CHYMOTRYPSIN MESSENGER-RNA EXPRESSION IN DIGESTIVE GLAND AMEBOCYTES - CELL SPECIFICATION OCCURS PRIOR TO METAMORPHOSIS AND GUT MORPHOGENESIS IN THE GASTROPOD, HALIOTIS-RUFESCENS

In the non-feeding larva of the marine gastropod, Haliotis rufescens, gut morphogenesis is initiated at metamorphosis. Intestine-specific chymotrypsin gene expression begins in amoebocytes located in the dorsoposterior region of the undifferentiated digestive gland prior to metamorphosis, 5 d post-fertilization. Transcript accumulates steadily in these cells over the next 6 d while the amoebocytes migrate slowly dorsally. Induction of metamorphosis dramatically accelerates the rates of chymotrypsin mRNA accumulation and amoebocyte migration, and is required for homing of the amoebocytes to the hindgut region. Induction of chymotrypsin gene expression occurs only in larvae that had developed competence to recognize an exogenous morphogenetic cue and initiate metamorphosis, with a more pronounced increase in chymotrypsin mRNA accumulation in occurring older larvae. Chymotrypsin mRNA accumulation patterns suggest that hindgut cell specification occurs prior to metamorphosis, but that completion of the morphogenetic program requires signaling events associated with metamorphosis.