Expression of actin mRNA in embryos of the leech Helobdella triserialis

A cDNA clone (Htr-actin) containing a 1.48 kb insert corresponding to actin was isolated from a stage 10-11 Helobdella triserialis cDNA library by cross hybidization to a Drosophila melanogaster actin clone. The cDNA is equivalent in size to the adult actin mRNA detected by Northern analysis. Genomic Southern blot analysis revealed that Htr-actin is a single copy gene and is one member of a family of actins in H. triserialis. Protein sequence comparisons revealed that Htr-actin was most similar to actin-1 of the earthworm and like other invertebrate actins, Htr-actin was found to be more similar to mammalian cytoplasmic actins than to mammalian muscle-specific actins. In situ hybridization revealed that at early stages the actin mRNA was distributed primarily in the yolk-deficient cytoplasm of all cells. At later stages, elevated levels were detected over the germinal bands and plate. Following muscle formation, a subset of the actin mRNA expression pattern resembled the immunostaining pattern obtained with the muscle specific antibody, Lan3-14. The Htr-actin transcript was undetectable in the segmental nervous system. These studies lay the groundwork for any future ectopic gene expression studies using the Htr-actin promoter in the leech embryo.