Identification of a neurogenic sublineage required for CNS segmentation in an Annelid

In embryos of leeches (phylum Annelida), metameric structures arise sequentially from a germinal plate comprising the descendants of five pairs of embryonic stem cells called teloblasts. It has been shown that transverse stripes of cells expressing ht-en (a homolog of engrailed, a Drosophila segment polarity gene), arise in the germinal plate prior to the appearance of segmental ganglia and that, in the main neurogenic lineage (derived from the N teloblasts), the stripe of cells expressing ht-en demarcates the boundary between prospective segmental ganglia. Previous lineage-tracing experiments had suggested that the clones of nf and ns primary blast cells in the N lineage are confined to within segmental borders. This conclusion was called into question by the observation that the cells expressing ht-en do not appear to be at the very posterior edge of the nf clone, from which they arise. To resolve this issue, we have injected individual primary blast cells with fluorescent lineage tracers; we find that cells in the nf clone actually straddle two adjacent ganglia. Moreover, using photoablation techniques, we find that the nf clone is required for proper morphogenesis of the segmentally iterated central nervous system (CNS).