Identification of leech embryonic neurons that express a Hox gene required for the differentiation of a paired, segment-specific motor neuron

Abstract

This study investigated the embryonic expression and function of the Hox gene Lox1 in the simple, well-characterized central nervous system (CNS) of the medicinal leech Hirudo medicinalis. Lox1 was expressed in an anterior–posterior domain, extending from the posterior aspect of the fourth segment (rostral neuromere 4, R4) to the seventeenth segment (midbody ganglion 13, M13). Lox1 expression was also found in both sex organ primordia (male and female). Lox1 expression was not detected in every cell of the ganglia included in its domain. It was detected in a specific subset that included several segmentally iterated neurons and segment-specific neurons. Several central neurons (neurons located in the central nervous system – CNS) that coexpressed both Lox1 and FMRFamide-like peptides were identified using antibody staining of leech embryos and epifluorescence and confocal microscopy. RNA interference was used to block the expression of Lox1. The expression pattern and the effect of RNA interference indicate that Lox1 is required for the differentiation of a segment-specific pair of motor neurons, the RPE (rostral penile evertor) neurons, which appear only in midbody ganglion 6 (M6) and innervate the male sex organ.