The CNS connectome of a tadpole larva of Ciona intestinalis (L.) highlights sidedness in the brain of a chordate sibling

Abstract

Left-right asymmetries in brains are usually minor or cryptic. We report brain asymmetries in the tiny, dorsal tubular nervous system of the ascidian tadpole larva, Ciona intestinalis. Chordate in body plan and development, the larva provides an outstanding example of brain asymmetry. Although early neural development is well studied, detailed cellular organization of the swimming larva’s CNS remains unreported. Using serial-section EM we document the synaptic connectome of the larva’s 177 CNS neurons. These formed 6618 synapses including 1772 neuromuscular junctions, augmented by 1206 gap junctions. Neurons are unipolar with at most a single dendrite, and few synapses. Some synapses are unpolarised, others form reciprocal or serial motifs; 922 were polyadic. Axo-axonal synapses predominate. Most neurons have ciliary organelles, and many features lack structural specialization. Despite equal cell numbers on both sides, neuron identities and pathways differ left/right. Brain vesicle asymmetries include a right ocellus and left coronet cells.

DOI: http://dx.doi.org/10.7554/eLife.16962.001

eLife digest

Brains are made up of a network of nerve cells (neurons) that are connected to each other by junctions called synapses. The neurons on the left and right sides of the brain form different patterns of connections, but this asymmetry can be difficult to spot because the brain is large and complex. Understanding how the whole network operates is key to understanding how the brain works. However, a full map of all the connections between neurons – known as a connectome – has only been described for one species so far, a nematode worm called C. elegans.

The tadpole larva of the common sea squirt has a fairly simple brain distantly related to our own but made up of only about 330 cells. Ryan et al. used a technique called electron microscopy to study thin sections from the brains of sea squirt larvae to reveal this animal’s connectome and investigate left-right asymmetry in the brain.

The analysis revealed 177 neurons in this larval brain, just over half of its brain cells. These can be split into at least 25 types and each neuron has a simple, mostly unbranched shape with, on average, 49 synapses with other cells. This means that, even though it has such a small number of neurons, the neuron network is still relatively complex. The shortest sensory pathway to any muscle connects via three synapses, although most pathways involve more. The left and right sides of the brain differ in the types of neurons they contain and the connections these form, even though both sides have the same number of cells.

The findings of Ryan et al. reveal the second animal connectome and lay the groundwork for future studies on how each neuron in the network influences the behaviour of the sea squirt’s larva. Further work is also required to find out how the patterns of synapses in the brain change as the larva ages, and whether the connectome differs between siblings.

DOI: http://dx.doi.org/10.7554/eLife.16962.002