Biology subjects

Kourakis, M. J.

Publications and source records attributed to Kourakis, M. J..

4 recordsLinked to original sources

bioRxiv2025

The dorsal/ventral subdivision of the hindbrain predates the tunicate/vertebrate split

While the CNSs of the chordate sister clades Tunicata and Vertebrata have unmistakable homology, the extent of their conservation is still being uncovered. We report that the hindbrain of the tunicate Ciona, like those of vertebrates, has functionally-distinct dorsal and ventral domains. The Ciona dorsal hindbrain functions as a relay and processing center for peripheral sensory neurons, while in vertebrates the dorsal hindbrain forms the cerebellum. Despite the different fates of dorsal hindbrains in Ciona versus vertebrates, we present evidence from analysis of gene expression, developmental mechanisms, and neural circuit architecture that they share a common origin. While it is generally agreed that the cerebellum evolved in vertebrates, our findings indicate that a dorsally-positioned precursor to the cerebellum has much deeper roots.

evolutionary biology↗

bioRxiv2023

An single oscillating proto-hypothalamic neuron gates taxis behavior in the primitive chordate Ciona

Ciona larvae display a number of behaviors, including negative phototaxis. In negative phototaxis, the larvae first perform short spontaneous rhythmic casting swims. As larvae cast in a light field, their photoreceptors are directionally shaded by an associated pigment cell, providing a phototactic cue. This then evokes an extended negative taxis swim. We report here that the larval forebrain of Ciona has a previously uncharacterized single slow-oscillating inhibitory neuron (neuron cor-assBVIN78) that projects to the midbrain, where it targets key interneurons of the phototaxis circuit known as the photoreceptor relay neurons. The anatomical location, gene expression and oscillation of cor-assBVIN78 suggest homology to oscillating neurons of the vertebrate hypothalamus. Ablation of cor-assBVIN78 results in larvae showing extended phototaxis-like swims, but which occur in the absence of phototactic cues. These results indicate that cor-assBVIN78 has a gating activity on phototaxis by projecting temporally-oscillating inhibition to the photoreceptor relay neurons. However, in intact larvae the frequency of cor-assBVIN78 oscillation does not match that of the rhythmic spontaneous swims, indicating that the troughs in oscillations do not themselves initiate swims, but rather that cor-assBVIN78 may modulate the phototaxis circuit by filtering out low level inputs while restricting them temporally to the troughs in inhibition.

neuroscience↗

bioRxiv2021

Left/right asymmetry disruptions and mirror-image reversals to behavior andbrain anatomy in Ciona

BackgroundLeft-right asymmetries are a common feature of metazoan nervous systems. This is particularly pronounced in the comparatively simple larval central nervous system (CNS) of the tunicate Ciona, whose swimming tadpole larva shows a clear chordate ground plan. While common pathway elements for specifying the left-right axis are found in the chordates, particularly a requirement for Nodal signaling, Ciona differs from its vertebrate cousins by specifying its axis at the neurula stage, rather than at gastrula. Additionally, Ciona, and other ascidians, have a requirement for an intact chorionic membrane for proper left/right specification. ResultsWe present here results showing that left-right asymmetry disruptions caused by removal of the chorion (dechorionation) are highly variable and present throughout the Ciona larval nervous system. While previous studies have documented disruptions to the conspicuously asymmetric sensory systems in the anterior brain vesicle, we document asymmetries in seemingly symmetric structures such as the posterior brain vesicle and motor ganglion. Moreover, defects caused by dechorionation include misplaced or absent neuron classes, loss of asymmetric gene expression, aberrant synaptic connectivity, and abnormal behaviors. In the motor ganglion, a brain structure that has been equated with the vertebrate hindbrain, we find that despite the apparent left/right symmetric distribution of interneurons and motor neurons, AMPA receptors are expressed exclusively on the left side, which equates with asymmetric swimming behaviors. We also find that within a population of dechorionated larvae, there is a small percentage with apparently normal left-right specification, and approximately equal population with inverted (mirror-image) asymmetry. We present a method based on a behavioral assay for isolating these larvae. When these two classes of larvae (normal and inverted) are assessed in a light dimming assay they display mirror-image behaviors, with normal larvae responding with counterclockwise swims, while inverted larvae respond with clockwise swims. ConclusionsOur findings highlight the importance of left-right specification pathways not only for proper CNS anatomy, but also for correct synaptic connectivity and behavior.

developmental biology↗

bioRxiv2020

Deep origins for the tectal visual processing centers in chordates

Visual processing transforms the complex visual world into useful information. Ciona, a close relative of vertebrates, has one of the simplest nervous systems known, yet has a range of visuomotor behaviors. Among them are negative phototaxis and a looming-shadow response. These two behaviors are mediated by separate photoreceptor groups acting via distinct, but overlapping, neural circuits. We show here that processing circuits underlying both behaviors transform visual inputs to generate fold-change detection (FCD) outputs. In FCD, the response scales with the relative fold changes in input, but is invariant to the overall magnitude of the stimulus. Several different circuit architectures can generate FCD responses. Both the behavioral outputs and the putative circuitry for the two visuomotor behaviors point to them using different FCD circuits. Pharmacological treatment points to circuits in the posterior brain vesicle of Ciona, a region we speculate shares homology with the vertebrate midbrain, as important for FCD.

neuroscience↗