GENOMIC ORGANIZATION AND EXPRESSION OF THE PLANARIAN HOMEOBOX GENES DTH-1 AND DTH-2

We have characterized the genomic organization of Dth-1 and Dth-2, pla narian homeobox-containing genes, previously described at the cDNA lev el (J. Garcia-Fernandez, J. Baguna and E. Salo (1991), Proc. Natl. Aca d. Sci. USA, 88, 7338-7342). Genomic analysis shows that Dth-1 and Dth -2 genes encode proteins of 533 and 363 amino acids respectively. The open reading frame of Dth-1 is interrupted by two large introns of 8 k b and 12 kb Dth-2 also shows two introns, but these are short (42 bp a nd 44 bp) and the second interrupts helix III at position 44-45, as is the case with other homeobox genes from such divergent animals as Dro sophila, honeybee, C elegans, ascidians, and mouse, which suggests an ancient evolutionary relationship between these genes. The spatial dis tribution of transcripts in adult tissues, determined by in situ hybri dization, demonstrates that Dth-1 is expressed at a high level in the gastrodermal cells, while Dth-2 is expressed in the peripheral parench yma, at higher levels in the dorsal than the ventral regions. Their sp ecific spatial distribution suggests a possible role for these homeobo x genes in determination and/or differentiation of specific cell types . The expression pattern of both genes is more or less continuous, but in Dth-1 clustered discontinuous labelling in areas surrounding the g astrodermis may indicate a specific expression of this gene in groups of undifferentiated cells (neoblasts) already committed or determined to gastrodermal cell fates. In situ hybridization analysis during earl y regeneration shows expression only in the postblastema (stump) diffe rentiated areas while no expression has been detected in the undiffere ntiated blastema, indicating that neither gene has a role in pattern f ormation mechanisms known to occur at the early stages of regeneration (0-3 days). Hence, Dth-1 and Dth-2 are planarian homeobox genes presu mably involved in specific cell or tissue determination and/or differe ntiation.