SEGMENTAL DIVERSIFICATION OF AN IDENTIFIED LEECH NEURON CORRELATES WITH THE SEGMENTAL DOMAIN IN WHICH IT EXPRESSES LOX2, A MEMBER OF THE HOX GENE FAMILY

The cellular colocalization of LOX2 protein and small cardioactive pep tide (SCP)-like immunoreactivity was studied in the nerve cord of the glossiphoniid leech Helobdella triserialis. Of the six neurons that ex press SCP in the midbody segments 7 to 17, only one, the MPS neuron, e xpresses LOX2 protein. The medial paired SCP (MPS) neurons are segment ally repeated and can be divided into three contiguous segmental domai ns according to cell body size and the timing and level of SCP express ion. MPS neurons located in the anterior and middle segmental domains express LOX2 protein. In the middle domain, large MPS neurons begin to accumulate SCP shortly after the end of embryonic development, wherea s in the anterior domain the MPS neurons are smaller and begin to expr ess SCP at a later stage. In the posterior domain the MPS neurons exhi bit a third phenotype-they have large cell bodies, express low levels of SCP starting from the midjuvenile stage, and do not show detectable LOX2 expression. Lineage tracer injections showed that the MPS neuron s arise from a stereotyped cell lineage and are descended from the O t eloblast stem cell. In midbody ganglia 2 to 6 and 18 to 21, there are lineally homologous neurons that do not express either LOX2 protein or SCP. Thus, the boundaries of LOX2 expression coincide precisely with two of the segmental boundaries of MPS differentiation, suggesting tha t expression of Lox2 at the level of this single identified neuron gov erns some, but not all, aspects of the neuron's segmental diversificat ion. (C) 1996 John Wiley & Sons, Inc.