Vertebrates develop an olfactory system that detects odorants and pheromones through their interaction with specialized cell surface receptors on olfactory sensory neurons. During development, the olfactory system forms from the olfactory placodes, specialized areas of the anterior ectoderm that share cellular and molecular properties with placodes involved in the development of other cranial senses. The early-diverging chordate lineages amphioxus, tunicates, lampreys and hagfishes give insight into how this system evolved. Here, we review olfactory system development and cell types in these lineages alongside chemosensory receptor gene evolution, integrating these data into a description of how the vertebrate olfactory system evolved. Some olfactory system cell types predate the vertebrates, as do some of the mechanisms specifying placodes, and it is likely these two were already connected in the common ancestor of vertebrates and tunicates. In stem vertebrates, this evolved into an organ system integrating additional tissues and morphogenetic processes defining distinct olfactory and adenohypophyseal components, followed by splitting of the ancestral placode to produce the characteristic paired olfactory organs of most modern vertebrates.

COE genes encode transcription factors that have been found in all metazoans examined to date. They possess a distinctive domain structure that includes a DNA-binding domain (DBD), an IPT/TIG domain and a helix-loop-helix (HLH) domain. An intriguing feature of the COE HLH domain is that in jawed vertebrates it is composed of three helices, compared to two in invertebrates. We report the isolation and expression of two COE genes from the brook lamprey Lampetra planeri and compare these to COE genes from the lampreys Lethenteron japonicum and Petromyzon marinus. Molecular phylogenetic analyses do not resolve the relationship of lamprey COE genes to jawed vertebrate paralogues, though synteny mapping shows that they all derive from duplication of a common ancestral genomic region. All lamprey genes encode conserved DBD, IPT/TIG and HLH domains; however, the HLH domain of lamprey COE-A genes encodes only two helices while COE-B encodes three helices. We also identified COE-B splice variants encoding either two or three helices in the HLH domain, along with other COE-A and COE-B splice variants affecting the DBD and C-terminal transactivation regions. In situ hybridisation revealed expression in the lamprey nervous system including the brain, spinal cord and cranial sensory ganglia. We also detected expression of both genes in mesenchyme in the pharyngeal arches and underlying the notochord. This allows us to establish the primitive vertebrate expression pattern for COE genes and compare this to that of invertebrate chordates and other animals to develop a model for COE gene evolution in chordates.

The larva of the ascidian Ciona intestinalis controls a small repertoire of behaviours with a simple nervous system in which each cell is identifiable. As such it offers the prospect of building a cohesive cell-level picture of how a nervous system integrates sensory inputs to produce specific behavioural outcomes. Here, we report the development of a microfluidic chip in which larvae can be immobilised and exposed to chemical stimuli. We generate transgenic larvae in which the calcium ion reporter GCaMP6m is expressed in a defined population of cells, allowing us to record real-time neural activity following stimulation. We then use this to establish that some cell populations can sense dissolved carbon dioxide. We also leverage genome and transcriptome data coupled with molecular evolutionary analysis to identify putative chemoreceptors of the MS4A family in Ciona . Our study demonstrates that Ciona larvae can respond to dissolved carbon dioxide, identifies the cells that are likely responsible for chemosensation, and establishes a chip based imaging platform coupled with transgenic technology that could be adapted to establish where other stimuli are sensed and how such incoming signals are processed in the brain to yield behavioural output.

18Hill stream loaches (family Balitoridae and Gastromyzontidae) are thumb-sized fish that effortlessly 19 exploit environments where flow rates are so high that potential competitors would be washed 20 away. To cope with these extreme flow rates hill stream loaches have evolved adaptations to stick to 21 the bottom, equivalent to the downforce generating wings and skirts of F1 racing cars, and scale 22 35