photoreceptor genes in a trechine beetle, Trechiama kuznetsovi, living in the upper hypogean zone

To address how organisms adapt to a new environment, subterranean organisms whose ancestors colonized subterranean habitats from surface habitats have been studied. Photoreception abilities have been shown to have degenerated in organisms living in caves and calcrete aquifers. Meanwhile, the organisms living in a shallow subterranean environment, which are inferred to reflect an intermediate stage in an evolutionary pathway to colonization of a deeper subterranean environment, have not been studied well. In the present study, we examined the photoreception ability in a trechine beetle, Trechiama kuznetsovi, which inhabits the upper hypogean zone and has a vestigial compound eye. By de novo assembly of genome and transcript sequences, we were able to identify photoreceptor genes and phototransduction genes. Specifically, we focused on opsin genes, where one long wavelength opsin gene and one ultraviolet opsin gene were identified. The encoded amino acid sequences had neither a premature stop codon nor a frameshift mutation, and appeared to be subject to purifying selection. Subsequently, we examined the internal structure of the compound eye and nerve tissue in the adult head, and found potential photoreceptor cells in the compound eye and nerve bundle connected to the brain. The present findings suggest that T. kuznetsovi has retained the ability of photoreception. This species represents a transitional stage of vision, in which the compound eye regresses, but it may retain the ability of photoreception using the vestigial eye.

Evidence of ancestral nocturnality, locomotor clock regression, and cave zone-adjusted sleep duration modes in a cave beetle

The small carrion beetle Ptomaphagus hirtus is an abundant inhabitant of the exceptionally biodiverse Mammoth Cave system. Previous studies revealed negative phototaxis and the expression of biological clock genes in this microphthalmic cave beetle. Here we present results from probing P. hirtus for the entrainment of locomotor rhythms using the TriKinetics activity monitor setup. Although curtailed by low adjustment frequency of animals to the test environment, the data obtained from successfully monitoring two animals in constant darkness (DD) and six animals exposed to 12 hour light-dark cycles (LD) revealed a strong effect of light on locomotor activity in P. hirtus. In LD, activity was prevalent during the artificial night phases while close to absent during the presumptive day phases, suggesting conserved nocturnality. Upon transitioning LD animals to constant darkness, none displayed detectable evidence of free-running activity rhythms, suggesting complete regression of the central circadian clock. Equally notable, overall locomotor activity of the two DD-monitored animals was about three-fold lower compared to LD animals due to longer rest durations in the former. We, therefore, propose the existence of cave zone-specific energy expenditure modes that are mediated through light schedule responsive modification of sleep duration in P. hirtus.

An LW-Opsin Mutation Changes the Gene Expression of the Phototransduction Pathway: A Cryptochrome1 Mutation Enhances the Phototaxis of Male Plutella xylostella (Lepidoptera: Plutellidae)

Plutella xylostella is a typical phototactic pest. LW-opsin contributes to the phototaxis of P. xylostella, but the expression changes of other genes in the phototransduction pathway caused by the mutation of LW-opsin remain unknown. In the study, the head transcriptomes of male G88 and LW-opsin mutants were compared. A GO-function annotation showed that DEGs mainly belonged to the categories of molecular functions, biological processes, and cell composition. Additionally, a KEGG-pathway analysis suggested that DEGs were significantly enriched in some classical pathways, such as the phototransduction-fly and vitamin digestion and absorption pathways. The mRNA expressions of genes in the phototransduction-fly pathway, such as Gq, ninaC, and rdgC were significantly up-regulated, and trp, trpl, inaD, cry1, ninaA and arr1 were significantly down-regulated. The expression trends of nine DEGs in the phototransduction pathway confirmed by a RT-qPCR were consistent with transcriptomic data. In addition, the influence of a cry1 mutation on the phototaxis of P. xylostella was examined, and the results showed that the male cry1 mutant exhibited higher phototactic rates to UV and blue lights than the male G88. Our results indicated that the LW-opsin mutation changed the expression of genes in the phototransduction pathway, and the mutation of cry1 enhanced the phototaxis of a P. xylostella male, providing a basis for further investigation on the phototransduction pathway in P. xylostella.

Parallel decay of vision genes in subterranean water beetles

In the framework of neutral theory of molecular evolution, genes specific to the development and function of eyes in subterranean animals living in permanent darkness are expected to evolve by relaxed selection, ultimately becoming pseudogenes. However, definitive empirical evidence for the role of neutral processes in the loss of vision over evolutionary time remains controversial. In previous studies, we characterized an assemblage of independently-evolved water beetle (Dytiscidae) species from a subterranean archipelago in Western Australia, where parallel vision and eye loss have occurred. Using a combination of transcriptomics and exon capture, we present evidence of parallel coding sequence decay, resulting from the accumulation of frameshift mutations and premature stop codons, in eight phototransduction genes (arrestins, opsins, ninaC and transient receptor potential channel genes) in 32 subterranean species in contrast to surface species, where these genes have open reading frames. Our results provide strong evidence to support neutral evolutionary processes as a major contributing factor to the loss of phototransduction genes in subterranean animals, with the ultimate fate being the irreversible loss of a light detection system.

Multiple mechanisms of photoreceptor spectral tuning in Heliconius butterflies

The evolution of color vision is often studied through the lens of receptor gain relative to an ancestor with fewer spectral classes of photoreceptor. For instance, in Heliconius butterflies, a genus-specific UVRh opsin duplication led to the evolution of UV color discrimination in Heliconius erato females, a rare trait among butterflies. However, color vision evolution is not well understood in the context of loss. In Heliconius melpomene and Heliconius ismenius lineages, the UV2 receptor subtype has been lost, which limits female color vision in shorter wavelengths. Here, we compare the visual systems of butterflies that have either retained or lost the UV2 photoreceptor using intracellular recordings, ATAC-seq, and antibody staining. We identify several ways these butterflies modulate their color vision. In H. melpomene, chromatin reorganization has downregulated an otherwise intact UVRh2 gene, whereas in H. ismenius, pseudogenization has led to the truncation of UVRh2. In species that lack the UV2 receptor, the peak sensitivity of the remaining UV1 photoreceptor cell is shifted to longer wavelengths. Across Heliconius, we identify the widespread use of filtering pigments and co-expression of two opsins in the same photoreceptor cells. Multiple mechanisms of spectral tuning, including the molecular evolution of blue opsins, have led to the divergence of receptor sensitivities between species. The diversity of photoreceptor and ommatidial subtypes between species suggests that Heliconius visual systems are under varying selection pressures for color discrimination. Modulating the wavelengths of peak sensitivities of both the blue- and remaining UV-sensitive photoreceptor cells suggests that Heliconius species may have compensated for UV receptor loss.

Mammoth Cave: A Hotspot of Subterranean Biodiversity in the United States

The Mammoth Cave System in the Interior Low Plateau karst region in central Kentucky, USA is a global hotspot of cave-limited biodiversity, particularly terrestrial species. We searched the literature, museum accessions, and database records to compile an updated list of troglobiotic and stygobiotic species for the Mammoth Cave System and compare our list with previously published checklists. Our list of cave-limited fauna totals 49 species, with 32 troglobionts and 17 stygobionts. Seven species are endemic to the Mammoth Cave System and other small caves in Mammoth Cave National Park. The Mammoth Cave System is the type locality for 33 cave-limited species. The exceptional diversity at Mammoth Cave is likely related to several factors, such as the high dispersal potential of cave fauna associated with expansive karst exposures, high surface productivity, and a long history of exploration and study. Nearly 80% of the cave-limited fauna is of conservation concern, many of which are at an elevated risk of extinction because of small ranges, few occurrences, and several potential threats.

The eyes have it: dim-light activity is associated with the morphology of eyes but not antennae across insect orders

The perception of cues and signals in visual, olfactory and auditory modalities underpins all animal interactions and provides crucial fitness-related information. Sensory organ morphology is under strong selection to optimize detection of salient cues and signals in a given signalling environment, the most well-studied example being selection on eye design in different photic environments. Many dim-light active species have larger compound eyes relative to body size, but little is known about differences in non-visual sensory organ morphology between diurnal and dim-light active insects. Here, we compare the micromorphology of the compound eyes (visual receptors) and antennae (olfactory and mechanical receptors) in representative pairs of day active and dim-light active species spanning multiple taxonomic orders of insects. We find that dim-light activity is associated with larger compound eye ommatidia and larger overall eye surface area across taxonomic orders but find no evidence that morphological adaptations that enhance the sensitivity of the eye in dim-light active insects are accompanied by morphological traits of the antennae that may increase sensitivity to olfactory, chemical or physical stimuli. This suggests that the ecology and natural history of species is a stronger driver of sensory organ morphology than is selection for complementary investment between sensory modalities.

Evidence for speciation underground in diving beetles (Dytiscidae) from a subterranean archipelago

Most subterranean animals are assumed to have evolved from surface ancestors following colonization of a cave system; however, very few studies have raised the possibility of "subterranean speciation" in underground habitats (i.e., obligate cave-dwelling organisms [troglobionts] descended from troglobiotic ancestors). Numerous endemic subterranean diving beetle species from spatially discrete calcrete aquifers in Western Australia (stygobionts) have evolved independently from surface ancestors; however, several cases of sympatric sister species raise the possibility of subterranean speciation. We tested this hypothesis using vision (phototransduction) genes that are evolving under neutral processes in subterranean species and purifying selection in surface species. Using sequence data from 32 subterranean and five surface species in the genus Paroster (Dytiscidae), we identified deleterious mutations in long wavelength opsin (lwop), arrestin 1 (arr1), and arrestin 2 (arr2) shared by a sympatric sister-species triplet, arr1 shared by a sympatric sister-species pair, and lwop and arr2 shared among closely related species in adjacent calcrete aquifers. In all cases, a common ancestor possessed the function-altering mutations, implying they were already adapted to aphotic environments. Our study represents one of the first confirmed cases of subterranean speciation in cave insects. The assessment of genes undergoing pseudogenization provides a novel way of testing modes of speciation and the history of diversification in blind cave animals.

Illuminating the impact of diel vertical migration on visual gene expression in deep-sea shrimp

Diel vertical migration (DVM) of marine animals represents one of the largest migrations on our planet. Migrating fauna are subjected to a variety of light fields and environmental conditions that can have notable impacts on sensory mechanisms, including an organism's visual capabilities. Among deep-sea migrators are oplophorid shrimp that vertically migrate hundreds of metres to feed in shallow waters at night. These species also have bioluminescent light organs that emit light during migrations to aid in camouflage. The organs have recently been shown to contain visual proteins (opsins) and genes that infer light sensitivity. Knowledge regarding the impacts of vertical migratory behaviour, and fluctuating environmental conditions, on sensory system evolution is unknown. In this study, the oplophorid Systellaspis debilis was either collected during the day from deep waters or at night from relatively shallow waters to ensure sampling across the vertical distributional range. De novo transcriptomes of light-sensitive tissues (eyes/photophores) from the day/night specimens were sequenced and analysed to characterize opsin diversity and visual/light interaction genes. Gene expression analyses were also conducted to quantify expression differences associated with DVM. Our results revealed an expanded opsin repertoire among the shrimp and differential opsin expression that may be linked to spectral tuning during the migratory process. This study sheds light on the sensory systems of a bioluminescent invertebrate and provides additional evidence for extraocular light sensitivity. Our findings further suggest opsin co-expression and subsequent fluctuations in opsin expression may play an important role in diversifying the visual responses of vertical migrators.

Developmental Transcriptomic Analysis of the Cave-Dwelling Crustacean, Asellus aquaticus

Cave animals are a fascinating group of species often demonstrating characteristics including reduced eyes and pigmentation, metabolic efficiency, and enhanced sensory systems. Asellus aquaticus, an isopod crustacean, is an emerging model for cave biology. Cave and surface forms of this species differ in many characteristics, including eye size, pigmentation, and antennal length. Existing resources for this species include a linkage map, mapped regions responsible for eye and pigmentation traits, sequenced adult transcriptomes, and comparative embryological descriptions of the surface and cave forms. Our ultimate goal is to identify genes and mutations responsible for the differences between the cave and surface forms. To advance this goal, we decided to use a transcriptomic approach. Because many of these changes first appear during embryonic development, we sequenced embryonic transcriptomes of cave, surface, and hybrid individuals at the stage when eyes and pigment become evident in the surface form. We generated a cave, a surface, a hybrid, and an integrated transcriptome to identify differentially expressed genes in the cave and surface forms. Additionally, we identified genes with allele-specific expression in hybrid individuals. These embryonic transcriptomes are an important resource to assist in our ultimate goal of determining the genetic underpinnings of the divergence between the cave and surface forms.

Evolution of Phototransduction Genes in Lepidoptera

Vision is underpinned by phototransduction, a signaling cascade that converts light energy into an electrical signal. Among insects, phototransduction is best understood in Drosophila melanogaster. Comparison of D. melanogaster against three insect species found several phototransduction gene gains and losses, however, lepidopterans were not examined. Diurnal butterflies and nocturnal moths occupy different light environments and have distinct eye morphologies, which might impact the expression of their phototransduction genes. Here we investigated: 1) how phototransduction genes vary in gene gain or loss between D. melanogaster and Lepidoptera, and 2) variations in phototransduction genes between moths and butterflies. To test our prediction of phototransduction differences due to distinct visual ecologies, we used insect reference genomes, phylogenetics, and moth and butterfly head RNA-Seq and transcriptome data. As expected, most phototransduction genes were conserved between D. melanogaster and Lepidoptera, with some exceptions. Notably, we found two lepidopteran opsins lacking a D. melanogaster ortholog. Using antibodies we found that one of these opsins, a candidate retinochrome, which we refer to as unclassified opsin (UnRh), is expressed in the crystalline cone cells and the pigment cells of the butterfly, Heliconius melpomene. Our results also show that butterflies express similar amounts of trp and trpl channel mRNAs, whereas moths express ∼50× less trp, a potential adaptation to darkness. Our findings suggest that while many single-copy D. melanogaster phototransduction genes are conserved in lepidopterans, phototransduction gene expression differences exist between moths and butterflies that may be linked to their visual light environment.

The Evolution of Gene Expression Underlying Vision Loss in Cave Animals

Dissecting the evolutionary genetic processes underlying eye reduction and vision loss in obligate cave-dwelling organisms has been a long-standing challenge in evolutionary biology. Independent vision loss events in related subterranean organisms can provide critical insight into these processes as well as into the nature of convergent loss of complex traits. Advances in evolutionary developmental biology have illuminated the significant role of heritable gene expression variation in the evolution of new forms. Here, we analyze gene expression variation in adult eye tissue across the freshwater crayfish, representing four independent vision-loss events in caves. Species and individual expression patterns cluster by eye function rather than phylogeny, suggesting convergence in transcriptome evolution in independently blind animals. However, this clustering is not greater than what is observed in surface species with conserved eye function after accounting for phylogenetic expectations. Modeling expression evolution suggests that there is a common increase in evolutionary rates in the blind lineages, consistent with a relaxation of selective constraint maintaining optimal expression levels. This is evidence for a repeated loss of expression constraint in the transcriptomes of blind animals and that convergence occurs via a similar trajectory through genetic drift.

The Kingdom of the Blind: Disentangling Fundamental Drivers in the Evolution of Eye Loss

Light is a fundamentally important biological cue used by almost every animal on earth, to maintain daily rhythms, navigate, forage, find mates, or avoid predators. But an enormous number of species live in darkness: in subterranean caves, deep oceans, underground burrows, and within parasitic host bodies, and the loss of eyes appears consistently across these ecosystems. However, the evolutionary mechanisms that lead to the reduction of the visual system remain the subject of great interest and debate more than 150 years after Darwin tackled the issue. Studies of model taxa have discovered significant roles for natural selection, neutral evolution, and pleiotropy, but the interplay between them remains unclear. To nail down unifying concepts surrounding the evolution of eye loss, we must embrace the enormous range of affected animals and habitats. The fine developmental details of model systems such as the Mexican cave tetra Astyanax mexicanus have transformed and enriched the field, but these should be complemented by wider studies to identify truly overarching patterns that apply throughout animals. Here, the major evolutionary drivers are placed within a conceptual cost-benefit framework that incorporates the fundamental constraints and forces that influence evolution in the dark. Major physiological, ecological, and environmental factors are considered within the context of this framework, which appears faithful to observed patterns in deep-sea and cavernicolous animals. To test evolutionary hypotheses, a comparative phylogenetic approach is recommended, with the goal of studying large groups exhibiting repeated reduction, and then comparing these across habitats, taxa, and lifestyles. Currently, developmental and physiological methods cannot feasibly be used on such large scales, but penetrative imaging techniques could provide detailed morphological data non-invasively and economically for large numbers of species. Comprehensive structural datasets can then be contextualized phylogenetically to examine recurrent trends and associations, and to reconstruct character histories through multiple independent transitions into darkness. By assessing these evolutionary trajectories within an energetic cost-benefit framework, the relationships between fundamental influences can be inferred and compared across different biological and physical parameters. However, substantial numbers of biological and environmental factors affect the evolutionary trajectory of loss, and it is critical that researchers make fair and reasonable comparisons between objectively similar groups.

Phototransduction Gene Expression and Evolution in Cave and Surface Crayfishes

In the absence of light in caves, animals have repeatedly evolved reduced eyes and visual systems. Whether the underlying genetic components remain intact in blind species remains unanswered across taxa. The freshwater crayfish have evolved to live in caves multiple times throughout their history; therefore, this system provides an opportunity to probe the genetic patterns and processes underlying repeated vision loss. Using transcriptomic data from the eyes of 14 species of cave and surface crayfishes, we identify the expression of 17 genes putatively related to visual phototransduction. We find a similarly complete repertoire of phototransduction gene families expressed in cave and surface species, but that the expression levels of those transcripts are consistently lower in cave species. We find statistical support for episodic positive selection, increased and decreased selection strength in caves, depending on the gene family. Analyses of gene expression evolution suggest convergent and possibly adaptive downregulation of these genes across eye-reduction events. Our results reveal a combination of evolutionary processes acting on the sequences and gene expression levels of vision-related genes underlying the loss of vision in caves.

Mitochondrial sequence data indicate “Vicariance by Erosion” as a mechanism of species diversification in North American Ptomaphagus (Coleoptera, Leiodidae, Cholevinae) cave beetles

Small carrion beetles (Coleoptera: Leiodidae: Cholevinae) are members of cave communities around the world and important models for understanding the colonization of caves, adaptation to cave life, and the diversification of cave-adapted lineages. We developed a molecular phylogeny to examine the diversification of the hirtus-group of the small carrion beetle genus Ptomaphagus. The hirtus-group has no surface-dwelling members; it consists of 19 short-range endemic cave- and soil-dwelling species in the central and southeastern United States of America. Taxonomic, phylogenetic and biogeographic data were previously interpreted to suggest the hirtus-group diversified within the past 350,000 years through a series of cave colonization and speciation events related to Pleistocene climate fluctuations. However, our time-calibrated molecular phylogeny resulting from the analysis of 2,300 nucleotides from five genes across three mitochondrial regions (cox1, cytb, rrnL-trnL-nad1) for all members of the clade paints a different picture. We identify three stages of diversification in the hirtus-group: (1) ~10 million years ago (mya), the lineage that develops into P.shapardi, a soil-dwelling species from the Ozarks, diverged from the lineage that gives rise to the 18 cave-obligate members of the group; (2) between 8.5 mya and 6 mya, seven geographically distinct lineages diverged across Kentucky, Tennessee, Alabama and Georgia; six of these lineages represent a single species today, whereas (3) the ‘South Cumberlands’ lineage in Tennessee and Alabama diversified into 12 species over the past ~6 my. While the events triggering diversification during the first two stages remain to be determined, the distributions, phylogenetic relationships and divergence times in the South Cumberlands lineage are consistent with populations being isolated by vicariant events as the southern Cumberland Plateau eroded and fragmented over millions of years.

Embryonic origin and genetic basis of cave associated phenotypes in the isopod crustacean Asellus aquaticus

Characteristics common to animals living in subterranean environments include the reduction or absence of eyes, lessened pigmentation and enhanced sensory systems. How these characteristics have evolved is poorly understood for the majority of cave dwelling species. In order to understand the evolution of these changes, this study uses an invertebrate model system, the freshwater isopod crustacean, Asellus aquaticus, to examine whether adult differences between cave and surface dwelling individuals first appear during embryonic development. We hypothesized that antennal elaboration, as well as eye reduction and pigment loss, would be apparent during embryonic development. We found that differences in pigmentation, eye formation, and number of segments of antenna II were all present by the end of embryonic development. In addition, we found that cave and surface hatchlings do not significantly differ in the relative size of antenna II and the duration of embryonic development. To investigate whether the regions responsible for eye and pigment differences could be genetically linked to differences in article number, we genotyped F2 hybrids for the four previously mapped genomic regions associated with eye and pigment differences and phenotyped these F2 hybrids for antenna II article number. We found that the region previously known to be responsible for both presence versus absence of pigment and eye size also was significantly associated with article number. Future experiments will address whether pleiotropy and/or genetic linkage play a role in the evolution of cave characteristics in Asellus aquaticus.

Hypothesis on monochromatic vision in scorpionflies questioned by new transcriptomic data

In the scorpionfly Panorpa, a recent study suggested monochromatic vision due to evidence of only a single opsin found in transcriptome data. To reconsider this hypothesis, the present study investigates opsin expression using transcriptome data of 21 species including representatives of all major lineages of scorpionflies (Mecoptera) and of three families of their closest relatives, the fleas (Siphonaptera). In most mecopteran species investigated, transcripts encode two opsins with predicted peak absorbances in the green, two in the blue, and one in the ultraviolet spectral region. Only in groups with reduced or absent ocelli, like Caurinus and Apteropanorpa, less than four visual opsin messenger RNAs have been identified. In addition, we found a Rh7-like opsin in transcriptome data derived from larvae of the mecopteran Nannochorista, and in two flea species. Peropsin expression was observed in two mecopterans. In light of these new data, we question the hypothesis on monochromatic vision in the genus Panorpa. In a broader phylogenetic perspective, it is suggested that the common ancestor of the monophyletic taxon Antliophora (Diptera, Mecoptera and Siphonaptera) possessed the full set of visual opsins, a Rh7-like opsin, and in addition a pteropsin as well as a peropsin. In the course of evolution individual opsins were likely lost in several lineages of this clade.

Ptomaphaginus troglodytes sp. n., the first anophthalmic species of Ptomaphaginina from China (Coleoptera, Leiodidae, Cholevinae, Ptomaphagini)

Ptomaphaginus troglodytessp. n., the first anophthalmic species of Ptomaphaginus Portevin, 1914 is described from two close caves in Libo Karst, south Guizhou Province, China.

Ptomaphaginus troglodytes sp. n., the first anophthalmic species of Ptomaphaginina from China (Coleoptera, Leiodidae, Cholevinae, Ptomaphagini)

Ptomaphaginustroglodytessp. n., the first anophthalmic species ofPtomaphaginusPortevin, 1914 is described from two close caves in Libo Karst, south Guizhou Province, China.

Retinal perception and ecological significance of color vision in insects

Color vision relies on the ability to discriminate different wavelengths and is often improved in insects that inhabit well-lit, spectrally rich environments. Although the Opsin proteins themselves are sensitive to specific wavelength ranges, other factors can alter and further restrict the sensitivity of photoreceptors to allow for finer color discrimination and thereby more informed decisions while interacting with the environment. The ability to discriminate colors differs between insects that exhibit different life styles, between female and male eyes of the same species, and between regions of the same eye, depending on the requirements of intraspecific communication and ecological demands.

Transcriptome analysis of molecular mechanisms responsible for light-stress response in Mythimna separata (Walker)

Light is an important environmental signal for most insects. The Oriental Armyworm, Mythimna separata, is a serious pest of cereal crops worldwide, and is highly sensitive to light signals during its developmental and reproductive stages. However, molecular biological studies of its response to light stress are scarce, and related genomic information is not available. In this study, we sequenced and de novo assembled the transcriptomes of M. separata exposed to four different light conditions: dark, white light (WL), UV light (UVL) and yellow light (YL). A total of 46,327 unigenes with an average size of 571 base pairs (bp) were obtained, among which 24,344 (52.55%) matched to public databases. The numbers of genes differentially expressed between dark vs WL, dark vs UVL, dark vs YL, and UVL vs YL were 12,012, 12,950, 14,855, and 13,504, respectively. These results suggest that light exposure altered gene expression patterns in M. separata. Putative genes involved in phototransduction-fly, phototransduction, circadian rhythm-fly, olfactory transduction, and taste transduction were identified. This study thus identified a series of candidate genes and pathways potentially related to light stress in M. separata.

First Insights into the Subterranean Crustacean Bathynellacea Transcriptome: Transcriptionally Reduced Opsin Repertoire and Evidence of Conserved Homeostasis Regulatory Mechanisms

Bathynellacea (Crustacea, Syncarida, Parabathynellidae) are subterranean aquatic crustaceans that typically inhabit freshwater interstitial spaces (e.g., groundwater) and are occasionally found in caves and even hot springs. In this study, we sequenced the whole transcriptome of Allobathynella bangokensis using RNA-seq. De novo sequence assembly produced 74,866 contigs including 28,934 BLAST hits. Overall, the gene sequences were most similar to those of the waterflea Daphnia pulex. In the A. bangokensis transcriptome, no opsin or related sequences were identified, and no contig aligned to the crustacean visual opsins and non-visual opsins (i.e. arthropsins, peropsins, and melaopsins), suggesting potential regressive adaptation to the dark environment. However, A. bangokensis expressed conserved gene family sets, such as heat shock proteins and those related to key innate immunity pathways and antioxidant defense systems, at the transcriptional level, suggesting that this species has evolved adaptations involving molecular mechanisms of homeostasis. The transcriptomic information of A. bangokensis will be useful for investigating molecular adaptations and response mechanisms to subterranean environmental conditions.

Emerging developmental genetic model systems in holometabolous insects

The number of insect species that are amenable to functional genetic studies is growing rapidly and provides many new research opportunities in developmental and evolutionary biology. The holometabolous insects represent a disproportionate percentage of animal diversity and are thus well positioned to provide model species for a wide variety of developmental processes. Here we discuss emerging holometabolous models, and review some recent breakthroughs. For example, flies and midges were found to use structurally unrelated long-range pattern organizers, butterflies and moths revealed extensive pattern formation during oogenesis, new imaging possibilities in the flour beetle Tribolium castaneum showed how embryos break free of their extraembryonic membranes, and the complex genetics governing interspecies difference in head shape were revealed in Nasonia wasps.

Life in a dark biosphere: a review of circadian physiology in "arrhythmic" environments

Most of the life with which humans interact is exposed to highly rhythmic and extremely predictable changes in illumination that occur with the daily events of sunrise and sunset. However, while the influence of the sun feels omnipotent to surface dwellers such as ourselves, life on earth is dominated, in terms of biomass, by organisms isolated from the direct effects of the sun. A limited understanding of what life is like away from the sun can be inferred from our knowledge of physiology and ecology in the light biosphere, but a full understanding can only be gained by studying animals from the dark biosphere, both in the laboratory and in their natural habitats. One of the least understood aspects of life in the dark biosphere is the rhythmicity of physiology and what it means to live in an environment of low or no rhythmicity. Here we describe methods that may be used to understand rhythmic physiology in the dark and summarise some of the studies of rhythmic physiology in "arrhythmic" environments, such as the poles, deep sea and caves. We review what can be understood about the adaptive value of rhythmic physiology on the Earth's surface from studies of animals from arrhythmic environments and what role a circadian clock may play in the dark.

Light triggers habitat choice of eyeless subterranean but not of eyed surface amphipods

Boundaries of species distributions are the result of colonization-extinction processes. Survival on the boundary depends on how well individuals discriminate optimal from suboptimal habitat patches. Such behaviour is called habitat choice and was only rarely applied to macroecology, although it links species ecological niche and species distribution. Surface and subterranean aquatic species are spatially strongly segregated, even in the absence of physical barriers. We explored whether a behavioural response to light functions as a habitat choice mechanism that could explain species turnover between surface and subterranean aquatic ecosystems. In a controlled laboratory experiment, we studied the behavioural response to light of ten pairs of surface and subterranean amphipods that permanently co-occur in springs. Surface species showed a weak photophobic, photoneutral, and in one case, photophilic response, whereas all subterranean species showed a strong photophobic response. Eyeless subterranean but not eyed surface amphipods appear to orient themselves with light cues. On a local scale, this difference possibly diminishes harmful interactions between the co-occurring amphipods, whereas on a regional scale, photophobia could explain limited dispersal and a high degree of endemism observed among subterranean species.

A Transcriptomic Analysis of Cave, Surface, and Hybrid Isopod Crustaceans of the Species Asellus aquaticus

Cave animals, compared to surface-dwelling relatives, tend to have reduced eyes and pigment, longer appendages, and enhanced mechanosensory structures. Pressing questions include how certain cave-related traits are gained and lost, and if they originate through the same or different genetic programs in independent lineages. An excellent system for exploring these questions is the isopod, Asellus aquaticus. This species includes multiple cave and surface populations that have numerous morphological differences between them. A key feature is that hybrids between cave and surface individuals are viable, which enables genetic crosses and linkage analyses. Here, we advance this system by analyzing single animal transcriptomes of Asellus aquaticus. We use high throughput sequencing of non-normalized cDNA derived from the head of a surface-dwelling male, the head of a cave-dwelling male, the head of a hybrid male (produced by crossing a surface individual with a cave individual), and a pooled sample of surface embryos and hatchlings. Assembling reads from surface and cave head RNA pools yielded an integrated transcriptome comprised of 23,984 contigs. Using this integrated assembly as a reference transcriptome, we aligned reads from surface-, cave- and hybrid- head tissue and pooled surface embryos and hatchlings. Our approach identified 742 SNPs and placed four new candidate genes to an existing linkage map for A. aquaticus. In addition, we examined SNPs for allele-specific expression differences in the hybrid individual. All of these resources will facilitate identification of genes and associated changes responsible for cave adaptation in A. aquaticus and, in concert with analyses of other species, will inform our understanding of the evolutionary processes accompanying adaptation to the subterranean environment.

Transcriptome-Wide Differential Gene Expression in Bicyclus anynana Butterflies: Female Vision-Related Genes Are More Plastic

Vision is energetically costly to maintain. Consequently, over time many cave-adapted species downregulate the expression of vision genes or even lose their eyes and associated eye genes entirely. Alternatively, organisms that live in fluctuating environments, with different requirements for vision at different times, may evolve phenotypic plasticity for expression of vision genes. Here, we use a global transcriptomic and candidate gene approach to compare gene expression in the heads of a polyphenic butterfly. Bicyclus anynana have two seasonal forms that display sexual dimorphism and plasticity in eye morphology, and female-specific plasticity in opsin gene expression. Nonchoosy dry season females downregulate opsin expression, consistent with the high physiological cost of vision. To identify other genes associated with sexually dimorphic and seasonally plastic differences in vision, we analyzed RNA-sequencing data from whole head tissues. We identified two eye development genes (klarsicht and warts homologs) and an eye pigment biosynthesis gene (henna) differentially expressed between seasonal forms. By comparing sex-specific expression across seasonal forms, we found that klarsicht, warts, henna, and another eye development gene (domeless) were plastic in a female-specific manner. In a male-only analysis, white (w) was differentially expressed between seasonal forms. Reverse transcription polymerase chain reaction confirmed that warts and white are expressed in eyes only, whereas klarsicht, henna and domeless are expressed in both eyes and brain. We find that differential expression of eye development and eye pigment genes is associated with divergent eye phenotypes in B. anynana seasonal forms, and that there is a larger effect of season on female vision-related genes.

Circadian molecular clockworks in non-model insects

The recent development of molecular genetic technology is promoting studies on the clock mechanism of various non-model insect species, revealing diversity and commonality of their molecular clock machinery. Like in Drosophila, their clocks generally consist of clock genes including period, timeless, Clock, and cycle, except for hymenopteran species which lack timeless in their genome. Unlike in Drosophila, however, some insects show vertebrate-like traits: The clock machinery involves mammalian type cryptochrome, cycle is rhythmically expressed, and Clock is constitutively expressed. Although the oscillatory mechanisms of the clock are still to be investigated in most insects, RNAi and genome editing technology should accelerate the study, leading toward understanding the origin of variable overt behavioral rhythms such as nocturnal, diurnal, and crepuscular activity rhythms.

Opsin transcripts of predatory diving beetles: a comparison of surface and subterranean photic niches

The regressive evolution of eyes has long intrigued biologists yet the genetic underpinnings remain opaque. A system of discrete aquifers in arid Australia provides a powerful comparative means to explore trait regression at the genomic level. Multiple surface ancestors from two tribes of diving beetles (Dytiscidae) repeatedly invaded these calcrete aquifers and convergently evolved eye-less phenotypes. We use this system to assess transcription of opsin photoreceptor genes among the transcriptomes of two surface and three subterranean dytiscid species and test whether these genes have evolved under neutral predictions. Transcripts for UV, long-wavelength and ciliary-type opsins were identified from the surface beetle transcriptomes. Two subterranean beetles showed parallel loss of all opsin transcription, as expected under 'neutral' regressive evolution. The third species Limbodessus palmulaoides retained transcription of a long-wavelength opsin (lwop) orthologue, albeit in an aphotic environment. Tests of selection on lwop indicated no significant differences between transcripts derived from surface and subterranean habitats, with strong evidence for purifying selection acting on L. palmulaoides lwop. Retention of sequence integrity and the lack of evidence for neutral evolution raise the question of whether we have identified a novel pleiotropic role for lwop, or an incipient phase of pseudogene development.

Opsin transcripts of predatory diving beetles: a comparison of surface and subterranean photic niches

The regressive evolution of eyes has long intrigued biologists yet the genetic underpinnings remain opaque. A system of discrete aquifers in arid Australia provides a powerful comparative means to explore trait regression at the genomic level. Multiple surface ancestors from two tribes of diving beetles (Dytiscidae) repeatedly invaded these calcrete aquifers and convergently evolved eye-less phenotypes. We use this system to assess transcription of opsin photoreceptor genes among the transcriptomes of two surface and three subterranean dytiscid species and test whether these genes have evolved under neutral predictions. Transcripts for UV, long-wavelength and ciliary-type opsins were identified from the surface beetle transcriptomes. Two subterranean beetles showed parallel loss of all opsin transcription, as expected under 'neutral' regressive evolution. The third species Limbodessus palmulaoides retained transcription of a long-wavelength opsin (lwop) orthologue, albeit in an aphotic environment. Tests of selection on lwop indicated no significant differences between transcripts derived from surface and subterranean habitats, with strong evidence for purifying selection acting on L. palmulaoides lwop. Retention of sequence integrity and the lack of evidence for neutral evolution raise the question of whether we have identified a novel pleiotropic role for lwop, or an incipient phase of pseudogene development.

Using phylogenetically-informed annotation (PIA) to search for light-interacting genes in transcriptomes from non-model organisms

Background

Tools for high throughput sequencing and de novo assembly make the analysis of transcriptomes (i.e. the suite of genes expressed in a tissue) feasible for almost any organism. Yet a challenge for biologists is that it can be difficult to assign identities to gene sequences, especially from non-model organisms. Phylogenetic analyses are one useful method for assigning identities to these sequences, but such methods tend to be time-consuming because of the need to re-calculate trees for every gene of interest and each time a new data set is analyzed. In response, we employed existing tools for phylogenetic analysis to produce a computationally efficient, tree-based approach for annotating transcriptomes or new genomes that we term Phylogenetically-Informed Annotation (PIA), which places uncharacterized genes into pre-calculated phylogenies of gene families.

Results

We generated maximum likelihood trees for 109 genes from a Light Interaction Toolkit (LIT), a collection of genes that underlie the function or development of light-interacting structures in metazoans. To do so, we searched protein sequences predicted from 29 fully-sequenced genomes and built trees using tools for phylogenetic analysis in the Osiris package of Galaxy (an open-source workflow management system). Next, to rapidly annotate transcriptomes from organisms that lack sequenced genomes, we repurposed a maximum likelihood-based Evolutionary Placement Algorithm (implemented in RAxML) to place sequences of potential LIT genes on to our pre-calculated gene trees. Finally, we implemented PIA in Galaxy and used it to search for LIT genes in 28 newly-sequenced transcriptomes from the light-interacting tissues of a range of cephalopod mollusks, arthropods, and cubozoan cnidarians. Our new trees for LIT genes are available on the Bitbucket public repository (http://bitbucket.org/osiris_phylogenetics/pia/) and we demonstrate PIA on a publicly-accessible web server (http://galaxy-dev.cnsi.ucsb.edu/pia/).

Conclusions

Our new trees for LIT genes will be a valuable resource for researchers studying the evolution of eyes or other light-interacting structures. We also introduce PIA, a high throughput method for using phylogenetic relationships to identify LIT genes in transcriptomes from non-model organisms. With simple modifications, our methods may be used to search for different sets of genes or to annotate data sets from taxa outside of Metazoa.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-014-0350-x) contains supplementary material, which is available to authorized users.

Evolution and development in cave animals: from fish to crustaceans

Cave animals are excellent models to study the general principles of evolution as well as the mechanisms of adaptation to a novel environment: the perpetual darkness of caves. In this article, two of the major model systems used to study the evolution and development (evo-devo) of cave animals are described: the teleost fish Astyanax mexicanus and the isopod crustacean Asellus aquaticus. The ways in which these animals match the major attributes expected of an evo-devo cave animal model system are described. For both species, we enumerate the regressive and constructive troglomorphic traits that have evolved during their adaptation to cave life, the developmental and genetic basis of these traits, the possible evolutionary forces responsible for them, and potential new areas in which these model systems could be used for further exploration of the evolution of cave animals. Furthermore, we compare the two model cave animals to investigate the mechanisms of troglomorphic evolution. Finally, we propose a few other cave animal systems that would be suitable for development as additional models to obtain a more comprehensive understanding of the developmental and genetic mechanisms involved in troglomorphic evolution.

The evolution of complexity in the visual systems of stomatopods: insights from transcriptomics

Stomatopod crustaceans have complex visual systems containing up to 16 different spectral classes of photoreceptors, more than described for any other animal. A previous molecular study of this visual system focusing on the expression of opsin genes found many more transcripts than predicted on the basis of physiology, but was unable to fully document the expressed opsin genes responsible for this diversity. Furthermore, questions remain about how other components of phototransduction cascades are involved. This study continues prior investigations by examining the molecular function of stomatopods' visual systems using new whole eye 454 transcriptome datasets from two species, Hemisquilla californiensis and Pseudosquilla ciliata. These two species represent taxonomic diversity within the order Stomatopoda, as well as variations in the anatomy and physiology of the visual system. Using an evolutionary placement algorithm to annotate the transcriptome, we identified the presence of nine components of the stomatopods' G-protein-coupled receptor (GPCR) phototransduction cascade, including two visual arrestins, subunits of the heterotrimeric G-protein, phospholipase C, transient receptor potential channels, and opsin transcripts. The set of expressed transduction genes suggests that stomatopods utilize a Gq-mediated GPCR-signaling cascade. The most notable difference in expression between the phototransduction cascades of the two species was the number of opsin contigs recovered, with 18 contigs found in retinas of H. californiensis, and 49 contigs in those of P. ciliata. Based on phylogenetic placement and fragment overlap, these contigs were estimated to represent 14 and 33 expressed transcripts, respectively. These data expand the known opsin diversity in stomatopods to clades of arthropod opsins that are sensitive to short wavelengths and ultraviolet wavelengths and confirm the results of previous studies recovering more opsin transcripts than spectrally distinct types of photoreceptors. Many of the recovered transcripts were phylogenetically placed in an evolutionary clade of crustacean opsin sequences that is rapidly expanding as the visual systems from more species are investigated. We discuss these results in relation to the emerging pattern, particularly in crustacean visual systems, of the expression of multiple opsin transcripts in photoreceptors of the same spectral class, and even in single photoreceptor cells.

Parallel reduction in expression, but no loss of functional constraint, in two opsin paralogs within cave populations of Gammarus minus (Crustacea: Amphipoda)

Background

Gammarus minus, a freshwater amphipod living in the cave and surface streams in the eastern USA, is a premier candidate for studying the evolution of troglomorphic traits such as pigmentation loss, elongated appendages, and reduced eyes. In G. minus, multiple pairs of genetically related, physically proximate cave and surface populations exist which exhibit a high degree of intraspecific morphological divergence. The morphology, ecology, and genetic structure of these sister populations are well characterized, yet the genetic basis of their morphological divergence remains unknown.

Results

We used degenerate PCR primers designed to amplify opsin genes within the subphylum Crustacea and discovered two distinct opsin paralogs (average inter-paralog protein divergence ≈ 20%) in the genome of three independently derived pairs of G. minus cave and surface populations. Both opsin paralogs were found to be related to other crustacean middle wavelength sensitive opsins. Low levels of nucleotide sequence variation (< 1% within populations) were detected in both opsin genes, regardless of habitat, and dN/dS ratios did not indicate a relaxation of functional constraint in the cave populations with reduced or absent eyes. Maximum likelihood analyses using codon-based models also did not detect a relaxation of functional constraint in the cave lineages. We quantified expression level of both opsin genes and found that the expression of both paralogs was significantly reduced in all three cave populations relative to their sister surface populations.

Conclusions

The concordantly lowered expression level of both opsin genes in cave populations of G. minus compared to sister surface populations, combined with evidence for persistent purifying selection in the cave populations, is consistent with an unspecified pleiotropic function of opsin proteins. Our results indicate that phototransduction proteins such as opsins may have retained their function in cave-adapted organisms because they may play a pleiotropic role in other important processes that are unrelated to vision.

An integrated transcriptome-wide analysis of cave and surface dwelling Astyanax mexicanus

Numerous organisms around the globe have successfully adapted to subterranean environments. A powerful system in which to study cave adaptation is the freshwater characin fish, Astyanax mexicanus. Prior studies in this system have established a genetic basis for the evolution of numerous regressive traits, most notably vision and pigmentation reduction. However, identification of the precise genetic alterations that underlie these morphological changes has been delayed by limited genetic and genomic resources. To address this, we performed a transcriptome analysis of cave and surface dwelling Astyanax morphs using Roche/454 pyrosequencing technology. Through this approach, we obtained 576,197 Pachón cavefish-specific reads and 438,978 surface fish-specific reads. Using this dataset, we assembled transcriptomes of cave and surface fish separately, as well as an integrated transcriptome that combined 1,499,568 reads from both morphotypes. The integrated assembly was the most successful approach, yielding 22,596 high quality contiguous sequences comprising a total transcriptome length of 21,363,556 bp. Sequence identities were obtained through exhaustive blast searches, revealing an adult transcriptome represented by highly diverse Gene Ontology (GO) terms. Our dataset facilitated rapid identification of sequence polymorphisms between morphotypes. These data, along with positional information collected from the Danio rerio genome, revealed several syntenic regions between Astyanax and Danio. We demonstrated the utility of this positional information through a QTL analysis of albinism in a surface x Pachón cave F(2) pedigree, using 65 polymorphic markers identified from our integrated assembly. We also adapted our dataset for an RNA-seq study, revealing many genes responsible for visual system maintenance in surface fish, whose expression was not detected in adult Pachón cavefish. Conversely, several metabolism-related genes expressed in cavefish were not detected in surface fish. This resource will enable powerful genetic and genomic analyses in the future that will better clarify the heritable genetic changes governing adaptation to the cave environment.