Maternal transcripts of mitotic checkpoint gene, Xbub3, are accumulated in the animal blastomeres of Xenopus early embryo.

Maternally transcribed mRNAs play the important role during early embryogenesis. Especially in patterning, distribution of the maternal transcripts has a causal relation to axis formation in the early embryo. We compared the quantity of mRNAs among four blastomeres of Xenopus 8-cell-stage embryos by the differential display method. A novel gene, Xbub3, was cloned by screening the oocyte cDNA library with an animal blastomere-enriched PCR product. Xbub3 is a homolog of the human mitotic checkpoint gene hBub3. A transcript of Xbub3 was 2940 bp and encoded a predicted protein of 330 amino acids with six WD repeats. Expression of Xbub3 was observed from oocyte to tadpole. Whole-mount in situ hybridization showed that Xbub3 mRNAs were uniformly distributed in the early stages of oogenesis but gradually localized to the animal hemisphere, especially in the perinuclear cytoplasm of full-grown oocytes. In the cleavage-stage embryos, the maternal transcripts of Xbub3 were recruited into each blastomere, associating closely with chromosomes. Zygotic expression of Xbub3 was widely detected in gastrula ectoderm and was gradually restricted to the central nervous systems, eyes, and branchial arches by the tadpole stage. This evidence contributes to understanding of the regulatory mechanism of the cell cycle and cell differentiation in the early embryo.