A NANOS HOMOLOG IN LEECH

From the glossiphoniid leech Helobdella robusta, we have cloned and de termined the complete coding sequence of Hro-nos, a gene homologous to the nanos gene from Drosophila melanogaster. Developmental northern b lots show that Hro-nos, like nanos, is a maternal transcript that deca ys rapidly during early development. A polyclonal antiserum raised aga inst the HRO-NOS protein was used in developmental western blots and f or immunostaining leech embryos of different developmental stages. The HRO-NOS protein is first detectable in 2-cell embryos (4-6 hours of d evelopment) 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 pr ecursor 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 i n the decision between ectodermal and mesodermal cell fates in leech.