|MadSci Network: Development|
As far as I can tell, no one has done the experiment that you describe. There are at least two reasons for this. First, there is a technical issue, because of the unusual biology of mammals with respect to egg development. The mature mammalian egg is arrested at metaphase of meiosis II until fertilization. Following fertilization, the second meiotic division is completed, and one of the resulting haploid nuclei is cast off as the second polar body. So until fertilization, there is no haploid egg nucleus that could be used as a source in the experiment that you describe. Please see 'Molecular Biology of the Cell' by Alberts et al., or any other comprehensive cell biology textbook, for a good description of the process with figures. You can see Alberts online, although it is usually slow: http://www.ncbi.nlm.nih.gov:80/books/mboc/mboc.cgi?code= 200304683501705 Second, even if we were to rescue such a haploid egg nucleus to substitute for the haploid sperm nucleus, the two nuclei must undergo karyogamy (or nuclear fusion) to make a diploid zygotic nucleus. It is not clear that two egg nuclei can do this. There might be some way to work this out technically, but the scientific purpose behind these experiments has been addressed by other means. Recall that in cloning experiments, the egg nucleus is removed, and a diploid nucleus from another cell is transferred. Cloning experiments address the question: are nuclei other than newly formed zygotic nuclei able to undergo development into normal adults? The answer is clearly yes. The much more interesting question that you have asked is whether there is anything lacking in the genetic material of an egg that is necessary for development. This question has been addressed by other means. At first glance, the answer to this question would appear to be no, based on the fundamental principles of genetics. Half of the genes of an individual come from each parent. Confining our discussion to mammals, the only thing that an egg lacks is the Y chromosome; if the sperm carries one, the zygote is male, while if the sperm carries an X, as the egg does, the zygote is female. So for a female zygote, the two chromosome complements from each parent appear to be identical in terms of their genetic content. The mitochondrial genome comes from the egg, of course, because the sperm carries virtually no cytoplasm. However, it has become clear that there are epigenetic modifications to the genome that make the maternal and paternal genomes nonequivalent. The epigenetic modifications are called 'imprinting'. Imprinting is epigenetic because the actual nucleotide sequence of genes is not changed, but there are modifications to the genetic material that are heritable; principally methylation. Early evidence for imprinting appeared from unusual cases in which a particular set of genes was not inherited from one parent. There is a human disease called Prader-Willi Syndrome, which is caused when a particular chromosomal deletion on chromosome 15 is inherited from the father. The identical deletion inherited from the mother causes a different syndrome. See the OMIM entry at: http://www3.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?176270 a> Even more unusual are those cases of Prader-Willi Syndrome in which the affected individuals are euploid, but have inherited both copies of chromosome 15 from the mother and no chromosome 15 through the father. In this case, there are no missing genes, extra genes, or mutated genes, yet the individuals have a disease identical to that caused by inheriting a chromosome 15 deletion from the father. A limited set of genes is known to be imprinted in mice. You can find out more at this site: http:// www.mgu.har.mrc.ac.uk/imprinting/implink.html Here is a good paper on the acquisition of developmental competence by the oocyte nucleus that involves nuclear transplantation. The issue of imprinting is raised: Bao S, Obata Y, Carroll J, Domeki I, Kono T (2000): Epigenetic modifications necessary for normal development are established during oocyte growth in mice. Biol Reprod 62:616-621. You might also find the MGI Glossary useful in reading from these sources, or even reading this answer: http:// www.informatics.jax.org/userdocs/glossary.shtml Good luck, and thank you for an interesting question. Yours, Paul Szauter Mouse Genome Informatics
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