Genesis of segmental identity in the leech nervous system.

The Antennapedia-class homeobox genes are likely to play a role in the specification of neuronal identities in invertebrates. The leech Hirudo medicinalis, a species well-suited for the analysis of these genes at the level of identified neurons, contains homologs of many Antennapedia-class and related genes. The expression pattern in the central nervous system of four leech homebox genes was examined in detail. Lox1 is expressed during early gangliogenesis in one pair of transient neurons present in every segment and, at later stages, in 15-20 pairs of neurons per segment. Lox2 is expressed in 25-30 pairs of neurons repeated in the posterior two-thirds of the midbody. Lox4 is present in 20-30 pairs of iterated neurons in the posterior half of the midbody, and in smaller subset of them in more anterior ganglia. Lox6 is expressed in 15-20 pairs of neurons of the third subesophageal neuromere and in fewer cells of more posterior ganglia. The subsets of neurons that express these homeobox genes are different but overlapping. Combinations of Lox genes could in theory generate enough variability to specify all central neurons in a leech ganglion.