The origin of the vertebrate jaw is one of the most long-standing and enigmatic topics in vertebrate evolution. To address this fundamental problem, lamprey larvae have long been the primary source of information on the cranial system of jawless vertebrates. However, their cranial anatomy remains contentious, largely because previous anatomical studies have relied primarily on conventional methodologies, leaving room for subjective identification. Here, we focused on cranial muscles whose morphology and innervation have been particularly controversial, performing high-resolution and objective imaging of the muscles and trigeminal nerve branches in three dimensions using various advanced techniques, including micro-computed tomography and light-sheet microscopy. Our detailed observations revealed several key characters that have previously been overlooked. First, an undescribed upper lip muscle is innervated by a sub-branch of the second major branch of the trigeminal nerve, which had long been thought to be purely sensory. Second, most lower lip muscles exhibit dual innervation by the second and third major branches of the trigeminal nerve. Last, the topographical arrangement of the lower lip muscle was elucidated in detail for the first time, suggesting that this muscle gives rise to the musculature of adult lingual apparatus through metamorphosis. Our three-dimensional data revises previous views on the cranial morphology of lamprey larvae and provides a solid foundation for comparative morphological and developmental studies across extinct and extant vertebrates. In addition, the cutting-edge imaging methods employed here could be applied to metamorphic and adult lampreys, offering a promising approach for elucidating the morphogenesis of the adult head. Taken together, this study opens new avenues for evolutionary developmental and paleontological research on the early evolution of vertebrate heads.
Bristly millipedes face many challenges regarding fossilisation (i.e., soft body, hidden living environments) and are clearly under-represented in the literature. Surprisingly, they are quite common in amber, offering an exceptional window to investigate their evolutionary history of the past 100 million years. Hundreds of inclusions in amber that cover the almost complete ontogenetic sequence of both Polyxenidae and Synxenidae have been investigated, and for each specimen distances between ten landmarks are measured. In contradiction to previous expectations, representatives of both Polyxenidae and Synxenidae have significantly changed through time, going from “dwarfs” during the Cretaceous ( 100 Ma) to “giants” during the Eocene ( 35 Ma); extant representatives are found in-between. This first attempt in investigating morphological changes in bristly millipedes may suggest the potential influence of the environment on the evolutionary trajectories within this group. The Cretaceous was a time of warmer climates, with tropical rainforests on the Burma Terrane, while the Eocene and modern-day representatives are from equable and temperate regions. Potentially, the environmental conditions on the Burma Terrane pressured bristly millipedes to reach “dwarf” sizes. The observed signals may be an indication of the Temperature-Size Rule, favouring quick development while slow growth in warmer ecosystems.
Understanding the concept of deep time is essential for comprehending evolutionary processes, yet research indicates that learners often struggle with grasping vast geological time scales. This study examines the understanding of deep time of first-year university students across 26 European countries and examines its relationship with knowledge about evolution and acceptance of evolution. We assessed students’ estimations of key historical events, including the origin of life, the existence of dinosaurs, and the emergence of humans. The results indicate that while most students correctly sequence these events, many hold substantial misconceptions regarding their absolute time frames and points in time. Statistical analyses reveal a weak correlation between the understanding of deep time and knowledge of evolution, while no significant correlation was found between the understanding of deep time and acceptance of evolution. These findings suggest that deep time is a distinct conceptual domain that requires targeted educational approaches. Additionally, notable differences between country samples highlight the potential influence of curriculum design, cultural factors, and prior education. Given the fundamental role of deep time in understanding evolutionary change, climate change, and biodiversity loss, there is a clear need for effective educational strategies and innovative teaching tools to enhance students’ comprehension.
The mammalian jaw joint between the dentary bone of the lower jaw and the squamosal/temporal bone of the upper jaw is a mammalian evolutionary innovation that enables complex movements essential for chewing and speaking. It comprises the mandibular condyle, the glenoid (mandibular) fossa, and an interposed fibrocartilaginous disc. The disc is critical for joint function, and its pathology underlies several temporomandibular disorders (TMD), which can affect up to a third of adult and elderly patients. The mammalian jaw joint is proposed to have evolved to stabilize the existing primary jaw joint, found in all non-mammalian jawed vertebrates, acting as a buttress for the lower jaw against the upper to prevent jaw dislocation. The origin of the disc is less well understood, but it has been hypothesized to have evolved either from the top layers of the fibrocartilaginous condyle, or as an independent structure, potentially evolved from an existing tendon. To resolve these relationships, the condyle and disc were analysed during murine development to shed light on their molecular and cellular identity during formation of the joint. Using Bulk RNA-seq we demonstrate that the forming temporomandibular joint (TMJ) disc exhibited a developmental trajectory distinct from that of the mandibular condyle, with Lgr5 expression serving as a marker of the developing disc from early developmental stages. Clonal tracing of cells at the interface between the condyle and disc, using Pcag-CreER;Confetti mice, further demonstrated that the disc cells formed a distinct compartment compared to the superficial layers of the condyle. Notably, disc identity was preserved in Wnt1-Cre; Sox9fl/fl mutants where the condyle failed to form. Together, these findings establish the TMJ disc as having a distinct molecular and cellular identity, initiating independently from the condyle, while sharing many features in common with a tendon. Together these findings support the TMJ disc as evolving from a tendon rather than from the condyle.
Course-based undergraduate research experiences (CUREs) have been transformative for enhancing undergraduate students’ opportunities in science. In response to the COVID-19 pandemic, traditional course methods were adapted to online and digital formats, and faculty explored methods for continued student learning despite limited access to physical laboratories, including CUREs. The current study presents the methods of development and refinement of a Digital Course-Based Undergraduate Research Experience (DCURE) as an accessible, long-term approach in STEM education. We define a DCURE as a CURE with a substantial digital component in data collection and data analysis, while still meeting the established criteria for a CURE (Auchincloss et al. in CBE Life Sci Educ 13(1):29–40, 2014). Importantly, we distinguish between digital (use of software and computational tools) and online (mode of course delivery), noting that DCUREs may be implemented in in-person, hybrid, or online formats. We describe the process of transitioning a plant evolutionary ecology CURE that examines the effects of mutations on plant traits across environments to a DCURE. Our DCURE model leverages common digital tools, such as ImageJ, for data collection and analysis, while preserving the core elements of CUREs, including hypothesis testing, data interpretation, and scientific collaboration. Through an iterative refinement process, we successfully transitioned the student experience from live plants in the classroom into a plant image-based format, demonstrating an example of a DCURE in evolutionary biology. Additionally, the DCURE model enhances student accessibility by reducing barriers associated with laboratory access, or resource limitations and geographic constraints when a network of faculty collaborate across institutions. This paper provides a detailed overview of the DCURE transition process and offers practical recommendations for evolutionary biology educators seeking to transition current laboratory activities or implement digital research experiences to meet the needs of students across academic settings.
Evolutionary scientists have recently expressed concern at the political weaponization of their research findings by online communities of laypeople. This is the case for genetics research among white supremacists, but also of evolutionary psychology among the online antifeminist communities of the “manosphere.” Yet, so far, there has been no empirical assessment of the evolutionary scientific literacy and scientific knowledge acquisition practices of these lay Darwinian aficionados. The present exploratory survey study (n = 148) aimed to start filling this gap by comparing results of English-speaking manosphere respondents on a science literacy questionnaire with those of a Prolific US respondent sample (n = 151). Manospherians exhibited high educational levels, including scientific higher education, and an almost unanimous acceptance of evolutionary theory. Moreover, they scored much better than a counterpart group of US respondents on the science literacy questionnaire (70.4
Ernst Haeckel’s biogenetic law was critically revised by several post-Haeckelian morphologists. Among the most significant between the two world wars were the German zoologist Victor Franz and the Russian morphologist Alexei Sewertzoff. Both were committed Darwinians who developed analogous theoretical systems consisting of two interconnected components: a revised version of the biogenetic law and a theory of evolutionary directions. The two biologists knew each other personally, and their developmental theories shared substantial common ground. Nevertheless, the trajectories of their ideas diverged sharply. Sewertzoff founded a powerful school of evolutionary morphology, and his student Ivan Schmalhausen integrated Sewertzoff’s concepts into the Soviet formulation of the Evolutionary Synthesis and subsequently into the broader international Darwinian movement. Franz, by contrast, attempted to employ his theory to provide a biological justification for Hitler’s dictatorship; after World War II, his theoretical contributions were largely forgotten and remained of interest only to historians of science. Despite the apparent similarity of their theories, a crucial difference distinguishes them. Sewertzoff’s theory was highly elaborated, grounded in an extensive body of empirical research, and articulated with maximal scientific precision. Franz, by contrast, remained speculative in both his conception of development and his understanding of evolution. Although he claimed to follow the intellectual lineage of Haeckel and Goethe in his vision of evolutionary progress, his actual argumentation remained decidedly speculative. The speculative nature of his theoretical system made it unusually easy to adapt to ideological purposes during the Nazi regime. Franz approached the problem of ontogeny and phylogeny in a manner similar to Sewertzoff; however, he remained at an abstract level and did not engage with the empirical detail thoroughly elaborated by Sewertzoff. As Schmalhausen had already pointed out, Franz’s theory of evolutionary directions was one-sided, taking into account only a single aspect: centralization accompanied by differentiation. Franz pushed this principle even further, extending it to the sphere of social evolution in an attempt to justify Hitler’s absolute dictatorship. The speculative nature of Franz’ hypothesis made it vulnerable for ideological distortions.
RNA interference (RNAi) is a natural antiviral defense mechanism in plants and animals. As a counter defense strategy, most viruses have evolved viral suppressors of RNAi (VSRs) to antagonize the RNAi pathway. Here, we utilized transgenic misexpression of a VSR from Cricket Paralysis virus (CrPV1A) to dampen RNAi in a temporal and life-stage specific way in order to overcome limitations of knocking down pleiotropic genes by the strong systemic RNAi response in the red flour beetle Tribolium castaneum. We found that ubiquitously driven VSR rescued the sterility of the females injected with Tc-axin or Tc-decapentaplegic double-stranded RNA, where sterility had previously hampered analysis. By overcoming sterility using this tool and by rescuing zygotic function with a heat-shock driven VSR, we were able to separate maternal from zygotic function for the Wnt pathway inhibitor Tc-Axin. Thereby, we could provide evidence that maternal Wnt signalling alone is responsible for axis formation in Tribolium. Our tool opens new experimental possibilities such as studying genes by parental RNAi, which would normally lead to sterility and separating maternal from zygotic gene functions. Further improvements are required to allow for studying zygotic gene function while rescuing maternal functions and for spatially restricting the RNAi effect.
Mutations in Myostatin (MSTN) coding-region are known to dramatically enhance muscle mass. In contrast, the impact of variations in MSTN cis-regulatory elements remains poorly understood. To shed light on this issue, we analysed the MSTN promoter/enhancer (MSTN P/E) using a phylogenetic framework, focused on comparing transcription factor binding site (TFBS) patterns across vertebrates. We identified a conserved core shared by jawed vertebrates, which arose in the MSTN P/E before the cartilaginous and bony vertebrates divergence. Within this core, we found clade-specific TFBS arrangements. In addition, we detected an ancient duplication of the MSTN P/E core, which generated additional layers of TFBS conservation across amniotes. The genetic mechanisms driving diversification in the core and duplicated regions include TFBS gain/loss, turnover, and overlapping. Notably, distinct MSTN expression in chicken and mouse embryos somites appears to correlate with structural variations in the MSTN P/E, suggesting adjustments in MSTN expression. Finally, we observed signatures of accelerated evolution in the stem branches of Sauropsida and Mammalia and in the branches leading to Aves, Rodentia and Chiroptera. Our work enables future studies to explore how variations in MSTN P/E may be linked to skeletal muscle trait variation and evolutionary innovation.
Arthropod segmentation involves not only embryonic patterning but also postembryonic segment addition through molting, a process known as anamorphosis. In millipedes (class Diplopoda), which universally undergo anamorphosis, new segmental units also known as “rings” are added anterior to the telson, yet the morphogenetic mechanisms underlying this process remain poorly understood. Here, we investigate ring addition during anamorphosis in the millipede Niponia nodulosa (Polydesmida, Cryptodesmidae), focusing on epidermal and muscular organization. Using X-ray micro-computed tomography and microscopic histological analysis, we reconstructed the sequence of tissue changes preceding molting. Our results show that a local invagination of the epidermis between the telson and the penultimate ring establishes a ring primordium, whose subsequent development gives rise to a new ring. In stadia where multiple rings are added with one molt, a corresponding number of ring primordia appeared. In addition, the muscle originally linking the penultimate ring to the telson detaches and reattaches to the newly added ring, while additional muscles differentiate within the new ring to form novel connections with the telson. These coordinated changes occur rapidly before molting, enabling ring addition within the restricted posterior body region. We provided a detailed three-dimensional description of the morphological changes accompanying ring addition. Epidermal morphogenesis and muscular rearrangement are thought to enable the addition of new rings while maintaining the functional integrity of the posterior body, including defecation. This study establishes a histological and morphological basis for understanding segmentation during postembryonic development in arthropods and paves the way for future developmental investigations.
Congenital Heart Disease (CHD) is currently the most common birth defect, affecting approximately 1% of live births. Understanding the genetic mechanisms underlying CHD is crucial for developing effective interventions. This paper explores the functional and evolutionary roles of the T-box transcription factors Tbx2a and Tbx2b in zebrafish heart development. Functional studies in zebrafish demonstrated that both homozygous and heterozygous mutations in tbx2a and tbx2b result in heart looping defects, challenging the assumption of their redundancy and indicating unique, non-overlapping functions essential for cardiac development. The observed phenotypic variability in heterozygous mutants suggests a complex interplay between these genes and highlights the sensitivity of cardiac development to precise gene dosage. Utilizing advanced bioinformatics techniques, we reconstructed the ancestral sequences of these genes to understand their evolutionary trajectory and functional divergence. Our analysis revealed strong evolutionary conservation of the T-box DNA-binding domain in Tbx2a and Tbx2b, suggesting that both proteins remain constrained to recognize the same core cis-regulatory elements. This conservation implies that their ability to bind essential cardiac target genes is functionally indispensable and under strong purifying selection. We observed unique and shared amino acid substitutions, indicating potential adaptive changes and conserved functions. Given their highly conserved DNA-binding domains, we hypothesized that Tbx2a and Tbx2b interact with different nuclear factors to regulate distinct sets of genes. Mass spectrometry-based proteomics provided insights into the unique nuclear interactions of Tbx2a and Tbx2b, supporting the hypothesis of their functional divergence. Overall, this research offers new insights into the functional and evolutionary roles of Tbx2a and Tbx2b in heart development, with implications for understanding the genetic basis of CHD.
Understanding the general relationship between animal ontogeny and phylogeny remains a long-standing issue in evolutionary developmental biology. Historically, observation of these two phenomena, which involve increasing complexity, led to a series of hypotheses culminating in Haeckel’s recapitulation theory in the 1860s. These classical theories posited the conservation of earlier developmental stages in what is sometimes referred to as the ‘funnel model’ of development. However, in the 21st century, a body of evidence accumulated that robustly supported the “developmental hourglass model”, first proposed by Duboule, in which the mid-embryonic phylotypic period is highly conserved. Despite this empirical progress, the mechanisms underlying such conservation have remained elusive. Recent empirical studies have begun to indicate that this conservation may stem from the reduced potential of embryos at this stage to create phenotypic variations. Nevertheless, the precise mechanisms underlying this lower evolutionary potential remain unclear. In this review, we outline historical and recent perspectives and findings on the relationship between ontogeny and phylogeny, contrasting historical assumptions with emerging evidence concerning the mechanisms of embryonic evolution. We particularly highlight the possible role of lower evolvability, referring to intrinsic properties of developmental systems that pontentially constrain phenotypic diversification.
The remarkable morphological disparity of the animal kingdom is underpinned by changes in embryonic development across the tree of life; as such, deciphering evolutionary patterns of developmental divergence depends on investigations of different species across a range of comparable developmental stages. Among the most influential ideas regarding such developmental divergences are von Baer's Laws of Development and Haeckel's Theory of Recapitulation. Here, we assess several predictions following from these ideas at the tissue-level by comparing skull osteogenesis in representatives of the bird clade Galloanserae. We investigated high-resolution µCT scans of embryonic series for four galloanseran species: chickens and quails, representing Galliformes (landfowl), and ducks and geese, representing Anseriformes (waterfowl). To compare skull osteogenesis across our taxon sample, we devised a skull-specific staging system based on ossification sequences to discretise the process into five stages. During skull osteogenesis, we found that the location of the onset of ossification within each element and the direction of ossification progression were the same in all species in our sample, implying a conserved developmental programme for induction and ossification progression across Galloanserae. Moreover, we found that the appearance of synapomorphies diagnostic of broader clades often overlapped with species-specific ones during osteogenesis. Indeed, many diagnostic features of deep clades, such as osteological synapomorphies of the phylogenetically inclusive clade Galloanserae, appear at surprisingly late stages of development. These observations fail to support several predictions of von Baer's Laws of Development and Haeckel's Theory of Recapitulation, instead suggesting what we term a 'braiding' pattern of developmental divergence in which degrees of interspecific morphological similarity wax and wane during development as a result of the interplay between developmental constraints and phyletic variation.
Hydnora (Hydnoraceae) comprises a few parasitic species exceptional among the autotrophic members of the perianth-bearing Piperales. Flowers in the genus are thick, fleshy, sapromyophilous, and develop into massive, polyspermous fruits. They are formed directly along underground rhizomes that parasitize species of Euphorbiaceae and Fabaceae. Due to its peculiar floral morphology and lack of leaves, Hydnora is often dubbed 'the strangest plant in the world'. Here we generated the first transcriptomes of Hydnora visseri from the dissected rhizome, perianth, osmophore, stamen, carpel, and fruit. Our results suggest that Hydnora possesses one of the simplest developmental genetic toolkits for flowering and floral organ identity among angiosperms, further emphasizing its uniqueness. We detected that most of the photoperiodic flowering integrators are expressed. In contrast, regulators of the autonomous pathway and circadian clock were notably absent from the transcriptomes. Conversely, we identified an intact genetic toolkit linked to floral organ identity and fruit development in Hydnora. Through positional homology and gene expression data, we inferred that the perianth of Hydnora corresponds to the calyx, and that the osmophores are late sepal elaborations. Additionally, the expression patterns of genes responsible for stamen, carpel, and ovule identity align with the canonical ABCDE model. Finally, we recorded large-scale duplications in putative perianth identity genes prior to the diversification of all perianth-bearing Piperales. This study serves as an additional comparative point for assessing the evolutionary onset of holoparasitic plants, as Hydnora and its sister genus Prosopanche are likely the earliest branching representatives of this lifestyle across angiosperms.
Within the symbiont-hosting Siboglinidae (Annelida), Osedax stands out as the sole genus capable of degrading bones and displaying pronounced sexual dimorphism (except O. priapus). While macroscopic, gutless females feed on whale falls with their symbiont-housing “roots”, males are microscopic and non-feeding. Yet, embryos and larvae look identical, and sex is suggested to be environmentally determined, i.e., larvae metamorphose into females on bare bone or into males when finding an adult female. However, we here describe a transient gut present in half of the late larvae and in juvenile females of O. japonicus. We confirm the gut-carrying larvae as being females from sex-specific in situ gene expression. Moreover, morphological evidence coupled with differential gene expression indicate that the ‘non-feeding’ transient gut may pattern the vascular system and/or act as a gas-exchange surface in juvenile females, before their branchial appendages develop. The transient gut of O. japonicus females reveals a genetic sex determination. Proposedly homologous across siboglinids, this vestigial gut is suggested to function in organ patterning and/or for gas-exchange during development of the gutless adult.
Some aspects of the life cycle of the scyphozoan jellyfish Cassiopea xamachana have been described in detail. Investigations of C. xamachana have largely focused on strobilation and the unusual pattern of planuloid budding at the polyp stage, in which the body wall of the polyp forms a swimming planuloid bud that shows morphological and behavioral similarities to the planula. Here, we fill gaps in our understanding of C. xamachana life history by characterizing embryonic development and planula settlement and metamorphosis. These processes happen in a manner similar to other scyphozoans studied. Gastrulation occurs by invagination, as in many other scyphozoans. Morphological observations of planula settlement and metamorphosis resemble observations of the process in Aurelia, the other well-studied scyphozoan, though some details about germ layer fates remain unclear. We also show that homeobox genes expressed during planula development are redeployed in a similar pattern in the planuloid bud. In the newly settled polyp, one of these genes is expressed in a pattern that breaks radial symmetry, extremely unusual in a scyphozoan. Our results set the stage for more detailed molecular dissections of morphogenesis in organisms with metagenic life cycles.
Lineage-specific adult structures form through modifications of pre-existing juvenile body parts during postembryonic development in insects. It remains unclear how these novel traits originate from ancestral structures within the constrained body plan. In the coffin-headed cricket Loxoblemmus equestris, an ancestral rounded head shape directly transforms into a flattened derived form in a sex-specific manner. To understand the origin of novel traits, we investigated the development of the adult head in L. equestris as a model of lineage-specific novelty. We found that head morphologies remained sexually monomorphic until the final molt, and the male-specific head shape emerged in the frons region during the transition to adulthood in L. equestris. Two- and three-dimensional morphological analyses revealed that the sexual dimorphism in the frons epithelial folding patterns appeared in the late final nymphal instar. These results suggest that the male-specific novel head development is linked to the final molt in L. equestris. We tested this hypothesis by knocking down the metamorphic gene network (MGN) comprised of Krüppel-homolog 1 (Kr-h1), broad (br), and Ecdysone induced protein 93F (E93). Despite the timing shifts of the nymph-to-adult transition caused by knockdown of the MGN, male-specific head structures are formed only after the final molt. These results demonstrate that the novel male head structures are formed during the final molt through the formation of sex-specific epithelial patterns in L. equestris. This highlights the unique metamorphic lifecycle with the final molt as a driver that has created lineage- and sex-specific adult forms in insects.
The development of the digits (fingers/toes) provides an excellent model for analyzing the molecular regulation of skeletal morphogenesis in vertebrates. Digits develop in the autopod as radial chondrogenic condensations separated by interdigital spaces containing undifferentiated skeletal progenitors destined to die by apoptosis. In avian species, leg digits are characterized by a differential size, with the first digit being short and the fourth largest. In vitro experiments using micromass cultures of digit progenitors demonstrated that RA controls the balance between cell death, cell proliferation, and cell differentiation in a dose-dependent fashion. In vivo, qPCR analysis revealed that the RA-synthesizing enzyme Raldh2 and the RA-degrading enzyme Cyp26a1 are expressed in the interdigits in an inverse gradient that correlates with the size of the digit adjacent to each interdigit. RA gain- and loss-of-function experiments via pharmacological approaches confirmed a close correlation between interdigital RA and digit size. A low concentration of RA applied to the first interdigits, when the phalanxes of the first digit are being formed, promoted mesodermal cell proliferation and caused elongation of digit 1, while blocking RA synthesis into the third interdigit inhibited cell proliferation, followed by a reduction in the size of digits 3 and 4. This study reveals a potential role for Retinoic Acid (RA) expressed in the interdigits in the regulation of the differential digit size. The morphological similarity of the digit patterns obtained in our experimental assays with those of other tetrapods suggests an evolutionary role of RA in determining digit morphology.
Evidence suggests that Pax6 genes are necessary for the specification of eyes in a variety of metazoans, including mandibulate arthropods. In these arthropods, Pax6 genes usually interact with a conserved set of genes, collectively called the retinal determination gene network (RDGN), to specify eye cells. However, recent data have argued that Pax6 genes lack a role in the development of the eyes in Chelicerata (= arachnids, horseshoe crabs, and sea spiders). A genome sequence of the eyeless mite Archegozetes longisetosus revealed that it retains two Pax6 paralogs, as well as singleton orthologs of all RDGN genes. We hypothesized that the retention of these two Pax6 paralogs could be due to their non-eye determining roles, and/or their expression in vestigial eye primordia. We therefore used hybridization chain reactions (HCRs) to follow the embryonic expression of these genes. To provide a basis for understanding RDGN expression patterns, we developed a staging system for A. longisetosus head development. This showed the presence of structures that in other arachnids form neural components of all eye types. We then showed that two genes in the RDGN of eyed arachnids, i.e., sine oculis and atonal, are expressed in a manner that are suggestive of vestigial eye primordia. We also found that the expression of the Pax6 paralogs was consistent with their roles in the development of the central nervous system. By co-staining for these genes with the conserved head-patterning gene orthodenticle, we observed early expression patterns of these genes in the brains of early A. longisetosus embryos that are comparable to those arachnids with embryonic eyes. Our data provide support for the hypothesis that the retention of Pax6 genes in A. longisetosus is due to their non-eye patterning roles. Furthermore, our survey of RDGN gene expression also provides support that A. longisetosus patterns vestigial eye primordia. Lastly, our data suggest that the Pax6 genes, with orthodenticle, acts to specify the ancestral arachnid brain. We then discuss our results considering eye loss in other arachnids.
