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A Neuron Type Atlas of the Annelid Brain: Development and Evolution of Chemosensory-Motor Circuits

Final Report Summary - BRAINEVODEVO (A Neuron Type Atlas of the Annelid Brain: Development and Evolution of Chemosensory-Motor Circuits)

BrainEvoDevo has paved the way towards a new, cellular level of comparative brain research, with the aim of elucidating brain evolution in the Bilateria (animals with bilateral symmetry such as vertebrates, insects and annelids). While classical efforts have focused on anatomical comparisons of brain parts between these groups, more recent studies have started to investigate and compare the genes involved in brain development. In our project, we have now set out to enable, for the first time, the molecular comparison of all cells that constitute the brain in bilaterian animals. In a pioneer study, we have investigated the early differentiating brain in a marine invertebrate, the polychaete annelid Platynereis dumerilii. For selected larval and juvenile stages we have generated a cellular gene expression atlas for the whole animal. We have then dissociated entire animals of the same stage and sequenced transcriptomes of hundreds of randomly captured single cells. Next, we have used specifically expressed genes to map these cells onto the cellular atlas. We can now combine this unique and novel resource with an atlas of the Platynereis ultrastructure that we have generated with block-face scanning electron microscope for the same stage. In the second part of the project, we have used this resource to investigate chemosensory-locomotor neural circuits that direct chemotaxis movement and feeding. We have determined chemosensory cues and olfactory receptors and investigated the genetic networks specifying chemosensory neurons and locomotor circuits. As part of these efforts, we have functionally dissected the neural circuits driving the sleep-wake cycle of ciliary swimming of planktonic larvae.