Male X-chromosome Research Articles

Msl1-Mediated Dimerization of the Dosage Compensation Complex Is Essential for Male X-Chromosome Regulation in Drosophila

The Male-Specific Lethal (MSL) complex regulates dosage compensation of the male X chromosome in Drosophila. Here, we report the crystal structure of its MSL1/MSL2 core, where two MSL2 subunits bind to a dimer formed by two molecules of MSL1. Analysis of structure-based mutants revealed that MSL2 can only interact with the MSL1 dimer, but MSL1 dimerization is MSL2 independent. We show that Msl1 is a substrate for Msl2 E3 ubiquitin ligase activity. ChIP experiments revealed that Msl1 dimerization is essential for targeting and spreading of the MSL complex on X-linked genes; however, Msl1 binding to promoters of male and female cells is independent of the dimer status and other MSL proteins. Finally, we show that loss of Msl1 dimerization leads to male-specific lethality. We propose that Msl1-mediated dimerization of the entire MSL complex is required for Msl2 binding, X chromosome recognition, and spreading along the X chromosome.

X-inactivation and X-reactivation: epigenetic hallmarks of mammalian reproduction and pluripotent stem cells.

X-chromosome inactivation is an epigenetic hallmark of mammalian development. Chromosome-wide regulation of the X-chromosome is essential in embryonic and germ cell development. In the male germline, the X-chromosome goes through meiotic sex chromosome inactivation, and the chromosome-wide silencing is maintained from meiosis into spermatids before the transmission to female embryos. In early female mouse embryos, X-inactivation is imprinted to occur on the paternal X-chromosome, representing the epigenetic programs acquired in both parental germlines. Recent advances revealed that the inactive X-chromosome in both females and males can be dissected into two elements: repeat elements versus unique coding genes. The inactive paternal X in female preimplantation embryos is reactivated in the inner cell mass of blastocysts in order to subsequently allow the random form of X-inactivation in the female embryo, by which both Xs have an equal chance of being inactivated. X-chromosome reactivation is regulated by pluripotency factors and also occurs in early female germ cells and in pluripotent stem cells, where X-reactivation is a stringent marker of naive ground state pluripotency. Here we summarize recent progress in the study of X-inactivation and X-reactivation during mammalian reproduction and development as well as in pluripotent stem cells.

Cold-induced alteration in the global structure of the male sex chromosome of In(1)B M2 (reinverted) of Drosophila melanogaster is associated with increased acetylation of histone 4 at lysine 16

In Drosophila melanogaster, dosage compensation occurs through hypertranscription of sex-linked genes in males. The hypertranscription involves acetylation of histone 4 at lysine 16 (H4K16) on amale X-chromosome, brought about by a histone acetyltransferase encoded by the dosage compensation gene, males absent on the first (mof). We report a phenomenon in the strain In(1)B(M2)(reinverted) of D. melanogaster where the global structure of the male X-chromosome can be altered at the third instar larval stage through a 4-h cold shock at 12+/-1 degrees C. We show that the cold shock results in a transient hyperacetylation of H4K16 and an increased expression of MOF. Control proteins H4 acetylated at lysine 5, and the dosage compensation gene msl-2, do not show any change in expression after cold shock. Cytology of the male X-chromosome at different time points during cold shock and recovery, suggests that the hyperacetylation of H4 at lysine 16 causes the X-chromosome to corkscrew into itself, thereby achieving the cold-induced change in the higher order structure of the male polytene X-chromosome. Our studies suggest a role for H4K16 in maintaining the structure of the male X-chromosome in Drosophila.

Structure and function of MYST1 histone acetyltransferase in the interactome of animal cells

The major function of protein MYST1 is acetylation of histone H4 at the K16 residue. This modification is essential for chromatin remodeling and is used for regulation of gene expression in eukaryotes. MYST1 is a part of multiprotein complexes that accomplish functions of male X-chromosome activation and thereby functions of dosage compensation in drosophila and, in mammals, global acetylation of histone H4 K16. Recently, novel functional links between MYST1 and proteins ATM and p53 have been observed, and it is recognized that MYST1 plays a role in tumor suppression mechanisms. In the present review, we examine novel data about functional composition and mechanisms of MYST1-containing complexes. Interplay between MYST1 and other components of the animal cell interactome is also discussed.

LTR retrotransposons and the evolution of dosage compensation in Drosophila

BackgroundDosage compensation in Drosophila is the epigenetic process by which the expression of genes located on the single X-chromosome of males is elevated to equal the expression of X-linked genes in females where there are two copies of the X-chromosome. While epigenetic mechanisms are hypothesized to have evolved originally to silence transposable elements, a connection between transposable elements and the evolution of dosage compensation has yet to be demonstrated.ResultsWe show that transcription of the Drosophila melanogaster copia LTR (long terminal repeat) retrotransposon is significantly down regulated when in the hemizygous state. DNA digestion and chromatin immunoprecipitation (ChIP) analyses demonstrate that this down regulation is associated with changes in chromatin structure mediated by the histone acetyltransferase, MOF. MOF has previously been shown to play a central role in the Drosophila dosage compensation complex by binding to the hemizygous X-chromosome in males.ConclusionOur results are consistent with the hypothesis that MOF originally functioned to silence retrotransposons and, over evolutionary time, was co-opted to play an essential role in dosage compensation in Drosophila.

Painting of fourth and chromosome-wide regulation of the 4th chromosome in Drosophila melanogaster

Drosophila melanogaster exhibits two expression-regulating systems that target whole, specific chromosomes: the dosage compensation system whereby the male-specific lethal complex doubles transcription of genes on the male X-chromosome and the chromosome 4-specific protein Painting of fourth, POF. POF is the first example of an autosome-specific protein and its presence raises the question of the universality of chromosome-specific regulation. Here we show that POF and heterochromatin protein 1 (HP1) are involved in the global regulation of the 4th chromosome. Contrary to previous conclusions, Pof is not essential for survival of diplo-4th karyotype flies. However, Pof is essential for survival of haplo-4th individuals and expression of chromosome 4 genes in diplo-4th individuals is decreased in the absence of Pof. Mapping of POF using chromatin immunoprecipitation suggested that it binds within genes. Furthermore, we show that POF binding is dependent on heterochromatin and that POF and HP1 bind interdependently to the 4th chromosome. We propose a balancing mechanism involving POF and HP1 that provides a feedback system for fine-tuning expression status of genes on the 4th chromosome.

Chromosomal anomalies in first‐trimester miscarriages

It is well known that a large proportion of first-trimester spontaneous abortions is caused by chromosomal disorders. The present study represents a unique material and the success rate of the karyotyping was high. Chromosomal analysis from chorionic villus sampling of 259 of 304 consecutive first-trimester miscarriages in the Uppsala County, Sweden is presented. An abnormal karyotype was found in 61% of the cases. Autosomal trisomies were most frequently detected (in 37% of the karyotyped samples), followed by polyploidies (9%) and monosomy X (6%). Cases with an extra sex chromosome constituted approximately 5% of the karyotyped abortions, with a remarkable high frequency of 47,XXY (3.4%), that is approximately 40 times greater the prevalence of Klinefelter syndrome among live birth. Similar to autosomal chromosome abnormalities, present finding indicates that the majority of sex chromosome abnormalities do not survive to term. Autosomal trisomies and an extra X-chromosome in males (47,XXY) were associated with an advanced maternal age, whereas monosomy X as well as polyploidy changes seems to be inversely related to the age of the mother. The single most common aberration was trisomy 16, which was found in 14% of the chromosomally abnormal abortions.

Characterizing allelic associations from unphased diploid data by graphical modeling

A method for estimating a graphical model to describe allelic associations between genetic loci is extended to use diploid genotypes rather than haploid data. It also provides haplotype frequency estimates, estimates of phase for sampled individuals, allows for missing or partial information, genotyping errors, and arbitrary penetrance functions. A data set of 688 unrelated individuals genotyped on 25 genetic loci is used to illustrate haplotype frequency estimation. The frequencies obtained are shown to be similar to those obtained by the PHASE program. We also illustrate how putative loci for traits can be included in the analysis in order to detect allele-phenotype associations. Haplotype reconstruction is illustrated on a standard set of 40 male X-chromosome haplotypes, randomly paired to simulate diploid genotype data. The novel method is shown to be comparable to most existing ones, though the PHASE method does consistently better on this problem. The graphical model approach is shown to have some advantages in terms of tractability and can be used to select informative subsets of loci and to map loci influencing phenotypes.

Initiation of dosage compensation in Drosophila embryos depends on expression of the roX RNAs

The roX1 and roX2 genes of Drosophila produce non-coding transcripts that localize to the X-chromosome. In spite of their lack of sequence similarity, they are redundant components of an RNA/protein complex that up-regulates the male X-chromosome, contributing to the equalization of X-linked gene expression between males and females. roX1 is detected at 2 h AEL, prior to formation of the complex, and is present in both sexes. Maternally provided MLE (Maleless) is required for roX1 stability. By contrast, roX2 is male-specific and is first observed at 6 h. Either roX transcript can support X-localization of the complex, but localization is delayed in roX1 mutants until roX2 expression. These results support a model for the ordered assembly of the complex in embryos.

Painting of fourth, a chromosome-specific protein in Drosophila.

Chromosome-specific gene regulation is known thus far only as a mechanism to equalize the transcriptional activity of the single male X chromosome with that of the two female X chromosomes. In Drosophila melanogaster, a complex including the five Male-Specific Lethal (MSL) proteins, "paints" the male X chromosome, mediating its hypertranscription. Here, with the molecular cloning of Painting of fourth (Pof), we describe a previously uncharacterized gene encoding a chromosome-specific protein in Drosophila. Unlike the MSL proteins, POF paints an autosome, the fourth chromosome of Drosophila melanogaster. Chromosome translocation analysis shows that the binding depends on an initiation site in the proximal region of chromosome 4 and spreads in cis to involve the entire chromosome. The spreading depends on sequences or structures specific to chromosome 4 and cannot extend to parts of other chromosomes translocated to the fourth. Spreading can also occur in trans to a paired homologue that lacks the initiation region. In the related species Drosophila busckii, POF paints the entire X chromosome exclusively in males, suggesting relationships between the fourth chromosome and the X and between POF complexes and dosage-compensation complexes.

Sequence-specific DNA Binding Activity of RNA Helicase A to the p16INK4a Promoter

p16(INK4a) is frequently altered in human cancer, often through epigenetically mediated transcriptional silencing. However, little is known about the transcriptional regulation of this gene. To learn more about such control, we initiated studies of proteins that bind to the promoter in cancer cells that do, and do not, express the gene. We identify RNA helicase A (RHA) as a protein that binds much better to the p16(INK4a) promoter in the expressing cells. RHA has not previously been characterized to manifest sequence-specific DNA interaction but does so to the sequence 5' CGG ACC GCG TGC GC 3' in the p16(INK4a) promoter. The Drosophila homologue to RHA, maleless (Mle), functions in the fly for 2-fold activation of male X-chromosome genes. In our experimental setting, RHA induces a similar modest up-regulation of the p16(INK4a) promoter that is dependent upon its sequence-specific interaction. Mle colocalizes with hyperacetylated H4Ac16 on the X-chromosome and some autosomal loci. The decreased binding of RHA to p16(INK4a) in our cells, where the gene is transcriptionally inactive, is associated with decreased amounts of RHA that immunoprecipitate with acetylated lysine antibodies. Finally, we show RHA to be a cellular substrate for caspase-3, which decreases its sequence-specific binding to p16(INK4a) by cleavage of the N terminus. Thus, we have identified a new protein interaction with the p16(INK4a) promoter that involves an important protein for transcriptional modulation. This interaction is decreased in cancer cells, where this gene is aberrantly transcriptionally silent.

XMR is associated with the asynapsed segments of sex chromosomes in the XY body of mouse primary spermatocytes.

The XMR (Xlr-related, meiosis-regulated) protein is an M(r) 30,000 nuclear protein closely associated with the XY body in mouse primary spermatocytes. It shows sequence similarity with several other meiosis-specific proteins. In the present study, we investigated the fine immunolocalization of XMR in the XY body by laser confocal and electron microscopy. It was found that XMR was associated with the asynapsed segments of sex chromosomes, including their axes and the surrounding chromatin loops. In contrast, the pseudoautosomal region and the opposite free end of the X were unlabeled for XMR. In mice with the reciprocal T(X;16)16H translocation, XMR was also associated with the heterochromatic translocation product that emerges from the XY body. These findings at the subchromosomal level point to a role for XMR in chromatin condensation and transcriptional inactivation. XMR is unique among proteins in being capable of association with the XY body. It could play a specific role in a mechanism of male X-chromosome inactivation in mammals.

Ionizing radiation and genetic risks VII. The concept of mutation component and its use in risk estimation for Mendelian diseases

The responsiveness of Mendelian diseases to an increase in the mutation rate is studied by using the concept of the mutation component (MC) of genetic diseases. Algebraic expressions to evaluate MC at any specific generation following either a one-time or a permanent increase in mutation rate are derived and are illustrated with numerical examples. For a one-time increase in mutation rate, the analysis shows that the first generation MC for autosomal dominant diseases is equal to the selection coefficient; this is also true for X-linked diseases (adjusted for the proportion of X-chromosomes in males). For autosomal recessive diseases the first generation MC is substantially smaller than that for autosomal dominants. In subsequent generations MC gradually decays to zero. Under conditions of a permanent increase in the mutation rate, the MC for autosomal dominant, X-linked and completely recessive autosomal disorders progressively increases to reach a value of one at the new equilibrium. For incompletely recessive autosomal disorders, however, the MC at equilibrium can be larger than one. The rates of approach to the new equilibrium are different for the different classes of diseases, dictated by selection and time (in generations) following radiation exposure. The effects of increases in mutation rate on MC are more pronounced for autosomal dominants, followed by X-linked and are far less for autosomal recessives. Even for autosomal dominants, the early generation effects of radiation exposures would not be appreciable unless the heterozygotes have a severely reduced fitness.

Drosophila male-specific lethal-2 protein: structure/function analysis and dependence on MSL-1 for chromosome association.

MSL-2 is required for the male-specific assembly of a dosage compensation regulatory complex on the X chromosome of Drosophila melanogaster. We found that MSL-2 binds in a reproducible, partial pattern to the male X chromosome in the absence of MLE or MSL-3, or when ectopically expressed at a low level in females. Moreover, the pattern of MSL-2 binding corresponds precisely in each case to that of MSL-1, suggesting that the two proteins function together to associate with the X. Consistent with this hypothesis, we isolated EMS-induced loss of function msl-1 and msl-2 alleles in a screen for suppressors of the toxic effects of MSL-2 expression in females. We also used site-directed mutagenesis to determine the importance of the MSL-2 RING finger domain and second cysteine-rich motif. The mutations, including those in conserved zinc coordinating cysteines, confirm that the RING finger is essential for MSL-2 function, while suggesting a less stringent requirement for an intact second motif.

Transposon insertions causing constitutive Sex-lethal activity in Drosophila melanogaster affect Sxl sex-specific transcript splicing.

Sex-lethal (Sxl) gene products induce female development in Drosophila melanogaster and suppress the transcriptional hyperactivation of X-linked genes responsible for male X-chromosome dosage compensation. Control of Sxl functioning by the dose of X-chromosomes normally ensures that the female-specific functions of this developmental switch gene are only expressed in diplo-X individuals. Although the immediate effect of X-chromosome dose is on Sxl transcription, during most of the life cycle "on" vs. "off" reflects alternative Sxl RNA splicing, with the female (productive) splicing mode maintained by a positive feedback activity of SXL protein on Sxl pre-mRNA splicing. "Male-lethal" (SxlM) gain-of-function alleles subvert Sxl control by X-chromosome dose, allowing female Sxl functions to be expressed independent of the positive regulators upstream of Sxl. As a consequence, SxlM haplo-X animals (chromosomal males) die because of improper dosage compensation, and SxlM chromosomal females survive the otherwise lethal effects of mutations in upstream positive regulators. Five independent spontaneous SxlM alleles were shown previously to be transposon insertions into what was subsequently found to be the region of regulated sex-specific Sxl RNA splicing. We show that these five alleles represent three different mutant types: SxlM1, SxlM3, and SxlM4. SxlM1 is an insertion of a roo element 674 bp downstream of the translation-terminating male-specific exon. SxlM3 is an insertion of a hobo transposon (not 297 as previously reported) into the 3' splice site of the male exon, and SxlM4 is an insertion of a novel transposon into the male-specific exon itself. We show that these three gain-of-function mutants differ considerably in their ability to bypass the sex determination signal, with SxlM4 being the strongest and SxlM1 the weakest. This difference is also reflected in effects of these mutations on sex-specific RNA splicing and on the rate of appearance of SXL protein in male embryos. Transcript analysis of double-mutant male-viable SxlM derivatives in which the SxlM insertion is cis to loss-of-function mutations, combined with other results reported here, indicates that the constitutive character of these SxlM alleles is a consequence of an alteration of the structure of the pre-mRNA that allows some level of female splicing to occur even in the absence of functional SXL protein. Surprisingly, however, most of the constitutive character of SxlM alleles appears to depend on the mutant alleles' responsiveness, perhaps greater than wild-type, to the autoregulatory splicing activity of the wild-type SXL proteins they produce.

Male-sex-ratio trait in Drosophila pseudoobscura: frequency of autosomal aneuploid sperm.

Males with the SR X chromosome show the "sex-ratio" (sr) phenotype in which they produce almost entirely daughters. The few sons (about 1%) are invariably sterile X/O males and result entirely from nullo-XY sperm. The "male-sex-ratio" (msr) phenotype is a modified form of sr in which SR/Y males produce a higher frequency of sterile X/O sons. The msr trait is due to the presence of the SR X-chromosome in males which are also homozygous for one or more autosomes from the L116 strain. Here the frequency of nullo-3 and diplo-3 sperm from msr males was measured by crossing to a compound-3 strain and found to be 13.8% and 3.2%, respectively, of the total viable sperm. The sr males produced very low levels of nullo-3 sperm at a frequency not different from control X/Y males and a slightly elevated frequency of diplo-3 sperm over X/Y males. The msr males were found to have only 12% the fecundity of sr males and in matings to cause a high frequency of brown inviable eggs. These results indicate that high rates of autosomal aneuploidy are not restricted to chromosome 3 but also occur for chromosomes 2, 4 and 5. The overall frequency of autosomal aneuploid sperm is estimated to be approximately 50%. Microscopic studies of meiosis in testes from msr males indicates meiotic nondisjunction and meiotic chromosome loss are responsible for the msr phenotype. Last, microscopic studies of sperm cysts from msr males reveal high levels of spermiogenic failure.

FREQUENCY-DEPENDENT SELECTION IN DROSOPHILA: ESTIMATION OF NET FITNESS IN PSEUDOHAPLOID POPULATIONS.

There is much evidence that the viability of Drosophila larvae depends on the genotypes of other larvae with which they develop. There is, however, little evidence concerning frequency dependence of net fitness. This report documents variation in net fitness among male D. melanogaster from 17 lines studied in 49 discrete-generation populations. The experimental design, which uses attached-X females, eliminates effects of dominance, heterosis, recombination, genetic background, and selection in females. Selection operates only on patroclinously inherited X-chromosomes in males, causing frequency dynamics to mimic those of haploid populations. Analysis of one-generation transitions shows a general pattern of frequency dependence of net fitnesses. Analysis of multigeneration trajectories suggests frequency dependence in 12 populations, but trajectories tend to be heterogeneous among replicates. For a deterministic model in which fitness w(p) is a function of allelic frequency (p), oscillatory dynamics can occur where |dw(p)/dp| > w(p̂) (Curtsinger, 1984a). The derivative measures the strength of frequency-dependent selection, and divides models into two categories: dynamically "well-behaved" and "complex." For the pooled data, this criterion is violated in about half of the frequency space, though several factors probably cause strength to be overestimated. The strength criterion is a useful predictor of evolutionary dynamics: observed oscillations tend to occur most often where frequency dependence is strong, and less often where it is weak.

Mechanistic and methodological aspects of chemically-induced somatic mutation and recombination in Drosophila melanogaster

This study presents the analysis of chemically-induced somatic mutations and chromosomal damage in the eye imaginal discs of Drosophila larvae, assayed later as twin (TS) and single light (LS) mosaic spots in the adult eyes. Regarding the question as to what kind of DNA alterations contribute to somatic cell mutagenicity, the approach followed here has been to investigate the possible differences in response between male (hemizgous for an X) and female (homozygous) larvae, rod-X / rod-X versus ring-X / rod-X genotypes and inversion-heterozygotes versus genotypes not carrying an inversion. The systems chosen for analysis were the white-coral/white (w co / w) and the white (w + / w) eye mosaic system. The principle findings with 12 mutagens of different modes of action are as follows: 1. (1) At least 98% of all TS and LS induced by cisplatin (DDP) in w co / w female larvae and about 95% of those by formaldehyde (FA) appear as the result of recombinogenic activity between the two homologous X-chromosomes. The corresponding estimates for MMS, EMS and ENU are 81%, 73% and 61%, respectively. 2. (2) The long sc S1L sc 8R inversion, which also contains In(1)dl-49, suppresses induction of TS to 83–93%. There was also a sharp decline in the frequency of LS in inversion heterozygotes for DDP (91%), FA (86%), MMS (52%) and EMS (47%). 3. (3) Ethylnitrosourea (ENU) was the mutagen for which introduction of the inverted chromosome reduced only slightly (23%) the frequency of LS, indicating that the majority of them were somatic mutations (and deletions) at the white locus. 4. (4) In w/RX females heterozygous for a ring-X chromosome, the frequency of LS was only approximately one tenth of that of the control ( w + / w) group, after exposure to MMS or DDP. The explanation is that exchange processes involving the ring frequently lead to genetic imbalance with subsequent cell killing. 5. (5) Low frequencies of TS in w co males (one fifth to one tenth of the rates in females) after exposure to MMS, EMS, DEN, ENNG, ENU and DDP are indicative of unequal sister-strand recombination in the X-chromosome of males. Unequal mitotic recombination had been predicted by Auerbach (1945) to occur in males, and this event was later demonstrated after exposure to X-rays and triethylenemelamine (Becker, 1975). 6. (6) Frequencies of LS were clearly higher in w co / w females than in w co males for MMS (2 : 1) and DDP (7 : 1) but not for EMS and ENU. After correcting for that portion of LS due to recombinogenic activity, there was no significant departure from a 1 : 1 ratio for MMS and ENU. EMS seems to represent a mutagen which is more genotoxic in males than in females, both in germinal and somatic tissue. Part of this study also dealt with methodological aspects: 1. (7) Four different routes of application, i.e., chronic exposure, acute treatment, surface treatment and inhalation, were examined in order to devise suitable protocols for chemicals with different properties. Exposure doses as low as 1.4 ppm 1,2-dibromoethane were still mutagenic in inhalation experiments with w co / w and w + / w female larvae. 2. (8) The white coral / white bioassay gave problems in the sense that small mosaic spots, i.e., size classes 1, 2 and 3–4, but also 5–8 (ommatidia), mostly escaped detection. This drawback could be overcome by replacing the w co marker by the wild-type allele w +. Thus, we recommend for mutagenicity testing the use of w + / w crosses because this test enables the quantitative scoring of both small and large mosaic sectors (LS). 3. (9) Comparative experiments with MMS and ENU revealed that eye mosaics can be induced in first, second and third larval instar. However, the high number of small spots produced when treating third-instar larvae could only be detected in wild-type ( w + / w) phenotypes.

Comparative study of the function of polytene chromosomes in laboratory stocks of Drosophila melanogaster and the l(3)tl mutant (lethal tumorous larvae). II. Changes of banding pattern and transcriptional activity in the salivary chromosome of l(3)tl.

The salivary chromosomes of the l(3)tl mutant of D. melanogaster are considerably thicker and shorter than those of normal larvae. In most nuclei, chromosome shortening is associated with morphological changes of two types. a) The bands lose their distinctive pattern and become loose. The chromosome appears as a granular mass. In extreme cases pompon-like chromosomes arise. Most frequently male X-chromosomes undergo such changes and begin to shorten as early as in the middle of third larval instar. "Pompon" transformation is not associated with a change in the relative intensity of RNA synthesis: the ratio of silver grain number over the male X-chromosome to that over regions 61A-63F is the same in pompon-like l(3)tl chromosomes and in the male and female X-chromosomes of the normal lines. b) Shortened chromosomes occassionally retain distinct band organization and, in these cases, chromosome shortening is observed to be due to the condensation of the chromatin of many puffs and interbands resulting in the fusion of a large number of bands into "new" chromatin blocks. In regions of fused bands, transcriptional activity is decreased as compared with regions where this union does not occur. The chromosomes of l(3)tl larvae lack ecdysone-stimulated puffs and other prominent puffs. In 144-192 hour larvae, puffs can be induced by ecdysone and until 384 hours by temperature shock. The capacity of puff induction decreases with larval age.

Anomalous development and differential DNA replication in the X-chromosome of a Drosophila hybrid.

Male hybrids of the cross D. azteca X D. athabasca are larger (hybrid giant males) than their parents, whereas hybrid females are of the same size as the parental species. Microspectrophotometric measurements have shown that the larval polytene salivary gland chromosomes of hybrid giant males undergo one more endoreplication than those of their sisters or parents. Replication patterns of the larval salivary gland chromosomes were compared after pulse labeling with 3H-thymidine and autoradiography. In females of either species as well as of hybrids X-chromosomes and autosomes are equally labeled, i.e. all chromosome arms replicate synchronously. In males, however. often fewer sites are labeled on the X-chromosome than on the autosomes. In addition, in a significant number of nuclei from D. athabasca males and also from hybrid giant males the converse can also be observed: i.e. more sites are labeled on the X-chromosome than on the autosomes. The modified labeling patterns are interpreted as an indication of a time-shift in the replication of hemizygous X-chromosomes in males, in relation to the autosomes.