Biology subjects

Evans, D. A.

Publications and source records attributed to Evans, D. A..

4 recordsLinked to original sources

bioRxiv2025

Disentangling Cephalopod Chromatophores Motor Units with Computer Vision

Cephalopod chromatophores are skin pigment organs that enable unmatched camouflage through rapid, flexible and neurally controlled deformation. Although their morphology is well known, the organization of their motor control is not entirely understood. Here, we combine high-resolution videography with a dedicated computer-vision pipeline (CHROMAS) to investigate chromatophore control and their likely innervation in Euprymna berryi and Sepia officinalis. By segmenting chromatophores into radial slices and analyzing anisotropic deformations, we applied dimensionality reduction (PCA) and source separation (ICA) to estimate the number and spatial influence of motor neurons responsible for the control of individual and groups of chromatophores. On average, four independent components were detected (suggesting innervation by four motor neurons), each forming contiguous petal-shaped domains rather than causing uniform expansion. Clustering thousands of components revealed motor units spanning multiple chromatophores, most involving fewer than 14 but occasionally spanning more widely. These motor units displayed a wide variety of geometries, ranging from compact local groups to elongated or fragmented structures; they often overlapped, with repeated co-innervation of chromatophore pairs occurring more often than expected by chance. Expansion was consistently faster and more stereotyped than relaxation, consistent with active contraction (corresponding to chromatophore expansion) and passive recoil (chromatophore contraction). Together, these results show that individual chromatophores are not singular or uniform pixels, but rather contrast elements that can be fractionated into smaller territories, themselves coordinated with those of other chromatophores. This geometry of neural control enables, among others, the generation of "virtual" chromatophores, i.e., functional groupings of adjacent chromatophore territories that act as single units, as well as that of noise in the distribution of pixel shapes.

neuroscience↗

bioRxiv2024

Rust Fungus Red Data List and Census Catalogue for Wales

The rust fungi are a group of specialised plant pathogens. Conserving them seems to fly in the face of reason. Yet as our population grows and food supplies become more precarious, controlling pathogens of crop plants becomes more imperative. Breeding resistance genes into such plants has proved to be the most cost effective solution. Such resistance genes evolve only in plants challenged by pathogens. We hope this report will assist in prioritising the conservation of natural ecosystems and traditional agro-ecosystems that are likely to be the richest sources of resistance genes. Despite its small size (11% of mainland Britain) Wales has supported 225 rust fungi taxa (including 199 species) representing 78% of the total British mainland rust species. For the first time using widely accepted international criteria and data collected from a number of mycologists and institutions, a Welsh regional threat status is offered for all native Welsh rust taxa. The results are compared with other published Red Lists for Wales. Information is also supplied in the form of a census catalogue, detailing the rust taxa recorded from each of the 13 Welsh vice-counties. Of the 225 rust taxa so far recorded from Wales 7 are probably extinct (3% of the total), and 39 (18%) are threatened with extinction. Of this latter total 13 taxa (6%) are considered to be Critically Endangered, 15 (7%) to be Endangered and 13 (6%) to be Vulnerable. A further 20 taxa (9%) are Near Threatened, whilst 15 taxa (7%) lacked sufficient data to permit evaluation. Just over a third of Welsh rust fungi taxa require some form of action, either to better understand their status, or reverse known ongoing declines. This is a similar figure to those presented elsewhere for bryophytes and lichens but higher than the 19% of vascular plants requiring action.

microbiology↗

bioRxiv2023

Tonically active GABAergic neurons in the dorsal periaqueductal gray control the initiation and execution of instinctive escape

To avoid predation, animals perform defensive actions that are both instinctive and adaptable to the environment. In mice, the decision to escape from imminent threats is implemented by a feed-forward circuit in the midbrain, where excitatory VGluT2+ neurons in the dorsal periaqueductal gray (dPAG) compute escape initiation and escape vigour from threat evidence. Here we show that GABAergic VGAT+ neurons in the dPAG dynamically control this process by modulating the excitability of excitatory escape neurons. Using in vitro patchclamp and in vivo neural activity recordings in freely behaving mice we found that VGAT+ dPAG neurons fire action potentials tonically in the absence of synaptic inputs and are a major source of synaptic inhibition to VGluT2+ dPAG neurons. Activity in these spontaneously firing VGAT+ cells transiently decreases at escape onset and increases during escape, peaking at escape termination. Optogenetically increasing or decreasing VGAT+ dPAG activity bidirectionally changes the probability of escape when the stimulation is delivered at the time of threat onset, and the duration of escape when delivered after escape initiation. We conclude that the activity of tonically firing VGAT+ dPAG neurons sets a threshold for escape initiation and controls the execution of the flight locomotor action.

neuroscience↗

bioRxiv2018

A synaptic threshold mechanism for computing escape decisions

Escaping from imminent danger is an instinctive behaviour fundamental for survival that requires classifying sensory stimuli as harmless or threatening. The absence of threat allows animals to forage for essential resources, but as the level of threat and potential for harm increases, they have to decide whether or not to seek safety1. Despite previous work on instinctive defensive behaviours in rodents2-13, little is known about how the brain computes the threat level for initiating escape. Here we show that the probability and vigour of escape in mice scale with the intensity of innate threats, and are well described by a theoretical model that computes the distance between threat level and an escape threshold. Calcium imaging and optogenetics in the midbrain of freely behaving mice show that the activity of excitatory VGluT2+ neurons in the deep layers of the medial superior colliculus (mSC) represents the threat stimulus intensity and is predictive of escape, whereas dorsal periaqueductal gray (dPAG) VGluT2+ neurons encode exclusively the escape choice and control escape vigour. We demonstrate a feed-forward monosynaptic excitatory connection from mSC to dPAG neurons that is weak and unreliable, yet necessary for escape behaviour, and which we suggest provides a synaptic threshold for dPAG activation and the initiation of escape. This threshold can be overcome by high mSC network activity because of short-term synaptic facilitation and recurrent excitation within the mSC, which amplifies and sustains synaptic drive to the dPAG. Thus, dPAG VGluT2+ neurons compute escape decisions and vigour using a synaptic mechanism to threshold threat information received from the mSC, and provide a biophysical model of how the brain performs a critical behavioural computation.

neuroscience↗