Mesodermal determinants in the Tubifex embryo

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The second (2d) and fourth (4d) micromeres of the D quadrant in the Tubifex embryo are specified as precursors of ectodermal and mesodermal embryonic stem cells, respectively. To gain an insight into the mechanisms of this stem cell specification, we performed a systematic screen of lineage specific maternal mRNAs, using subtraction hybridization between cDNAs from 2d and 4d. We initially identified 16 genes, which include Impact, HMGB, Nucleolin, TERE1, ELAV and RNA binding proteins. Whole-mount in situ hybridization with riboprobes synthesized from these genes revealed that maternally supplied transcripts "behave" in a similar way to the pole plasms (yolk-deficient cytoplasm). That is, these transcripts are first segregated to the animal pole and the vegetal pole at the end of the first mitosis, partitioned to D quadrants, and finally localized to 2d and 4d cells. During the course of this study, we did not come across such genes as would be segregated exclusively to either 2d or 4d. Besides the above-mentioned subtraction experiments, we isolated and characterized several development genes (dorsal, hairy, decapentaplegic caudal and vasa) in Tubifex. Except caudal that is expressed zygotically, these genes are expressed maternally and transcripts stored in oocytes are found to behave in a very similar fashion to the gene transcripts discovered in the subtraction experiments.