Spiders are among the world's most species-rich animal lineages, and their visual systems are likewise highly diverse. These modular visual systems, composed of four pairs of image-forming "camera" eyes, have taken on a huge variety of forms, exhibiting variation in eye size, eye placement, image resolution, and field of view, as well as sensitivity to color, polarization, light levels, and motion cues. However, despite this conspicuous diversity, our understanding of the genetic underpinnings of these visual systems remains shallow. Here, we review the current literature, analyze publicly available transcriptomic data, and discuss hypotheses about the origins and development of spider eyes. Our efforts highlight that there are many new things to discover from spider eyes, and yet these opportunities are set against a backdrop of deep homology with other arthropod lineages. For example, many (but not all) of the genes that appear important for early eye development in spiders are familiar players known from the developmental networks of other model systems (e.g., Drosophila). Similarly, our analyses of opsins and related phototransduction genes suggest that spider photoreceptors employ many of the same genes and molecular mechanisms known from other arthropods, with a hypothesized ancestral spider set of four visual and four nonvisual opsins. This deep homology provides a number of useful footholds into new work on spider vision and the molecular basis of its extant variety. We therefore discuss what some of these first steps might be in the hopes of convincing others to join us in studying the vision of these fascinating creatures.

A widely accepted model for the evolution of cave animals posits colonization by surface ancestors followed by the acquisition of adaptations over many generations. However, the speed of cave adaptation in some species suggests mechanisms operating over shorter timescales. To address these mechanisms, we used Astyanax mexicanus, a teleost with ancestral surface morphs (surface fish, SF) and derived cave morphs (cavefish, CF). We exposed SF to completely dark conditions and identified numerous altered traits at both the gene expression and phenotypic levels. Remarkably, most of these alterations mimicked CF phenotypes. Our results indicate that cave-related traits can appear within a single generation by phenotypic plasticity. In the next generation, plasticity can be further refined. The initial plastic responses are random in adaptive outcome but may determine the subsequent course of evolution. Our study suggests that phenotypic plasticity contributes to the rapid evolution of cave-related traits in A. mexicanus. INTRODUCTION 1A major problem in modern biology is understanding how organisms adapt to an 2 environmental change and how complex, adaptive phenotypes originate. Phenotypic 3 evolution can result from standing genetic variation, new mutations, or phenotypic plasticity 4 followed by genetic assimilation, but these processes are often difficult to distinguish in 5 slowly changing environments. This difficulty can be overcome by studying adaptation to 6 more abrupt environmental changes, such as the dramatic transition from life on the Earth's 7 surface to subterranean voids and caves. 8 A unifying feature of subterranean environments is complete darkness (1,2). Cave-adapted 9 animals have evolved a range of unusual and specialized traits, often called troglomorphic 10 traits, which enable survival in challenging conditions of the subsurface. In cave dwelling 11 animals, visual senses and protection from the effects of sunlight are unnecessary, and 12 consequently eyes and pigmentation are usually reduced or absent. To compensate for lack 13 of vision, other traits, especially those related to chemo-and mechano-receptor sensations, 14 are enhanced. Circadian rhythms that fine-tune organismal physiology with day-night cycles 15 are also distorted, and light dependent behaviors, as well as the neural and endocrine 16 circuits controlling these behaviors, are modified. Because photosynthetic organisms are not 17 present in caves, primary productivity is absent and nutrient availability is usually limited. 18 Survival under conditions of reduced and/or sporadic food resources is possible due to the 19 evolution of modified feeding behaviors and adaptive changes in metabolism, such as lower 20 metabolic rate, increased starvation resistance, and changes in carbohydrate and lipid 21 metabolism (1). 22 The ancestors of cave dwelling animals originally lived on the surface. Regardless of whether 23 the pioneering animals entered the subsurface accidently (by capture of surface waters or by 24 falling int...

A widely accepted model for the evolution of cave animals posits colonization by surface ancestors followed by the acquisition of adaptations over many generations. However, the speed of cave adaptation in some species suggests mechanisms operating over shorter timescales. To address these mechanisms, we used Astyanax mexicanus, a teleost with ancestral surface morphs (surface fish, SF) and derived cave morphs (cavefish, CF). We exposed SF to completely dark conditions and identified numerous altered traits at both the gene expression and phenotypic levels. Remarkably, most of these alterations mimicked CF phenotypes. Our results indicate that many cave-related traits can appear within a single generation by phenotypic plasticity. In the next generation, plasticity can be further refined. The initial plastic responses are random in adaptive outcome but may determine the subsequent course of evolution. Our study suggests that phenotypic plasticity contributes to the rapid evolution of cave-related traits in A. mexicanus.

We sequenced the complete mitochondrial genomes of two bat fly species within the Nycteribiidae (Diptera: Hippoboscoidea) – Dipseliopoda setosa (Cyclopodiinae) and Basilia ansifera (Nycteribiinae). Both mitogenomes were complete and contained 13 protein-coding genes, 22 tRNAs, and two rRNAs. Relative to the inferred ancestral gene order of dipteran mitochondrial genomes, no rearrangements were identified in either species. There were large differences in size between the two genomes, with D. setosa having a larger genome (19,164 bp) than B. ansifera (16,964 bp); both species had larger genomes than two previously published Streblidae bat fly species (e.g., Paradyschiria parvula and Paratrichobius longicrus ). The increased genome sizes were due to expansions in the control region and the non-coding region downstream of the light-strand origin of replication. Additional differences between the two mitogenomes included a significantly longer cox3 gene in B. ansifera and a longer nad1 gene in D. setosa . Interestingly, both genomes also had the lowest GC content ( D. setosa – 15.9%; B. ansifera – 17.0%) of any available Hippoboscoidea mitochondrial genome (18.8–23.9%). These mitogenomes represent the first sequences from species within the bat fly family Nycteribiidae. The sequence data here will provide a foundation for continued studies of genome evolution more generally within obligate blood-feeding ectoparasites, and specifically for the bat flies as vectors of significant ‘bat-associated’ viruses and microorganisms.

Copepod crustaceans are an abundant and ecologically significant group whose basic biology is guided by numerous visually guided behaviors. These behaviors are driven by copepod eyes, including naupliar eyes and Gicklhorn's organs, which vary widely in structure and function among species. Yet little is known about the molecular aspects of copepod vision. In this study we present a general overview of the molecular aspects of copepod vision by identifying phototransduction genes from newly generated and publicly available RNA-sequencing data and assemblies from 12 taxonomically diverse copepod species. We identify a set of 10 expressed transcripts that serve as a set of target genes for future studies of copepod phototransduction. Our more detailed evolutionary analyses of the opsin gene responsible for forming visual pigments found that all of the copepod species investigated express two main groups of opsins: middle-wavelength-sensitive (MWS) opsins and pteropsins. Additionally, there is evidence from a few species (e.g., Calanus finmarchicus, Eurytemora affinis, Paracyclopina nana, and Lernaea cyprinacea) for the expression of two additional groups of opsins-the peropsins and rhodopsin 7 (Rh7) opsins-at low levels or distinct developmental stages. An ontogenetic analysis of opsin expression in Calanus finmarchicus found the expression of a single dominant MWS opsin, as well as evidence for differences in expression across development in some MWS, pteropsin, and Rh7 opsins, with expression peaking in early naupliar through early copepodite stages.