Maternal expression of a NANOS homolog is required for early development of the leech Helobdella robusta

The gene nanos (nos) is a maternal posterior group gene required for normal development of abdominal segments and the germ line in Drosophila. Expression of nos-related genes is associated with the germ line in a broad variety of other taxa, including the leech Helobdella robusta, where zygotically expressed Hro-nos appears to be associated with primordial germ cells. The function of maternally inherited Hro-nos transcripts remains to be determined, however. Here, the function of maternal Hro-nos is examined using an antisense morpholino (MO) knockdown strategy, as confirmed by immunostaining and western blot analysis. HRO-NOS knockdown embryos exhibit abnormalities in the distribution of micromeres during cleavage. Subsequently, their germinal bands are positioned abnormally with respect to the embryonic midline and the micromere cap, epiboly fails, and the HRO-NOS knockdown embryos die. This lethality can be rescued by injection of mRNA encoding an eGFP::HRO-NOS fusion protein. HRO-NOS knockdown embryos make their normal complements of mesodermal and ectodermal teloblasts, and the progeny of these teloblasts segregate into distinct mesodermal and ectodermal layers. These results suggest that maternal Hro-nos is required for embryonic development. However, contrary to previous suggestions, maternal inherited Hro-nos does not appear necessary for ectoderm specification.