SINGLE-ACTIVE-X HYPOTHESIS: CYTOLOGICAL EVIDENCE FOR RANDOM INACTIVATION OF X-CHROMOSOMES IN A FEMALE MULE COMPLEMENT

Embryonic loss of human females with partial trisomy 19 identifies region critical for the single active X

To compensate for the sex difference in the number of X chromosomes, human females, like human males have only one active X. The other X chromosomes in cells of both sexes are silenced in utero by XIST, the Inactive X Specific Transcript gene, that is present on all X chromosomes. To investigate the means by which the human active X is protected from silencing by XIST, we updated the search for a key dosage sensitive XIST repressor using new cytogenetic data with more precise resolution. Here, based on a previously unknown sex bias in copy number variations, we identify a unique region in our genome, and propose candidate genes that lie within, as they could inactivate XIST. Unlike males, the females who duplicate this region of chromosome 19 (partial 19 trisomy) do not survive embryogenesis; this preimplantation loss of females may be one reason that more human males are born than females.

Random X inactivation in the mule and horse placenta

In eutherian mammals, dosage compensation of X-linked genes is achieved by X chromosome inactivation. X inactivation is random in embryonic and adult tissues, but imprinted X inactivation (paternal X silencing) has been identified in the extra-embryonic membranes of the mouse, rat, and cow. Few other species have been studied for this trait, and the data from studies of the human placenta have been discordant or inconclusive. Here, we quantify X inactivation using RNA sequencing of placental tissue from reciprocal hybrids of horse and donkey (mule and hinny). In placental tissue from the equid hybrids and the horse parent, the allelic expression pattern was consistent with random X inactivation, and imprinted X inactivation can clearly be excluded. We characterized horse and donkey XIST gene and demonstrated that XIST allelic expression in female hybrid placental and fetal tissues is negatively correlated with the other X-linked genes chromosome-wide, which is consistent with the XIST-mediated mechanism of X inactivation discovered previously in mice. As the most structurally and morphologically diverse organ in mammals, the placenta also appears to show diverse mechanisms for dosage compensation that may result in differences in conceptus development across species.

Circadian and seasonal rhythms of melatonin production in mules (Equus asinus x Equus caballus)

The present study describes the patterns of melatonin production in the mule (Equus asinus x Equus caballus). Blood was sampled hourly for 24 h from eight mule mares in spring and fall. The data obtained show the presence of a circadian rhythm of production of melatonin, with highest values during the dark phase both in spring and fall. In fall the nightly rise of melatonin production begins earlier in the dark phase and reaches higher quantitative levels than in spring. The morning decline of melatonin production is similar in the two seasons. Maximal levels of nightly melatonin production in the mule are about 10 x higher than those described in the horse. The results reported here indicate the persistence of brain structures able to receive and transduce environmental signals in the mule, a genetically sterile mammalian hybrid.

Stochastic models for X chromosome inactivation

A multivariate Gaussian model for mammalian development is presented with the associated biological and mathematical assumptions. Many biological investigations use the female mammal X chromosome to test hypotheses and to estimate parameters of the developmental system. In particular, Lyon's (1961) hypotheses are used as a basis of the mathematical model. Experimental mouse data and three sets of human experimental data are analyzed using the hypothesized Gaussian model. The estimated biological parameters are consistent with some current biological theories.

Preferential late replication of one of the two morphologically distinguishable X-chromosomes in a female muntjac

In a female barking deer, Muntiacus muntjak, whose 2 X-chromosomes are mutually distinguishable from each other, one X has been found to be late replicating in 57.8% cells compared to the other which is late replicating in 42.2% cells. These data are suggestive of preferential inactivation of one X-chromosome. These findings have been discussed in the light of Lyon's hypothesis of random X-inactivation in eutherian mammals.

Nonrandom X-chromosome inactivation--an artifact of cell selection (mouse chimera)

The present study shows that a high degree of variability in the distribution of XX and XY cells exists among various tissues of artificially assembled or spontaneously occurring mouse chimeras. This variation from the expected equal distribution in various tissues of an animal may have resulted from random distribution, unequal segregation, or selective differences of such cells during development. This variation may also explain the observed intertissue and interanimal variations in the distribution of inactive paternal and maternal X-chromosomes in some mammalian females.

Cytological and biochemical correlation of late X-chromosome replication and gene inactivation in the mule

The correlation between late X-chromosome replication and the quantitation of different X-linked glucose-6-phosphate dehydrogenase electrophoretic forms was studied in a natural hybrid, the female mule. In all four animals examined, a significant deviation from the expected 1: 1 ratio of random X-chromosome inactivation was observed, with the donkey X-chromosome the more frequently late-replicating one. The relative amounts of horse and donkey enzymes activities in lysates of mule skin fibroblasts and in peripheral blood were in agreement with this finding: the donkey enzyme was the minor component. Although random expression of the enzymes from the two parent species was not observed, sampling, selection, or adaptation may actually be responsible for the apparent "preferential inactivation". These studies support the hypothesis that late DNA replication indicates genetic inactivation.