Current Biology

Eye loss has long fascinated evolutionary biologists and occurs across the animal kingdom. Spiders have two parallel visual systems -- two primary and six secondary eyes -- but eye losses, leaving six, four, two, or no eyes, have occurred in multiple lineages. Despite their significance, reports of eye loss are scattered, limiting broader analysis. Here we present the first comprehensive analysis of eye loss across all known spider lineages. We show that eye loss occurs in [~]12% of extant species, mainly within the clade Synspermiata. Six-eyed spiders are most common (>5,300 species), while four-eyed, two-eyed, and eyeless forms are rarer and often linked to troglobitic lifestyles. Principal eye loss is widespread, occurring in 49 families across nearly all major lineages. Using a recent phylogeny of the order Araneae, we demonstrate a strong correlation between eye loss and occupancy of low-light environments, but this is complicated by differential effects across eye types and phylogenetic groups through geological time. These findings reveal striking lability in eye number and lay groundwork for future research into ecological, developmental, and neurological drivers of eye loss. [hidden Markov models, ancestral state reconstruction, Araneae, discrete character evolution, principal eyes, secondary eyes, low light environments].

Carnivorous plants have been the subject of fascination and research ever since Darwin codified the subject in his 1875 book Insectivorous Plants. The origin of complex trapping mechanisms from structures adapted for photosynthesis is of particular interest. While Darwin proposed a plausible scenario for the origin of the snap traps of the Venus flytrap from simpler adhesive traps, the origin of the tiny and complex bladder traps of the genus Utricularia mystified Darwin and many subsequent workers, despite Utricularia being the most diverse genus of carnivorous plants. In this study, we test the "pitcher hypothesis," which proposes that Utricularia bladder traps evolved gradually from an adhesive trap ancestor, via an extinct pitcher trap intermediate. To overcome the lack of any fossil evidence for this scenario, we constructed a variety of continuous-time Markov chain (CTMC) models, each of which consists of a transition matrix allowing or disallowing certain transitions between 11 types of traps. We assembled available phylogenetic trees for 436 carnivorous plant species and noncarnivorous outgroups, classified each species by trap type, and statistically compared the fit of 18 CTMC models using Maximum Likelihood and statistical model comparison with Akaike Information Criterion. The best-fitting model (PH-7R-AAI), consistent with our pitcher hypothesis, had an AIC weight of 60%, with two similar models accounting for the remaining 40%. These results support a circuitous stepwise evolutionary pathway to the bladder trap, and demonstrate how a detailed stepwise evolutionary scenario may be statistically tested even without direct fossil evidence of key intermediate stages.

Acoustic communication signals often contain complex temporal structure, yet the features underlying signal recognition remain poorly understood, particularly in eavesdropping receivers. The parasitoid fly Ormia ochracea localises host crickets by eavesdropping on their calling songs. In Florida, preferred host songs consist of sound pulses repeated at ~50 pulses/s, and flies exhibit matching preferences. However, it remains unclear whether this preference reflects sensitivity to individual temporal features (e.g., pulse duration, interpulse interval) or to derived temporal relationships (e.g., pulse rate, pulse period, duty cycle) that emerge from their combination. We independently varied pulse duration and interpulse interval across a broad stimulus space and quantified tethered-walking phonotaxis using a switch-following paradigm. Behavioural responses formed a structured tuning surface, with high performance along a diagonal corresponding to 50 pulses/s, as well as elevated responses for a restricted range of pulse durations across a wide range of interpulse intervals. Responses failed to collapse across stimuli sharing the same pulse rate or pulse period, indicating that these features alone do not determine recognition. Instead, behaviour was best explained by the interacting effects of pulse duration and interpulse interval. These results demonstrate that song recognition in O. ochracea is multidimensional, with pulse rate tuning emerging from an underlying feature space rather than a single encoded parameter.

The sizes and shapes of organs are established by the combined actions of cell proliferation and cell growth. In plants, development of the determinate planar leaf is initiated by primordia formation and establishment of abaxial/adaxial polarity [1,2,3]. Lamina outgrowth is driven by cell division and growth along proximo-distal (PD) and medio-lateral (ML) axes [4], established by mutually repressive PD gradients of miRNA and target transcription factors [5,6,7,8]. These gradients generate proximal regions of competence for cell division and increased growth, with distal regions of reduced growth, endoreduplication and differentiation. The transition from proliferation to growth and differentiation is marked by a cell cycle arrest front, which moves basipetally during leaf growth, progressively restricting proximal proliferative zones as the leaf grows [9,10,11]. Intersection of proximal proliferation-promoting gradients with distal differentiation-promoting gradients may delineate the arrest front, but its dynamics remain poorly understood. We reasoned that mutants affecting cell proliferation patterns may provide insights into the formation, maintenance and dissolution of the arrest front. Spatio-temporal modelling of live imaging data of loss of function mutants of the regulatory peptidase DA1 and its E3 ligase activator Big Brother (BB), which increase cell proliferation [12,13], showed that these proteins effectively establish a threshold cell size at division as a function of distance from the base of the growing leaf and the duration of growth. Loss of BB and DA1 activities increased the persistence of cell divisions and dissolved the arrest front. This suggested that the arrest front emerges from the interactions of threshold areas of cell division with the cessation of division over time, and not from an independently-specified boundary.

Mosquitoes undergo development as aquatic larvae and pupae before emerging as terrestrial adults. Accordingly, blood-fed and mated female mosquitoes must select an appropriate egg-laying site to maximize the fitness of their offspring. Female yellow fever mosquitoes (Aedes aegypti) lay their eggs above the waterline of small containers or natural bodies of water, where they can remain dormant for many months until they are submerged and hatch. Here, we show that female mosquitoes use surface texture as a powerful cue to guide egg-laying decisions, selecting rougher textures over smooth when choosing among containers and when selecting specific sites within a given substrate. In addition, we identify an interaction between substrate texture and water salinity with respect to egg-laying decisions, demonstrating that female mosquitoes integrate competing cues to determine the ultimate suitability of an egg-laying site. Finally, we explore the dynamics of local egg-laying search behaviour, demonstrating that texture modulates traversal speed while mosquitoes search for appropriate egg-laying sites.

Hummingbirds are known for sustained hovering powered by rapid and continuous wingbeats. Here, we describe and quantify a novel flight behavior--intermittent hovering--in which hovering hummingbirds momentarily pause their wing motion mid-air but maintain their vertical position in space, keeping their wings fully extended at the end of the upstroke. We present the first systematic account of flap-pauses and wing coloration across hummingbirds, and evaluate potential morphological and ecological correlates, as well as evolutionary patterns in the expression of this behavior. Slow-motion footage from 86 species spanning all nine major hummingbird clades shows that at least 45 species exhibited flap-pauses during sustained hovering. Phylogenetic comparative analyses revealed that hovering pauses are evolutionarily conserved and significantly associated with both greater body mass and longer wings. Furthermore, we found that the 16 species in our study with colored underwings also exhibit significantly longer wings. The convergence of intermittent hovering, wing elongation, and chromatic traits leads us to hypothesize that this flight behavior plays a role in visual and/or auditory communication.

Planarian flatworms represent one of the most evolutionarily informative nervous systems for an account of ancient bilaterian brains. Likewise, the unparalleled regenerative ability of planarians makes possible certain investigations of neural development, memory, and behavior that are simply impossible with other model organisms. Despite these facts, learning and memory are today underexplored in planarians, likely due in part to the shadow of controversial 20th-century experiments on the transfer of memories between individual flatworms. Here, we attempted to replicate and extend the classical conditioning experiments in planarians that were the basis of the later memory transfer work. We failed to find evidence for classical conditioning in any of our procedural variations and obtained similar results using computer vision methods to avoid subjectivity in manual video annotation. Our results cast doubt on the suitability of planarian flatworms for studying primitive learning processes and the molecular basis of memory using classical conditioning.

Sensory systems provide animals with essential information about their environment and are critical for generating appropriate behaviours. Mechanosensation is a fundamental component of this sensory repertoire, and disruption of mechanosensory pathways can have severe functional consequences. In the nematode Caenorhabditis elegans, mechanosensory circuits have been extensively characterized and mediate touch-driven navigation and avoidance. These circuits rely on conserved molecular components including the stomatin-like protein MEC-2 along with MEC-6, which function together in the mechanotransduction complex. In contrast, the predatory nematode Pristionchus pacificus has repurposed mechanosensory pathways to also enable prey detection, a derived ecological behaviour. As we previously demonstrated that Ppa-mec-6 is required for efficient predation, here we assessed if Ppa-mec-2 plays a similar role in P. pacificus prey detection. We find that while Ppa-mec-2 is required for the aversive touch response, it is dispensable for prey detection. This functional divergence reflects differential neuronal expression as Ppa-mec-2 is absent from the IL2 neurons that mediate prey detection and also robustly express Ppa-mec-6. These findings reveal that partitioning of mechanosensory components across neuronal types enables functional specialization, demonstrating how conserved sensory machinery can support distinct behavioural functions across evolution.

Parasitic nematodes infect over a billion people worldwide and cause some of the most prevalent neglected tropical diseases1-5. Many of these parasites are skin penetrating and have both extra-host life stages that inhabit host feces and surrounding soil, and intra-host life stages that inhabit host niches such as skin, vasculature, and intestine2,6-8. Across life stages, these parasites encounter oxygen (O2) levels that range from ~21% at the soil-air interface to near-anaerobic levels in the host intestine9-12. However, whether parasitic nematodes detect and respond to changes in O2 levels was unknown. Here, we examine O2 sensation in skin-penetrating parasitic nematodes and find that they show robust responses to changes in O2 levels. Moreover, their O2-evoked behaviors differ from those of the free-living nematode Caenorhabditis elegans. We then investigate the molecular and neural mechanisms of O2 sensing in Strongyloides stercoralis, a genetically tractable human-infective nematode, and find that parasite-specific behavioral responses to O2 arise in part from evolutionary changes in their soluble guanylate cyclase repertoire. Finally, we find that neuronal O2 sensing regulates intra-host development in S. stercoralis. Our results demonstrate that skin-penetrating nematodes exhibit neuronally mediated O2 responses that are critical for multiple steps of their parasitic life cycle.

The ability of flying insects to locate distant food and mates by tracking odor plumes through turbulent and unsteady flow represents a remarkable feat of sensorimotor integration. Successful navigation requires not only extracting a reliable directional estimate from an intermittent olfactory signal, but also contending with the challenging dynamics of variable winds. While prior work has established that insects integrate the history of odor encounters to shape search decisions, whether they also retain a working memory of recently experienced wind conditions has remained unknown. Here, we use optogenetics combined with controlled wind perturbations in a free-flight wind tunnel to investigate how wind history modulates the olfactory search behavior of Drosophila melanogaster. By introducing lateral "gust" flow via auxiliary fans and independently delivering olfactory stimuli, we show that the wind experienced during an olfactory stimulus shapes both the immediate surge response and the subsequent spatial search. Flies that received an olfactory stimulus while being displaced by a crosswind gust were significantly more likely to return to the gust zone during the post-stimulus search phase compared to flies that received the same odor cue in steady laminar flow. Meanwhile, surge responses and course directions exhibited during search indicate that moment-to-moment flight kinematics may be driven more by instantaneous flow. These results reveal that wind experience is tracked in addition to olfactory experience, and provide evidence that Drosophila maintain a short-term working memory of ambient wind conditions to guide olfactory navigation.

The evolutionary origin of nervous systems in animals remains elusive and is largely hidden from the fossil record. Ctenophores, one of the earliest-branching animals possessing neurons, are instrumental to our understanding of nervous system origin, and a few rare ctenophore fossils preserve traces of nervous tissue as carbonaceous remains. Cambrian ctenophores appear to exhibit a more diverse neuroanatomy than that of modern species, suggesting secondary loss in extant ctenophores. However, much remains unknown about the origin and ontogeny giving rise to the structural organization of modern ctenophore nervous systems. Here, by investigating the neural anatomy of the ctenophore Mnemiopsis leidyi during development, we identified a ladder-like nerve net (LNN) beneath the comb rows that converges into condensed neurites and connects to the aboral organ. Examination of carbon-rich areas of Ctenorhabdotus capulus, an extinct ctenophore from the Burgess Shale, reveals a pattern similar to that of M. leidyi, consistent with a shared neural organization. Furthermore, M. leidyi exhibits a condensed comb nerve, resembling the longitudinal nerve preserved in the Cambrian ctenophore Fasciculus vesanus and the giant axon of extant Euplokamis dunlapae. Our study reveals conserved evolutionary constraints shaping nervous system architectures linked to locomotory organs and indicates that the different modes of nervous system organization observed in Cambrian ctenophores are variably retained in modern species.

Pennaceous feathers are fundamental to avian flight, yet their early function in non-volant dinosaurs remains unknown. Early-diverging pennaraptorans had simple pennaceous feathers on proto-wings and tails, which were unsuitable for flight but may have enhanced visual signals. However, the visual display hypothesis has not been empirically tested. To address this, we used computer animations of early pennaraptoran displays to measure responses in a well-established animal model of a visually sensitive neural pathway. We show that pennaceous proto-wings and tails enhance the efficiency of motion-based displays across a range of anatomically plausible movements. Integrating these results with comparative and paleontological evidence, we suggest that early pennaceous feathers functioned in diverse signaling contexts and were subsequently exapted for aerodynamic use.

Many animals navigate their world largely by seeing and feeling it. To disentangle these visual and mechanosensory contributions, we developed a virtual reality assay targeting the optomotor response in adult wild-type zebrafish swimming against flow. By projecting dynamic visual patterns onto the walls of a variable-speed flow tank, we decoupled wide-field optic flow from hydrodynamic velocity. We then tested fish responses to abrupt visual perturbations while they held station in the unsteady wake behind a bluff body. These perturbations reliably elicited compensatory optomotor responses, with fish aligning to the direction of the moving stimulus. Notably, this behavior was absent in uniform flows, suggesting that fish prioritize visual input when predictive lateral line signaling is compromised. We propose that this sensory shift serves to optimize swimming energetics in turbulent wakes. Extending this framework, we further show that zebrafish swimming against flow, whether alone or in groups, exhibit heightened escape responses to looming visual stimuli. Together, our findings reveal that fish sensory strategies are not fixed but dynamically tuned to hydrodynamic context: favoring visual cues in turbulent environments and lateral line input in uniform flows. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=84 SRC="FIGDIR/small/715425v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@1d7ba00org.highwire.dtl.DTLVardef@1f456f1org.highwire.dtl.DTLVardef@7826c4org.highwire.dtl.DTLVardef@391a68_HPS_FORMAT_FIGEXP M_FIG C_FIG

Repeated execution of individual behavioural units is a common feature of many learned motor behaviours such as dance, music, and birdsong. Little is known about the neuronal control of such learned motor sequences, and specifically, how the number of variable repetitions is determined. The songs of Bengalese finches (Lonchura striata domestica) consist of individual syllables which can repeat a variable number of times (repeat number) to form a repeat phrase. Like vocal sequences in other animals, Bengalese finch song syntax is typically modelled as a Markov chain, where the choice to repeat the same syllable type or switch to a different one is made stochastically after each syllable, before the next syllable is produced. Here, we report that repeat numbers of adjacent and distant repeat phrases in the song can be correlated across specific pairs of phrases. These hidden links between distinct phrases challenge existing models of song syntax where the number of repetitions is independently determined for each syllable type. Instead, they suggest an organisation where a joint factor can control multiple nonadjacent phrases in a song. Repeat phrases in Bengalese finches may therefore be particularly suited to study the neuronal mechanisms underlying long-range dependencies in complex vocal sequences.

How animals evaluate reward quality is a fundamental question in neuroscience and behavioral biology. Here we show that in honey bees (Apis mellifera), the value of a sucrose reward is not processed in absolute terms but relative to prior experience, and that this subjective evaluation strongly influences long-term memory formation. Using appetitive olfactory conditioning of the proboscis extension reflex (PER), we demonstrate that memory performance is determined by the contrast between a previously experienced reward and the reward used during training, rather than by the absolute concentration of sucrose received. This effect operates across multiple timescales, from contrasts between successive trials within a single session to differences between rewards experienced 24 hours apart. We further show that prior exposure to sucrose solutions of different concentrations modulates gustatory responsiveness and alters the sensitivity of antennal gustatory receptor neurons, suggesting that peripheral sensory plasticity contributes to experience-dependent changes in reward evaluation. Dissociating pre- and post-ingestive reward components revealed that the contrast between the sucrose concentration sensed by the antennae and the concentration ingested is sufficient to modulate memory formation. Together, our results indicate that bees form an internal expectation of reward quality based on experience, and that this expectation rescales the perceived value of subsequent rewards, thereby shaping associative memory strength. These findings provide a mechanistic framework for understanding how invertebrates perform relative reward comparisons across multiple temporal scales, with implications for flexible foraging strategies in dynamic environments.

Heterotrimeric kinesin 2 is the canonical motor protein for anterograde intraflagellar transport (IFT), driving movement of protein complexes towards the tip of cilia and flagella. Here, we show that all members of the Euglenozoa group lack genes for heterotrimeric kinesins and instead possess a variable number of genes for two homodimeric kinesins termed KIN2A and KIN2B. When expressed in vitro, both Trypanosoma brucei kinesins form homodimers and move processively along brain microtubules, KIN2A being faster than KIN2B. Studies in T. brucei and Leishmania mexicana show anterograde and retrograde IFT of both kinesins, with KIN2A travelling throughout the whole length of the flagellum, while KIN2B is concentrated at its base. In the proximal portion of the flagellum, most KIN2B molecules travel without IFT proteins, except for a few particles that are associated with IFT proteins and reach the tip. Surprisingly, the absence of KIN2A has mild effects on IFT and flagellum assembly, whereas KIN2B is essential for both. Investigation of trypanosome flagella deprived of KIN2B revealed that IFT proteins do not access these flagella but that KIN2A can still circulate. These results support a division-of-labour model where KIN2B is responsible for the import of IFT proteins while KIN2A is responsible for most of the anterograde transport.

A central challenge in characterizing species niches is ensuring that foraging data accurately capture both the resources used and their relative importance, and the role of resource abundance in shaping foraging patterns. Most studies infer diet breadth and resource-use patterns from observational records, yet such data can mask resource-specific decisions when animals forage with different goals. Here, we test this experimentally using individually identifiable bees in controlled resource communities to quantify foraging decisions between nectar (for sustenance) and pollen (for offspring provisioning). Combining observations, pollen DNA metabarcoding, and pollen microscopy, we show that observed visitation patterns misrepresent the floral resources most important for offspring provisioning, which ultimately determines offspring survival and population persistence. We further show that interaction patterns are structured from processes beyond resource abundance. Our results demonstrate that commonly used observational approaches can mischaracterize diet breadth, potentially challenging conclusions about species generalization.

The vascular system is central to plant ecology and evolution. Here, we show that more than 100 species across 27 genera and four subfamilies of Fabaceae have evolved atypical vascular architectures and that these species occur in all biogeographical regions except Antarctica. Because Fabaceae includes many ecologically and economically important species exhibiting these novel vasculatures, the family emerges as an ideal system for assessing the implications of vascular innovation in both fundamental and applied research.

Dumbbell-shaped stomata in grasses represent an evolutionary novelty, as their distinctive guard cell morphology is absent from most plant lineages. Stomata exhibit two major morphological forms: the kidney-shaped type found in most plants, and the dumbbell-shaped type that evolved in grasses. Dumbbell-like forms occur in sedges (Cyperaceae), providing an opportunity to examine how changes in developmental trajectories contribute to morphological evolution. By integrating analyses of cellulose microfibril organization, guard cell length to width ratio, and nuclear morphology, we demonstrate partial convergence between sedge and grass stomatal development. Specifically, cellulose microfibril organization in sedges represents an intermediate developmental state between kidney-shaped stomata and the grass dumbbell-shaped stomata. We further document differences in nuclear architecture: in contrast to kidney-shaped stomata, which have rounded nuclei in central guard cell regions, sedge nuclei are partially elongated and localize within bulbous regions, whereas grass nuclei exhibit fully elongated shapes along the cell axis. Notably, we identified secondary plasmodesmata between guard cells in one sedge species, suggesting a convergent route to symplastic communication achieved through secondary plasmodesmata formation rather than the incomplete cytokinesis characteristic of grasses. Together, these findings reveal convergent developmental solutions underlying similar stomatal morphologies.

The integrity of the axoneme - the microtubule (MT)-based core of the cilium - and intraflagellar transport (IFT) are interdependent. The mechanisms determining axoneme structure and dynamics have remained largely unknown, especially in primary cilia with a more variable architecture and longer MT singlet parts. Using fluorescence imaging in the phasmid neurons of C. elegans, we here demonstrate that {beta}-tubulin isotype TBB-4 diffuses through the dendrite and employs a combination of anterograde IFT and diffusion to reach the sites of incorporation in the steady-state axoneme. Disrupting tubulins ability to bind to the IFT significantly reduces its share in the axoneme. We suggest that, in phasmid cilia, a constant supply of tubulin by IFT is required for steady-state length maintenance, in order to elevate soluble tubulin concentration near the axonemal tips and to promote MT stability.