Proposed role of W chromosome inactivation and the absence of dosage compensation in avian sex determination

Three features of avian sex chromosomes-female heterogamety (ZZ male, ZW female), the apparently inactive state of the W chromosome, and dose-dependent expression of Z-linked genes-are examined in regard to their possible relation to sex determination. It is proposed that the W chromosome is facultatively heterochromatic and that the Z and W chromosomes carry one or more homologous sex-determination genes. The absence of dosage compensation in ZZ embryos, and W inactivation in ZW embryos, would then bring about a 2n(ZZ)-n(ZW) inequality in the effective copy number of such genes. The absence of dosage compensation of Z-linked genes in ZZ embryos is viewed as a means by which two copies of Z-W homologous sex determination genes are kept active to meet the requirements of testis determination. W inactivation may promote ovarian development by reducing the effective copy number of these genes from 2n to n. If there is a W-specific gene for femaleness, spread of heterochromatization to this gene in cells forming the right gonadal primordium may explain the latter's normally undifferentiated state; reversal of heterochromatization may similarly explain the development of the right gonad into a testis following left ovariectomy

The evolution of genomic imprinting

We explore three possible pathways for the evolution of genomic imprinting. (1) Imprinting may be advantageous in itself when imprinted and unimprinted alleles of a locus confer different phenotypes. If a segment of DNA is imprinted in the gametes of one sex but not in those of the other, it might lead to effects correlated with sexual dimorphism. More fundamentally, in certain organisms, sex determination might have evolved because of imprinting. When imprinting leads to chromosome elimination or inactivation and occurs in some embryos but not in others, two classes of embryos, differing in the number of functional gene copies, would result. A model for sex determination based on inequality in the actual or effective copy-number of particular noncoding, regulatory sequences of DNA has been proposed (Chandra, Proc. natn. Acad. Sci. U.S.A. 82. 1165-1169 and 6947-6949, 1985). Maternal control of offspring sex is another possible consequence of imprinting; this would indicate a potential role for imprinting in sex ratio evolution. (2) Genes responsible for imprinting may have pleiotropic effects and they may have been selected for reasons other than their imprinting ability. Lack of evidence precludes further consideration of this possibility. (3) Imprinting could have co-evolved with other traits. For instance, gamete-specific imprinting could lead to a lowered fitness of androgenetic or gynogenetic diploids relative to the fitness of 'normal' diploids. This in turn would reinforce the evolution of anisogamy. The reversibility of imprinting raises the possibility of occasional incomplete or improper erasure. If the site of imprinting is the egg - as appears to be the case with the human X (Chandra and Brown, Nature 253. 165-168, 1975) - either improper imprinting or improper erasure could lead to unusual patterns of inheritance (as in the fragile-X syndrome) or fitness effects skipping generations

The inactive X chromosome in the human female is enriched in 5-methylcytosine to an unusual degree and appears to contain more of this modified nucleotide than the remainder of the genome

By employing a procedure that combines ELISA and photoacoustic spectroscopy, we have examined the content of 5-methylcytosine (m5C) in DNA of individuals who differed from one another in the number of X chromosomes in their genomes. The results show that the human inactive X chromosome (Xi) contains very high amounts of this modified nucleotide. We estimate that in the 46,XX female there is more m5C in Xi (~3.6 × 107) than in all the remaining chromosomes put together (~2.1 × 107). Our results also suggest that nearly one-fifth of all cytosines in Xi are methylated and that, in addition to CpG methylation, there is extensive non-CpG methylation as well

A rapid and gentle method for the salt extraction of chromatin core histones H2A, H2B, H3 and H4 from rat liver nuclei

A complex of histones H2A, H2B, H3 and H4 has been isolated from purified rat liver nuclei by a method which is both gentle and rapid. Nuclei were homogenised in 0.25 I sucrose and the residual nuclear material obtained after centrifligation was adsorbed on calcium phosphate gel. After removing histone H1 from the adsorbed material by washing with 1M NaCl in 25 mM sodium phosphate buffer, pH 6.0, histones H2A, H2B, H3 and H4 were eluted together, with 2 I NaCl in 25 mM sodium phosphate buffer, pH 7.0. The core histones so obtained migrated as a single sharp band on polyacrylamide gel electrophoresis under non-denaturing conditions. Fractionation of the freshly prepared core histones on a Sephadex G-100 column yielded two major protein peaks. The peak having the larger elution volume contained histones H2A and H2B in equal amounts while the peak with the smaller elution volume contained all the four histones. Histones H3 and H4 were present in larger proportions in the second peak

Characterizing MRO in atomistic models of vitreous SiO2_2 generated using ab-initio molecular dynamics

Vitreous silica is the most versatile material for scientific and commercial applications. Although large-scale atomistic models of vitreous-SiO$_2$ (v-SiO$_2$) having medium-range order (MRO) have been successfully developed by melt-quench through classical molecular dynamics, the MRO is not well studied for the smaller-scale models developed by melt-quench using ab-initio molecular dynamics (AIMD). In this study, we obtain atomistic models of v-SiO$_2$ by performing melt-quench simulation using AIMD. The final structure is compared with the experimental data and some recent atomistic models, on the basis of the structural properties. Since AIMD allows for the estimation of electronic structure, a detailed study of electronic properties is also done. It shows the presence of defect states mainly due to dangling bonds in the band-gap region of electronic density of states, whereas the edge-shared type of defective structures in the glassy models are found to contribute mainly in the valence band. In addition, Oxygen and Silicon vacancies as well as bridging Oxygen type of defects were created and their contributions to the band-gap were studied.Comment: 28 pages, 12 figures, preprin