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 ch ymotrypsin gene expression begins in amoebocytes located in the dorsop osterior region of the undifferentiated digestive gland prior to metam orphosis, 5 d post-fertilization. Transcript accumulates steadily in t hese cells over the next 6 d while the amoebocytes migrate slowly dors ally. Induction of metamorphosis dramatically accelerates the rates of chymotrypsin mRNA accumulation and amoebocyte migration, and is requi red 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 me tamorphosis, with a more pronounced increase in chymotrypsin mRNA accu mulation in occurring older larvae. Chymotrypsin mRNA accumulation pat terns suggest that hindgut cell specification occurs prior to metamorp hosis, but that completion of the morphogenetic program requires signa ling events associated with metamorphosis.