vasa mRNA accumulates at the posterior pole during blastoderm formation in the flour beetle Tribolium castaneum.

The correct specification of germ cells during embryogenesis is a fundamental step in life that ensures the existence of the next generation. Although different species display various cellular modes of generating germ cells, the product of the vasa gene proves to be a reliable marker of primordial germ cells in metazoans [Extavour and Akam (2003) 130:5869-5884; Raz (2000) 1:1017]. Here, I report the isolation of the vasa ortholog from the red flour beetle Tribolium castaneum, named Tc-vasa, and describe its sequence, its genomic organisation and its expression pattern during early embryogenesis. Unlike in Drosophila where vasa messenger RNA (mRNA) is ubiquitously distributed in the egg, Tc-vasa mRNA gradually accumulates at the posterior egg pole during blastoderm formation. Shortly before gastrulation, Tc-vasa mRNA marks a group of intra-blastodermal cells at the posterior pole. In the germ rudiment, a ball-like group of vasa-positive cells adheres to the growth zone at the posterior end of the embryo. These vasa-positive cells likely represent the primordial germ cells that have not been described in Tribolium prior to gonad formation. At the beginning of germ growth, a small band of vasa-positive cells starts to migrate along the dorsal side of the growth zone. vasa transcription ceases during further germ band extension. In contrast to Drosophila, Tc-vasa transcripts cannot be detected in the germ cells within the gonadal anlage after segmentation is completed.