Transplantation of pronuclei between one-cell-stage embryos was used to construct diploid mouse embryos with two female pronuclei ( biparental gynogenones ) or two male pronuclei ( biparental androgenones ). The ability of these embryos to develop to term was compared with control nuclear-transplant embryos in which the male or the female pronucleus was replaced with an isoparental pronucleus from another embryo. The results show that diploid biparental gynogenetic and androgenetic embryos do not complete normal embryogenesis, whereas control nuclear transplant embryos do. We conclude that the maternal and paternal contributions to the embryonic genome in mammals are not equivalent and that a diploid genome derived from only one of the two parental sexes is incapable of supporting complete embryogenesis.

A comprehensive analysis of transposable element (TE) expression in mammalian full-grown oocytes reveals that LTR class III retrotransposons make an unexpectedly high contribution to the maternal mRNA pool, which persists in cleavage stage embryos. The most abundant transcripts in the mouse oocyte are from the mouse transcript (MT) retrotransposon family, and expression of this and other TE families is developmentally regulated. Furthermore, TEs act as alternative promoters and first exons for a subset of host genes, regulating their expression in full-grown oocytes and cleavage stage embryos. To our knowledge, this is the first example of TEs initiating synchronous, developmentally regulated expression of multiple genes in mammals. We propose that differential TE expression triggers sequential reprogramming of the embryonic genome during the oocyte to embryo transition and in preimplantation embryos.

The importance of embryonic stage-specific molecules in the regulation of cell interactions and cell sorting during development and differentiation has been postulated; however, experimental confirmation of this postulate is scarce. Several candidates for stage-specific molecules have been found by using antisera, raised by both syngeneic (1, 2) and xenogeneic (3) immunization with embryonal carcinoma cells (ECG) or by xenogeneic immunization (4) with mouse embryos. Indeed, inhibition of development of preimplantation mouse embryos by Fab fragments isolated from antisera to the murine teratocarcinoma cell F9 has recently been reported (5). However, such sera contain antibodies to multiple antigenic determinants, which precludes precise definition of embryo stage-specific antigens. In order to circumvent this difficulty and to study the stage-specific molecules in a methodical fashion, we are producing monoclonal antibodies (6) reactive with teratocarcinoma cells and embryos. Production and characterization of one such antibody is described here. MATERIALS AND METHODSPreparation of Monoclonal Reagents. BALB/c mice were immunized by weekly intraperitoneal injection of 107 irradiated F9 cells. The mice were tail-bled 7 days after each injection, and sera were tested for reactivity on F9 cells. Three days after the seventh immunization, the spleen was removed from one mouse, which showed a high titer of antibody reactivity. Splenic lymphocytes were isolated and fused with the P3-X63-Ag8 mouse myeloma cell line as described (6-8). Hybrid cell lines were isolated by growth of the fusion mixtures in Dulbecco's modification of Eagle's minimal essential medium containing 10% fetal bovine serum, 2 mM glutamine, and the HAT components (hypoxanthine/aminopterin/thymidine) (9). Supernatants from Linbro wells containing growing colonies were tested for reactivity on F9 cells by an indirect antibody binding radioimmunoassay (RIA). One positive colony was transferred to mass culture and cloned. Supernatants from clones were also tested in RIA and the positive clones were maintained in tissue culture. Supernatants were collected from dense cultures of specific antibody-producing hybrids, clarified by centrifugaThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S.

Methylation of DNA is an essential epigenetic control mechanism in mammals. During embryonic development, cells are directed toward their future lineages, and DNA methylation poses a fundamental epigenetic barrier that guides and restricts differentiation and prevents regression into an undifferentiated state. DNA methylation also plays an important role in sex chromosome dosage compensation, the repression of retrotransposons that threaten genome integrity, the maintenance of genome stability, and the coordinated expression of imprinted genes. However, DNA methylation marks must be globally removed to allow for sexual reproduction and the adoption of the specialized, hypomethylated epigenome of the primordial germ cell and the preimplantation embryo. Recent technological advances in genome-wide DNA methylation analysis and the functional description of novel enzymatic DNA demethylation pathways have provided significant insights into the molecular processes that prepare the mammalian embryo for normal development.