The ontogeny of nanos homologue expression in the oligochaete annelid Tubifex tubifex.

We have cloned and characterized the expression of a nanos homologue (designated Ttu-nos) from the oligochaete annelid Tubifex tubifex. Ttu-nos mRNA is distributed broadly throughout the early cleavage stages. Ttu-nos is expressed in most if not all of the early blastomeres, in which Ttu-nos RNA associates with pole plasms. Ttu-nos transcripts are concentrated to 2d and 4d cells. Shortly after 2d(111) (derived from 2d cell) divides into a bilateral pair of NOPQ proteloblasts, Ttu-nos RNA vanishes from the embryo, which is soon followed by the resumption of Ttu-nos expression in nascent primary blast cells produced by teloblasts. The resumption of Ttu-nos expression occurs only in a subset of teloblast lineages (viz., M, N and Q). After Ttu-nos expression is retained in the germ band for a while, it disappears in anterior-to-posterior progression. At the end of embryogenesis, there is no trace of Ttu-nos expression. Thereafter, growing juveniles do not show any sign of Ttu-nos expression, either. The first sign of Ttu-nos expression is detected in oocytes in the ovary of young adults (ca 40 days after hatching), and its expression continues in growing oocytes that undergo yolk deposition and maturation in the ovisac.