Lox2, a putative leech segment identity gene, is expressed in the same segmental domain in different stem cell...

The segmented tissues of the adult leech arise from a set of five, bilaterally paired embryonic stem cells via a stereotyped sequence of cell lineage. Individual segments exhibit unique patterns of cell differentiation, and previous studies have suggested that each stem cell lineage establishes at least some aspects of its own segmental specificity autonomously. In this paper, we describe a putative leech segment identity gene, Lox2, and examine its expression in the various stem cell lineages. Both sequence analysis and the segmental pattern of Lox2 expression suggest a specific homology to the fruitfly segment identity genes Ubx and abdA. In situ hybridization reveals a cellular accumulation of Lox2 RNA over a contiguous domain of 16 midbody segments (M6-M21), including postmitotic neurons, muscles and the differentiating genitalia. Lox2 transcripts were not detected at the stage when segment identities are first established, suggesting that Lox2 gene products may not be part of the initial specification process. Individual stem cell lineages were labeled by intracellular injection of fluorescent tracers, and single cell colocalization of lineage tracer and hybridization reaction product revealed expression of Lox2 RNA in the progeny of four different stem cells. The segmental domain of Lox2 RNA was very similar in the various stem cell lineages, despite the fact that some stem cells generate one founder cell/segment, whereas other stem cells generate two founder cells/segment.