Neural crest origin of sympathetic neurons at the dawn of vertebrates
he neural crest is an embryonic stem cell population unique to vertebrates whose expansion and diversification are thought to have promoted vertebrate evolution by enabling emergence of novel cell types and structures like jaws and peripheral ganglia2. While basal vertebrates have sensory ganglia, convention has it that trunk sympathetic chain ganglia arose only in jawed vertebrates. In contrast, here we report the presence of trunk sympathetic neurons in the sea lamprey, Petromyzon marinus, an extant jawless vertebrate. These neurons arise from sympathoblasts near the dorsal aorta that undergo noradrenergic specification via a transcriptional program homologous to that described in gnathostomes. Lamprey sympathoblasts populate the extracardiac space and extend along the length of the trunk in bilateral streams, expressing the catecholamine biosynthetic pathway enzymes tyrosine hydroxylase and dopamine ?-hydroxylase. CM-DiI lineage tracing analysis further confirmed that these cells derive from the trunk neural crest. RNA-seq of isolated ammocete trunk sympathoblasts revealed gene profiles characteristic of sympathetic neuron function. Our findings challenge prevailing dogma which posits that sympathetic ganglia are a gnathostome innovation, instead suggesting that a late-developing rudimentary sympathetic nervous system may have been characteristic of the earliest vertebrates. Overall design: To gain insight into the molecular identity of lamprey sympathetic neurons, we harvested bilateral tracts of cells—presumed to be the sympathetic chains—from the trunks of lamprey ammocetes. We further selected for TH+ and TH- cells and subjected them to RNA sequencing using Fixed and Recovered Intact Single Cell RNA-seq (FRISCR) followed by differential expression analysis.