ScienceWeek

PLANT BIOLOGY: ON MATERNAL CONTROL OF EMBRYOGENESIS

The following points are made by Frederic Berger (Science 2004 303:483):

1) The conquest of the terrestrial environment by plants and mammals is linked to the parallel evolution of a predominantly maternal control over embryogenesis. This evolution of maternal control spurred the development of an intriguing epigenetic mechanism, called "imprinting", for controlling gene expression. Imprinted genes are those in which expression depends on their parental origin. For example, imprinted genes are expressed only from the maternal allele, the paternal allele having been silenced by methylation of cytosines in the gene sequence. Kinoshita et al (1) have proposed a mechanism for establishing one-way control of imprinting in plants that is distinct from imprinting in mammals.

2) The nutrition and protection of the embryo by the mother requires the invention of a specific interface between the two. In mammals, this interface derives from extra-embryonic tissues, which form the placenta. In plants, the embryo is connected to the maternal tissues by the endosperm, the product of a second fertilization event. The plant endosperm and the mammalian placenta are both subjected to imprinting, resulting in the preferential expression of maternal copies of genes, most notably those involved in the control of growth (2-4). In the mouse, disruption of the imprinting of specific genes, such as Igfr2r, leads to aberrant development of the trophoblast (which contributes to the placenta) (5).

3) In plants, the polycomb-group gene MEDEA, a master regulator of endosperm development, is known to be imprinted in the endosperm but not in the embryo. Kinoshita et al. now report that a second plant gene, FWA, which encodes a homeodomain transcription factor, is imprinted in the endosperm of the model plant Arabidopsis and is expressed only from the maternal allele (1). Expression of FWA in endosperm coincides with an overall reduction in the amount of methylated cytosine residues in the direct repeat sequences of the 5' region of this gene. The level of FWA methylation remains high in other tissues of the plant, preventing transcription of FWA in these tissues.

References (abridged):

1. T. Kinoshita et al., Science 303, 521 (2004); published online 20 November 2003 (10.1126/science. 1089835)

2. F. Berger, V. Gaudin, Chromosome Res. 11, 277 (2003)

3. C. Baroux, C. Spillane, U. Grossniklaus, Adv. Genet. 46, 165 (2002)

4. M. Spielman, R. Vinkenoog, H. G. Dickinson, R. J. Scott, Trends Genet. 17, 705 (2001)

5. M. Constancia et al., Nature 417, 945 (2002) [Medline]. M. Charalambous et al., Proc. Natl. Acad. Sci. U.S.A. 100, 8292 (2003)

Science http://www.sciencemag.org

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MECHANISMS AND CONTROL OF EMBRYONIC GENOME ACTIVATION IN MAMMALIAN EMBRYOS.

The following points are made by K.E. Latham (Int Rev Cytol 1999 193:71):

1) Activation of transcription within the embryonic genome (EGA) after fertilization is a complex process requiring a carefully coordinated series of nuclear and cytoplasmic events which collectively ensure that the two parental genomes can be faithfully reprogrammed and restructured before transcription occurs.

2) Available data indicate that inappropriate transcription of some genes during the period of nuclear reprogramming can have long-term detrimental effects on the embryo. Therefore, precise control over the time of EGA is essential for normal embryogenesis.

3) In most mammals, genome activation occurs in a stepwise manner. In the mouse, for example, some transcription occurs during the second half of the one-cell stage, and then a much greater phase of genome activation occurs in two waves during the two-cell stage, with the second wave producing the largest onset of de novo gene expression.

4) Changes in nuclear structure, chromatin structure, and cytoplasmic macromolecular content appear to regulate these periods of transcriptional activation. A model is presented in which a combination of cell cycle-dependent events and both translational and posttranslational regulatory mechanisms within the cytoplasm play key roles in mediating and regulating EGA.

International Review of Cytology http://www.elsevier-international.com/serials/cytology

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COMPLETION OF MOUSE EMBRYOGENESIS REQUIRES BOTH THE MATERNAL AND PATERNAL GENOMES.

The following points are made by J. McGrath and D. Solter (Cell 1984 37:179):

1) The authors report that 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).

2) 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.

3) The results show that diploid biparental gynogenetic and androgenetic embryos do not complete normal embryogenesis, whereas control nuclear transplant embryos do.

4) The authors 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.

Cell http://www.cell.com

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