Spatiotemporal, allelic, and enforced expression of Ximpact, the Xenopus homolog of mouse imprinted gene Impact

Mouse Impact is an imprinted gene encoding an evolutionarily conserved protein of unknown function. We isolated cDNA for the Xenopus homolog of Impact (Ximpact), since the clawed frog not only provides an excellent model for the study of gene function in early development but also allows the generation of interspecific F1 hybrids required for the examination of allelic expression status. The predicted product of Ximpact shows an extreme sequence similarity to those of mouse Impact and its homologs in nematoda, fission yeast, and budding yeast. The transcript of Ximpact is present in oocytes as well as in early embryos, and its spatial distribution is ubiquitous in both embryonic and adult stages. An RT-PCR-RFLP assay using the reciprocal interspecific F1 hybrids and a DNA polymorphism between X. laevis and X. borealis showed that Ximpact is expressed biallelically when analyzed as a whole embryo. Overexpression of Ximpact by RNA microinjection resulted in a higher than normal rate of gastrulation defects, suggesting the need for tight control of its dosage in early development.