These findings suggest that Xist RNA is both necessary and sufficient to trigger heterochromatin formation and transcriptional silencing. A gene mapping to the sex-determining region of the mouse Y chromosome is a member of a novel family of embryonically expressed genes. X-chromosome territories were marked by fluorescent in situ hybridization FISH probes; one identified the duplicated region red , and a second identified the rest of X blue. A signal, the ratio of X chromosomes to sets of autosomes the ploidy; Figure 3 ; Madl and Herman, ; Miller et al. Until recently, it was thought that the initiation of imprinted and random X inactivation were both linked to cellular differentiation Monk and Harper
Polycomb family repressors elaborated in Secs. In organisms where more is known about molecular mechanisms of dosage compensation, specific protein complexes containing noncoding RNAs are targeted to the X chromosome. Despite the association of hydrocarbon signals and Drosophila courtship, absence of these signals did not alter courtship behaviours per se. Male development of chromosomally female mice transgenic for Sry. In some organisms, the Y chromosome was completely lost, for example, in the nematode Caenorhabditis elegans. Whereas cVA decreases the probability that females will re-mate, wild-caught females produce offspring from multiple sires, indicating that polyandry is common and that the effect of cVA is not absolute. Fourth, a high density of X recognition sites could recruit the complex, but no spreading would occur, implying direct, short-range regulation by the complex. SDC-2 activates hermaphrodite sexual development by binding directly to regulatory regions of the male sex-determination gene her-1 , repressing its transcription fold, thereby directing the sex determination pathway to the hermaphrodite mode Dawes et al. Genetic control of sex determination and dosage compensation in C. It can be confidently predicted that they will continue to do so. Molecular identification of discrete X-recognition elements Small, discrete X-recognition elements responsible for recruitment activity were identified by a general strategy that assessed binding of the dosage compensation complex in vivo Csankovszki et al. This latter hypothesis, although incorrect in fruit flies, is a main regulatory mechanism in C. Xist RNA is tightly associated with the nuclear matrix in the interphase nucleus and localization is retained following removal of chromatin by micrococcal nuclease, suggesting that Xist RNA does not contact underlying DNA sequences directly Clemson et al. The basis for the reversal of Xp inactivation is unknown, but may involve an ICM-specific program that represses Xp Xist expression see Sec. The specific X-linked genes that contribute to this phenotype have not been determined. Abstract Article Hilfiker, A. Xce was mapped to the approximate location of the Xic on the mouse X chromosome, suggesting the two loci could be synonymous. Despite the inability to autonomously recruit the complex, these regions, typified by stDp2 , harbor dosage compensated genes when present on the native X chromosome Figure 6. Males of species with non-sexually dimorphic hydrocarbons generally do not court females from dimorphic species, indicating that the dienes might act as reproductive isolation barriers between these species groups. The alkenes are expressed sex-specifically, and have been associated with both sex and species discrimination. In these two model organisms, sex is determined by the number of X chromosomes per diploid genome rather than by a Y-linked gene as in mammals. First, a single site on X could recruit the complex and nucleate long-range spreading of the dosage compensation complex across the entire chromosome. Genes and regulatory elements in the X inactivation-center region. Together, these data indicate that females lacking hydrocarbons are more attractive than those with a normal hydrocarbon profile. Abstract Article Hodgkin, J. SDC-2 also activates the hermaphrodite program of sexual development by repressing the male-specific sex-determination gene her-1 by fold.
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