A novel leech homeobox gene, Lox10, is shown to encode a homeodomain sequence characteristic of a phyletically

Homeobox genes have been implicated in a number of regulatory processes associated with the embryonic patterning and cell differentiation of eukaryotes (McGinnis and Krumlauf, 1992). The homeobox is a 180 bp DNA sequence which encodes a 60 amino acid protein motif, the homeodomain, that has been shown to serve as the DNA binding site for a phyletically and functionally diverse class of transcription factors (Gehring et al., 1990). The amino acid sequence and three-dimensional conformation of the homeodomain are highly conserved, but many distinct classes of homeobox gene have been defined on the basis of sequence, genomic organization, expression pattern and/or function (Scott et al., 1989; McGinnis and Krumlauf, 1992). In this paper, we describe a novel homeobox gene, Lox10, isolated from the glossiphoniid leech Helobdella triserialis. Lox10 is characteristic of a recently discovered group of NK2-like homeobox genes whose evolutionary history and functional significance are unclear. The best characterized of these genes encodes thyroid transcription factor 1 (TTF1), which activates the thyroid-specific transcription of thyroglobulin and thyroperoxidase genes in rat (Guazzi et al., 1990; Civitareale et al., 1989). The TTF-1 gene has since been cloned in mouse (designated Nkx-2.1), and shown to be one of four members of the murine Nkx-2 gene family (Price et al., 1992). Expression studies reveal that TTF-1 RNA is restricted to the thyroid, lung and a discrete region of the fetal forebrain (Lazzaro et al., 1991). The closely related Nkx-2.2 gene is expressed in a distinct region of the murine forebrain, and it has been proposed that the Nkx-2 gene family plays a developmental role in forebrain regionalization (Price et al., 1992). Several related genes have been described in invertebrate organisms, but their expression patterns are largely unknown. The reported sites of TTF-1 and Nkx-2.2 expression thyroid, lung and forebrain are organs specific to the vertebrate lineage (Beklemishev, 1969; Gans and Northcutt, 1983), and it is of considerable interest to learn how the homologous genes are utilized in invertebrates. The murine Nkx-2 gene family was named for its similarity to the Drosophila gene NK-2, which was isolated by DNA hybridization (Kim and Nirenberg, 1989). Additional NK-2like homeobox genes have since been found in planaria (Garcia-Fernàndez et al., 1991) and tapeworm (Oliver et al., 1992). To gain further insight into the function and phyletic 877 Development 118, 877-892 (1993) Printed in Great Britain © The Company of Biologists Limited 1993