Summary

From the glossiphoniid leech Helobdella robusta, we have cloned and determined the complete coding sequence of Hro-nos, a gene homologous to the nanos gene from Drosophila melanogaster. Developmental northern blots show that Hro-nos, like nanos, is a maternal transcript that decays rapidly during early development. A polyclonal antiserum raised against the HRO-NOS protein was used in developmental western blots and for immunostaining leech embryos of different developmental stages. The HRO-NOS protein is first detectable in 2-cell embryos (4-6 hours of development) and exhibits a transient expression peaking during fourth cleavage (9-12 cells; 8–14 hours of development). The HRO-NOS protein exhibits a graded distribution along the primary embryonic axis and is partitioned unequally between the sister cells DNOPQ and DM, progeny of macromere D' at fourth cleavage: DNOPQ is the segmental ectoderm precursor cell and exhibits levels of HRO-NOS protein that are at least two-fold higher than in cell DM, the segmental mesoderm precursor cell. The observed expression pattern suggests that Hro-nos plays a role in the decision between ectodermal and mesodermal cell fates in leech.