Expanded olfactory system in ray-finned fishes capable of terrestrial exploration.

BACKGROUND: Smell abilities differ greatly among vertebrate species due to distinct sensory needs, with exceptional variability reported in the number of olfactory genes and the size of the odour-processing regions of the brain. However, key environmental factors shaping genomic and phenotypic changes linked to the olfactory system remain difficult to identify at macroevolutionary scales. Here, we investigate the association between diverse ecological traits and the number of olfactory chemoreceptors in approximately two hundred ray-finned fishes. RESULTS: We found independent expansions producing large gene repertoires in several lineages of nocturnal amphibious fishes, generally able to perform active terrestrial exploration. We reinforced this finding with on-purpose genomic and transcriptomic analysis of Channallabes apus, a catfish species from a clade with chemosensory-based aerial orientation. Furthermore, we also detected an augmented information-processing capacity in the olfactory bulb of nocturnal amphibious fishes by estimating the number of cells contained in this brain region in twenty-four actinopterygian species. CONCLUSIONS: Overall, we report a convergent genomic and phenotypic magnification of the olfactory system in nocturnal amphibious fishes. This finding suggests the possibility of an analogous evolutionary event in fish-like tetrapod ancestors during the first steps of the water-to-land transition, favouring terrestrial adaptation through enhanced aerial orientation.

Sensorimotor integration in Caenorhabditis elegans: a reappraisal towards dynamic and distributed computations.

The nematode Caenorhabditis elegans is a tractable model system to study locomotion, sensory navigation and decision-making. In its natural habitat, it is thought to navigate complex multisensory environments in order to find food and mating partners, while avoiding threats like predators or toxic environments. While research in past decades has shed much light on the functions and mechanisms of selected sensory neurons, we are just at the brink of understanding how sensory information is integrated by interneuron circuits for action selection in the worm. Recent technological advances have enabled whole-brain Ca2+ imaging and Ca2+ imaging of neuronal activity in freely moving worms. A common principle emerging across multiple studies is that most interneuron activities are tightly coupled to the worm's instantaneous behaviour; notably, these observations encompass neurons receiving direct sensory neuron inputs. The new findings suggest that in the C. elegans brain, sensory and motor representations are integrated already at the uppermost sensory processing layers. Moreover, these results challenge a perhaps more intuitive view of sequential feed-forward sensory pathways that converge onto premotor interneurons and motor neurons. We propose that sensorimotor integration occurs rather in a distributed dynamical fashion. In this perspective article, we will explore this view, discuss the challenges and implications of these discoveries on the interpretation and design of neural activity experiments, and discuss possible functions. Furthermore, we will discuss the broader context of similar findings in fruit flies and rodents, which suggest generalizable principles that can be learnt from this amenable nematode model organism.This article is part of a discussion meeting issue 'Connectome to behaviour: modelling C. elegans at cellular resolution'.

A global brain state underlies C. elegans sleep behavior.

How the brain effectively switches between and maintains global states, such as sleep and wakefulness, is not yet understood. We used brainwide functional imaging at single-cell resolution to show that during the developmental stage of lethargus, the Caenorhabditis elegans brain is predisposed to global quiescence, characterized by systemic down-regulation of neuronal activity. Only a few specific neurons are exempt from this effect. In the absence of external arousing cues, this quiescent brain state arises by the convergence of neuronal activities toward a fixed-point attractor embedded in an otherwise dynamic neural state space. We observed efficient spontaneous and sensory-evoked exits from quiescence. Our data support the hypothesis that during global states such as sleep, neuronal networks are drawn to a baseline mode and can be effectively reactivated by signaling from arousing circuits.

The Apoptotic Engulfment Machinery Regulates Axonal Degeneration in C. elegans Neurons.

Axonal degeneration is a characteristic feature of neurodegenerative disease and nerve injury. Here, we characterize axonal degeneration in Caenorhabditis elegans neurons following laser-induced axotomy. We show that this process proceeds independently of the WLD(S) and Nmnat pathway and requires the axonal clearance machinery that includes the conserved transmembrane receptor CED-1/Draper, the adaptor protein CED-6, the guanine nucleotide exchange factor complex Crk/Mbc/dCed-12 (CED-2/CED-5/CED-12), and the small GTPase Rac1 (CED-10). We demonstrate that CED-1 and CED-6 function non-cell autonomously in the surrounding hypodermis, which we show acts as the engulfing tissue for the severed axon. Moreover, we establish a function in this process for CED-7, an ATP-binding cassette (ABC) transporter, and NRF-5, a lipid-binding protein, both associated with release of lipid-vesicles during apoptotic cell clearance. Thus, our results reveal the existence of a WLD(S)/Nmnat-independent axonal degeneration pathway, conservation of the axonal clearance machinery, and a function for CED-7 and NRF-5 in this process.