Embryo-lethal Phenotype Research Articles

Disruption of plE2, the gene for the E2 subunit of the plastid pyruvate dehydrogenase complex, in Arabidopsis causes an early embryo lethal phenotype.

The pyruvate dehydrogenase multi-enzyme complex is the main source of acetyl-CoA formation in the plastids of plants and is composed of multiple copies of four different subunits, E1alpha, E1beta, E2, and E3. A T-DNA insertion into the gene for the plastidic E2 (dihydrolipoyl acetyltransferase) subunit, plE2, of the complex in Arabidopsis destroys the expression of that gene. The resulting mutation has no apparent phenotype in the heterozygous state, but the homozygous mutation is lethal. Haploid sperm and eggs that contain only the disrupted plE2 gene function normally resulting in the formation of an embryo that is homozygous for the mutation. This embryo only develops to an early stage before the development arrests resulting in an early embryo-lethal phenotype. While the mutation could not be complemented with the cDNA for the plE2 gene under control of the 35S, the AtSERK1, or the napin promoter, it could be complemented using the endogenous plE2 promoter to drive expression of the plE2 cDNA. This verifies the essential nature of the plastidic pyruvate dehydrogenase complex and its role in embryo formation.

The embryology of the common arterial trunk

The developmental background of common arterial trunk still remains to be elucidated. In this anomaly both the coronary, systemic and pulmonary circulation arise from a common arterial stem with a common orifice. The contribution of the embryonic ectomesenchymal neural crest seems to be essential for a normal separation of the aorta, pulmonary trunk and the adjoining myocardium lined outflow tract. Experimental neural crest ablation studies in chicken have proved that there is a role for neural crest cells. Several molecular biological studies, however, have shown that we are not solely dealing with the influence of neural crest. Many genes and gene cascades have been manipulated in mice and have shown that e.g. the endothelin cascade, Foxc1 and Foxc2, and more recently Sema3C and neuropilin-1 are factors, that if disturbed, can lead to common arterial trunk often in combination with aortic arch anomalies such as interruption type B. In humans common arterial trunk is one of the characteristics of the 22q11 deletion syndrome. In the mouse several of these genes, located on the syntenic chromosome 16 have been studied for this effect, but most result in an embryolethal phenotype. Neural crest tracing studies in both chicken–quail chimeras and transgenic neural crest reporter mice have revealed that there is a difference in neural crest contribution to the aortic and pulmonary truncal wall. The orifice of the common arterial trunk reflects this finding in that the often abnormally high positioned coronary orifices reside in the aortic part of the orifice. Combining current data the common denominator for common arterial trunk seems to be a defective interaction of neural crest, endothelial cells and the surrounding mesenchyme. Disturbances of genes in any of the three cell types can lead to the malformation.

The Arabidopsis PILZ group genes encode tubulin-folding cofactor orthologs required for cell division but not cell growth.

Plant microtubules are organized into specific cell cycle-dependent arrays that have been implicated in diverse cellular processes, including cell division and organized cell expansion. Mutations in four Arabidopsis genes collectively called the PILZ group result in lethal embryos that consist of one or a few grossly enlarged cells. The mutant embryos lack microtubules but not actin filaments. Whereas the cytokinesis-specific syntaxin KNOLLE is not localized properly, trafficking of the putative auxin efflux carrier PIN1 to the plasma membrane is normal. The four PILZ group genes were isolated by map-based cloning and are shown to encode orthologs of mammalian tubulin-folding cofactors (TFCs) C, D, and E, and associated small G-protein Arl2 that mediate the formation of alpha/beta-tubulin heterodimers in vitro. The TFC C ortholog, PORCINO, was detected in cytosolic protein complexes and did not colocalize with microtubules. Another gene with a related, although weaker, embryo-lethal phenotype, KIESEL, was shown to encode a TFC A ortholog. Our genetic ablation of microtubules shows their requirement in cell division and vesicle trafficking during cytokinesis, whereas cell growth is mediated by microtubule-independent vesicle trafficking to the plasma membrane during interphase.

Heritability of the maternal meiotic drive system linked to Om and high-resolution mapping of the Responder locus in mouse.

Matings between (C57BL/6 x DDK)F(1) females and C57BL/6 males result in a significant excess of offspring inheriting maternal DDK alleles in the central region of mouse chromosome 11 due to meiotic drive at the second meiotic division. We have shown previously that the locus subject to selection is in the vicinity of D11Mit66, a marker closely linked to the Om locus that controls the preimplantation embryo-lethal phenotype known as the "DDK syndrome." We have also shown that observation of meiotic drive in this system depends upon the genotype of the sire. Here we show that females that are heterozygous at Om retain the meiotic drive phenotype and define a 0.32-cM candidate interval for the Responder locus in this drive system. In addition, analysis of the inheritance of alleles at Om among the offspring of F(1) intercrosses indicates that the effect of the sire is determined by the sperm genotype at Om or a locus linked to Om.

The Role of DNA Methylation in Plant Development

DNA methylation has been implicated in a wide range of biological processes including gene regulation (Razin and Cedar 1991; Finnegan et al. 1993), determination of chromatin structure (Lewis and Bird 1991), timing of DNA replication (Lewis and Bird 1991), genomic imprinting (Sasaki et al. 1993), increasing the spontaneous mutation rate (Coulondre et al. 1978; Duncan and Miller 1980), as the basis for epigenetic phenomena (Holliday 1993), and as a defense mechanism to protect against foreign or invading DNA (Bird 1986; Bestor 1990; Doerfler 1991). Despite this, the existence of organisms such as yeast and Drosophila that lack DNA methylation has led to debate about the role of DNA methylation. There can be no doubt that DNA methylation plays a fundamental role in mouse development following the establishment of a targeted mutation of the DNA methyltransferase gene in the germ line of transgenic mice. Embryos that were homozygous for the mutation had methylation levels reduced to about 30% of wild-type levels. These embryos were stunted and delayed in development compared to their normal siblings and spontaneously aborted in mid-gestation. Although the underlying cause for the embryo-lethal phenotype has not been determined, it has been proposed that inappropriate gene expression resulting from the reduced level of DNA methylation is involved (Li et al. 1992). In contrast, mutants of Neurospora crassa that abolish all detectable methylation showed only minor changes in vigor and growth morphology, suggesting that DNA methylation is not absolutely essential for development of this organism (Foss et al. 1993). This...