Germ cell-specific expression of a gene encoding eukaryotic translation elongation factor 1α (eEF-1α) and generation of eEF-1α; retropseudogenes in Xenopus laevis

We have studied by in situ hybridization the expression of the genes encoding the somatic form and the oocyte form of Xenopus laevis eEF-1α. The somatic form of eEF-1α (eEF-1αS) mRNA is virtually undetectable in male and female germ cells of the adult gonad but is very abundant in embryonic cells after the neurula stage. In contrast, another form of eEF-1α (eEF-1αO) mRNA is highly concentrated in oogonia and in previtellogenic oocytes but is undetectable in eggs and embryos. eEF-1αO mRNA is also present in spermatogonia and spermatocytes of adult testis. The latter finding identifies eEF-1αO mRNA as a germ cell-specific gene product. Although germ cells contain very little eEF-1αS mRNA, several eEF-1αS retropseudogenes exist in X. laevis chromosomes. These genes are thought to arise in germ cells from reverse transcription of mRNA and subsequent integration of the cDNA copies into chromosomal DNA. We suggest that eEF-1αS pseudogenes are generated in primordial germ cells of the embryo before they differentiate into oogonia or spermatogonia.