Interrelationship between the states of chromosome compaction, replication and transcription patterns in the polytene-X chromosome of Drosophila melanogaster.
01 Sep 1990-Vol. 99, Iss: 5, pp 377-386
TL;DR: Findings while favour a positive correlation between the state of chromosome compaction and replication patterns, are at odds with the idea that for an altered level of transcription, a modulation in the stateof chromosome compACTION and replication is an obligatory pre-requisite.
Abstract: We studied by autoradiography the replication and transcription patterns of polytene X chromosomes in different segmental aneuploid and hyperploid conditions in a constant diploid autosomal background. In all karyotypes, the patterns of X chromosome compaction and DNA synthesis were observed to be set strictly either at a “male-level” or a “female-level”. Addition of X-fragments to 1X progeny exceeding a certain limit (62% with proximal duplication) changed the pattern of chromatin compaction and replication of X chromosomes from male to female level, while similar additional X-fragments had no comparable effect on 2X hyperdiploids. In contrast, different levels of X-transcription per segment were observed in these aneuploids with variable X-load and in consequence, an equivalent amount of total X-transcription (relative to autosomal transcription) was visualized. These findings while favour a positive correlation between the state of chromosome compaction and replication patterns, are at odds with the idea that for an altered level of transcription, a modulation in the state of chromosome compaction and replication is an obligatory pre-requisite.
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TL;DR: The identity and equality of genie expression between the two sexes are not associated, a priori, with the development of sex, but rather involves a system of genes, plus and minus modifiers, which regardless of sex tends to repress the action of extra doses of sex-linked genes.
Abstract: INVESTIGATIONS of the expression of various sex-linked genes in Drosophila have shown that for most of them their phenotypic manifestation is identical in male and female. Stern1, Muller2 and later Muller et al.3 by use of mutations such as bobbed and apricot in various doses showed that the identity and equality of genie expression between the two sexes are not associated, a priori, with the development of sex, but rather involves a system of genes, plus and minus modifiers, which regardless of sex tends to repress the action of extra doses of sex-linked genes. Muller proposed the name ‘dosage compensation’ for this effect and concluded that for each sex-linked gene there is a set of ‘compensator genes’4.
185 citations
TL;DR: It is reported here that, reasoning that sex- specific lethal mutations may represent lesions in the processes controlling the transcription of X-linked loci, several male-specific lethal mutations are sought and recovered and it is noted that they affect the levels ofX-linked enzyme activities in crude extracts of homozygous male larvae.
Abstract: In Drosophila, a large group of structural genes exhibit coordinate regulation, not because they function in a common developmental pathway but because they happen to reside on the X chromosome. These genes are subjected to the regulatory mechanism of dosage compensation which insures that their phenotypic products are identical in the sex with one and in the sex with two X chromosomes. This equalization of gene products is achieved by regulating the level of transcription of both X chromosomes in females and of the single X chromosome in males1. We report here that, reasoning that sex-specific lethal mutations may represent lesions in the processes controlling the transcription of X-linked loci, we sought and recovered several male-specific lethal mutations and noted that they affect the levels of X-linked enzyme activities in crude extracts of homozygous male larvae. Autoradiographic monitoring of RNA synthesis in larval polytene chromosomes of males homozygous for one of these mutations, mlets, reveals a significant reduction in the rate of X chromosome transcription.
177 citations
TL;DR: The results demonstrate that the Sxl gene plays an important role in controlling X-chromosome activity and the relationship among the various genes known to act in sex differentiation and dosage compensation is discussed.
Abstract: The rate of 3H-uridine incorporation into X-chromosome and autosomal RNA was measured as an indicator of relative transcription activity in larvae carrying various Sxl mutant alleles. Hyperactivity of X chromosomes was found in heteroallelic Sxlf#1/Sxlfhv#1 and homozygous Sxlf#2 female larvae. Sxlfhv#1 homozygotes, Sxlf#1/Sxl+ heterozygotes, heteroallelic Sxlf#2/Sxlf#2 as well as homozygous Sxlf#ba female larvae exhibited normal X chromosome transcription. Except for Sxlf#ba, there is a correlation between the viability of the mutants and the degree to which X-chromosome activity is elevated. Male larvae carrying the dominant male-specific lethal mutation SxlM#1 displayed X chromosomes only half as wide as those of control larvae. However, it could not be determined whether this property is the result of a lower transcription rate or of underreplication of the mutated X chromosomes. The results demonstrate that the Sxl gene plays an important role in controlling X-chromosome activity. The relationship among the various genes known to act in sex differentiation and dosage compensation is discussed.
131 citations
TL;DR: The comparative lack of sexual dimorphism of apricot and other [sex-linked] genes studied is due to "sex-limitation", i.e., to a compensatory influence of the dosage difference between the sexes in respect to other genes in the X-chromosome.
Abstract: The comparative lack of sexual dimorphism of apricot and other [sex-linked] genes studied is due to “sex-limitation”, i.e., to a compensatory influence of the dosage difference between the sexes in respect to other genes in the X-chromo-some. The facts are of particular interest from an evolutionary standpoint [Muller et al., 1931, Anat. Rec. Suppl.51, p. 110].
130 citations
TL;DR: It is concluded that the model sequence of replication in polytene chromosomes follows a continuous to discontinuous labeling sequence, and that the single X in male completes its replication earlier than either the autosomes in male or the X's in female, substantiating the hypothesis of hyperactivity of the singleX in male as the chromosomal basis of dosage compensation in Drosophila.
Abstract: Thymidine-3H labeling patterns on the X (section 1 A to 12 E of Bridges' map) and 2 R (section 56 F to 60 F of Bridges' map) segments in the salivary gland chromosomes of Drosophila melanogaster have been analyzed in male and female separately The observed patterns fit, with a few exceptions, in a continuous to discontinuous labeling sequence In nuclei with similar labeling patterns on the 2R segment in both sexes, the number of labeled sites on the X in male is always less than in female X's The labeling frequency of the different sites on the male X is considerably lower than those on the female X's, while the sites on the 2R segment have very similar frequency in the two sexes The rate of thymidine-3H incorporation (as judged by visual grain counting) is relatively higher in male X than in female X's It is concluded that the model sequence of replication in polytene chromosomes follows a continuous to discontinuous labeling sequence, and that the single X in male completes its replication earlier than either the autosomes in male or the X's in female This asynchronous and faster rate of replication by the polytene X-chromosome in male substantiates the hypothesis of hyperactivity of the single X in male as the chromosomal basis of dosage compensation in Drosophila
70 citations
"Interrelationship between the state..." refers background in this paper
...subdivisions of the X chromosome and 56F to 60F subdivisions of the 2R of Bridges' map (Lindsley and Grell 1968), respectively were considered ( Lakhotia and Mukherjee 1970 )....
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...Decondensed nature and early replicating properties of the X chromosomes were observed to be concomitant with its hyperactive nature ( Lakhotia and Mukherjee 1970; Das et al 1982;Mutsuddi et al1985), and thus was speculated to have some unspecified role in elevation of the level of X-transcription in heterogametic sexes....
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...puffs, interbands and bands) ( Lakhotia and Mukherjee 1970; Das et al 1982)....
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