Patterns of replication in the neo-sex chromosomes ofDrosophila nasuta albomicans

Drosophila nasuta albomicans (with 2n = 6), contains a pair of metacentric neo-sex chromosomes. Phylogenetically these are products of centric fusion between ancestral sex (X, Y) chromosomes and an autosome (chromosome 3). The polytene chromosome complement of males with a neo-X- and neo-Y-chromosomes has revealed asynchrony in replication between the two arms of the neo-sex chromosomes. The arm which represents the ancestral X-chromosome is faster replicating than the arm which represents ancestral autosome. The latter arm of the neo-sex chromosome is synchronous with other autosomes of the complement. We conclude that one arm of the neo-X/Y is still mimicking the features of an autosome while the other arm has the features of a classical X/Y-chromosome. This X-autosome translocation differs from the other evolutionary X-autosome translocations known in certain species ofDrosophila.     

A 45,X male with Y-specific DNA translocated onto chromosome 15.

A 20-year-old male patient with chromosomal constitution 45,X, testes and normal external genitalia was examined. Neither mosaicism nor a structurally aberrant Y chromosome was observed when routine cytogenetic analysis was performed on both lymphocytes and skin fibroblasts. Y chromosome-specific single-copy and repeated DNA sequences were detected in the patient's genome by means of 11 different recombinant-DNA probes of known regional assignment on the human Y chromosome. Data indicated that the short arm, the centromere, and part of the long-arm euchromatin of the Y chromosome have been retained and that the patient lacks deletion intervals 6 and 7 of Yq. High-resolution analysis of prometaphase chromosomes revealed additional euchromatic material on the short arm of one of the patient's chromosomes 15. After in situ hybridization with the Y chromosome-specific probe pDP105, a significant grain accumulation was observed distal to 15p11.2, suggesting a Y/15 chromosomal translocation. We conclude that some 45,X males originate from Y-chromosome/autosome translocations following a break in the proximal long arm of the Y chromosome.     

X and Y chromosomal ribosomal DNA of Drosophila: comparison of spacers and insertions.

In Drosophila melanogaster, the genes coding for 18S and 28S ribosomal RNA (rDNA) are clustered at one locus each on the X and the Y chromosomes. We have compared the structure of rDNA at the two loci. The 18S and 28S rRNAs coded by the X and Y chromosomes are very similar and probably identical (Maden and Tartof, 1974). In D. melanogaster, many rDNA repeating units are interrupted in the 28S RNA sequence by a DNA region called the insertion. There are at least two sequence types of insertions. Type 1 insertions include the most abundant 5 kilobase (kb) class and homologous small (0.5 and 1 kb) insertions. Most insertions between 1.5 and 4 kb have no homology to the 5 kb class and are identified as type 2 insertions. In X rDNA, about 49% of all rDNA repeats have type 1 insertions, and another 16% have type 2 insertions. On the Y chromosome, only 16% of all rDNA repeats are interrupted, and most if not all insertions are of type 2.rDNA fragments derived from the X and Y chromosomes have been cloned in E. coli. The homology between the nontranscribed spacers in X and Y rDNA was studied with cloned fragments. Stable heteroduplexes were found which showed that these regions on the two chromosomes are very similar.The evolution of rDNA in D. melanogaster might involve genetic exchange between the X and Y chromosomal clusters with restrictions on the movement of type 1 insertions to the Y chromosome.     

Genomic changes following the reversal of a Y chromosome to an autosome in Drosophila pseudoobscura

Robertsonian translocations resulting in fusions between sex chromosomes and autosomes shape karyotype evolution by creating new sex chromosomes from autosomes. These translocations can also reverse sex chromosomes back into autosomes, which is especially intriguing given the dramatic differences between autosomes and sex chromosomes. To study the genomic events following a Y chromosome reversal, we investigated an autosome‐Y translocation in Drosophila pseudoobscura. The ancestral Y chromosome fused to a small autosome (the dot chromosome) approximately 10–15 Mya. We used single molecule real‐time sequencing reads to assemble the D. pseudoobscura dot chromosome, including this Y‐to‐dot translocation. We find that the intervening sequence between the ancestral Y and the rest of the dot chromosome is only ～78 Kb and is not repeat‐dense, suggesting that the centromere now falls outside, rather than between, the fused chromosomes. The Y‐to‐dot region is 100 times smaller than the D. melanogaster Y chromosome, owing to changes in repeat landscape. However, we do not find a consistent reduction in intron sizes across the Y‐to‐dot region. Instead, deletions in intergenic regions and possibly a small ancestral Y chromosome size may explain the compact size of the Y‐to‐dot translocation.     

Genetic events associated with an insertion mutation in yeast

The his4-912 mutation shares similar genetic properties with mutations promoted by procaryotic insertion elements. This mutation lacks all three his4 functions. Many different classes of His+ revertants have been obtained from his4-912. The most frequent class of His+ revertants results from a site mutation which confers a cold-sensitive His- phenotype. Other classes of revertants contain translocations (one between chromosomes I and III and the other between chromosomes III and XII), a transposition of the his4 region to chromosome VIII, and an inversion of most of the left arm of chromosome III. Another class contains deletions which extend from his4-912 into the his4 region. In each of these classes of revertants, the his4 region is closely linked to the chromosomal aberration. Many of these revertants contain additional changes in chromosome structure (duplication, deletion and aneuploidy) that are unrelated to the reversion of his4-912 to His4+.     

Age-related changes in crossing over in Drosophila resemble the picture of interchromosomal effect of chromosome rearrangement on crossing over

Crossing over in the left arm of chromosome 2 (2L) was studied in successive broods of Drosophila melanogaster females carrying intact chromosomes (+/+), inversion Muller-5 in the X chromosome (M-5/+), and insertion of the Y-chromosome material into region 34A (Is(2L)/+). The regions net-dp, dp-b, b-pr and pr-cn were examined in 14 two-day-old broods of females +/+ and M-5/+ and in 10 broods of females Is(2L)/+. In all lines, the highest level of crossing over was in the first three broods (eggs laid during the first 6 days of oviposition) and the lowest level in the broods 7-8 (eggs laid at days 14-16). A high rate of crossing over in the first broods of females +/+ and M-5/+ was due to an increment of exchanges in the proximal euchromatin regions (b-pr and pr-cn) and to an increase in the number of tetrads with double exchanges. These changes are similar to a pattern of the interchromosomal effect on crossing over (IEC) in structurally normal chromosomes. In Is(2L)/+ females, a high level of crossing over was due to extensive exchanges in the interstitial regions net-dp and dp and an increase in the number of tetrads with single exchanges. These changes resembled the IEC in rearranged chromosomes (in this case, in chromosomes bearing an insertion). Thus, the age changes of crossing over are similar to the consequences of the presence or absence of IEC. Age changes in crossing over in a chromosome depended both on the local rearrangements in this chromosome (the local effect on crossing over, LEC) and on rearrangements in nonhomologous chromosomes (IEC). In the first broods, both LEC and IEC decreased with an increase in the level of crossing over. In subsequent broods, the reduced level of crossing over was accompanied by an increase in both LEC and IEC. This suggests that the mechanisms responsible for the age changes in crossing over and IEC may have common steps. The contact model of crossing over may explain the similarity between the age changes in crossing-over and IEC. It is suggested that both phenomena result from delayed determination of crossing over in a meiotic cell. This may occur due to the retarded formation of the local contacts in one of the homologous chromosome pairs or because a higher number of local contacts is required to trigger crossing over in a meiotic cell (of early age).     

Biased distribution of repetitive elements: a landmark for neo-Y chromosome evolution in Drosophila miranda

It is generally assumed that the sex chromosomes developed from a pair of homologs. Over evolution, the proto-Y chromosome, with a very short differential segment, matured in its final stage into a heterochromatic and, for the most part, genetically eroded Y chromosome. The constraints on the evolution of the proto-Y chromosome have been speculated upon since the sex chromosomes were discovered. Several models have been suggested. Drosophila miranda has proved to be a unique and potent model system to study Y-chromosome evolution. We use selected test genes distributed along the neo-Y chromosome as entry gates to analyze the molecular mechanisms involved in the process of Y-chromosome evolution. Here, we report our findings on the Krüppel gene (Kr), which is located distally on the neo-sex chromosome pair.     

Use of repetitive DNA for diagnosis of chromosomal rearrangements

Summary We have used two repeated DNA fragments (3.4 and 2.1 kb) released from Y chromosome DNA by digestion with the restriction endonuclease Hae III to analyze potential Y chromosome/autosome translocations. Two female patients were studied who each had an abnormal chromosome 22 with extra quinacrine fluorescent material on the short arm. The origin of the 22p+ chromosomes was uncertain after standard cytologic examinations. Analysis of one patient's DNA with the Y-specific repeated DNA probes revealed the presence of both the 3.4 and 2.1 kb Y-specific fragments. Thus, in this patient, the additional material was from the Y chromosome. Analysis of the second patient's DNA for Y-specific repeated DNA was negative, indicating that the extra chromosomal segment was not from the long arm of the Y chromosome. These two cases demonstrate that repeated DNA can distinguish between similar appearing aberrant chromosomes and may be useful in karyotypic and prenatal diagnosis.     

Molecular cloning and mapping of 10 new probes on the human Y chromosome

We have developed a novel positive cloning vector whose use precludes the cloning of any fragments less than 0.8 kb as well as 3.4-kb EcoRI fragments of DYZ1, the largest repeating-DNA family on the long arm of the human Y chromosome. Using this vector, we subcloned inserts of a Y-chromosome-specific phage library constructed from EcoRI-digested flow-sorted Y-chromosome DNA. Ten novel Y-specific fragments were obtained. Their localization on the Y chromosome was determined by deletion mapping using clinical samples with structurally abnormal Y chromosomes. The long arm of the Y chromosome was divided into 12 segments by the novel probes in combination with established probes. The amelogenin-like sequence, mapped on the long arm in Human Gene Mapping 10, has been mapped on the short arm.     

The Entire Compound Autosomes of DROSOPHILA MELANOGASTER

Three new unusual compound chromosomes have been synthesized in Drosophila melanogaster. They consist of two homologous autosomes joined together in the new order: right arm, left arm, centromere, left arm, right arm, for each of the two major autosomes, and one in which chromosomes 2 and 3 have been combined in the order: right arm of 2, left arm of 2, centromere, left arm of 3, right arm of 3. The attachments of the autosomal arms were accomplished by obtaining chromosome breaks at or very close to the ends of the left arms of the autosomes such that no essential chromosome material has been removed; the compounds derived from them are therefore referred to as entire compounds. These large chromosomes are recovered in progeny with frequencies lower than expectation partly because of zygote mortality associated with these chromosomes, and partly because of a failure of spermiogenesis.     

Drosophila ribosomal RNA genes function as an X-Y pairing site during male meiosis

In Drosophila melanogaster males, the sex chromosomes pair during meiosis in the centric X heterochromatin and at the base of the short arm of the Y (YS), in the vicinity of the nucleolus organizers. X chromosomes deficient for the pairing region segregate randomly from the Y. In this report we show that a single ribosomal RNA (rRNA) gene stimulates X-Y pairing and disjunction when inserted onto a heterochromatically deficient X chromosome by P element-mediated transformation. We also show that insert-containing X chromosomes pair at the site of insertion, that autosomal rDNA inserts do not affect X-Y pairing or disjunction, and that the strength of an X pairing site is proportional to the dose of ectopic rRNA genes. These results demonstrate that rRNA genes can promote X-Y pairing and disjunction and imply that the nucleolus organizers function as X-Y pairing sites in wild-type Drosophila males.     

Satellited Y chromosomes: Structure,origin, and clinical significance

Summary Three cases of inherited satellited Y chromosomes (Yqs) were analysed using several cytogenetic techniques. The cytogenetic data of the 14 cases of Yqs chromosomes described to date were reviewed. All Yqs chromosomes carry an active nucleolus organizer region (NOR) in their long arm and must have developed from translocations involving the short arms of the acrocentric autosomes. The structure of the heterochromatic satellite region in the Yqs chromosomes shows conspicuous inter-familial differences; this permits the reconstruction of the translocations from which the various Yqs were derived. Some causal factors leading to the development of Yqs chromosomes are considered: the specific localization of the four satellite DNAs and highly methylated DNA sequences in the karyotype, and some new experimental data on the spatial arrangement of heterochromatic regions in interphase nuclei. These provide distinct evidence for a preferential involvement of the autosomes 15 and 22 in the translocations with the Y heterochromatin. All clinical reports documenting Yqs males born with malformations were reviewed. It appears that the presence of an extra NOR and NOR-associated heterochromatin in the Yqs chromosomes does not cause any phenotypic abnormalities (as long as the Y euchromatin is intact). The possibility that a Yqs chromosome predisposes to non-disjunction and/or to translocations of other chromosomes is discussed.     

Y chromosome-specific mutations induced by a giant transposon in Drosophila hydei

Summary The w ^m Co duplication of Drosophila hydei (Dp (1; Y) 16B2-17B1) contains 13–16 bands in salivary gland chromosomes. The duplication resides preferentially in the X heterochromatin or on the Y chromosome. In some stocks frequent (up to 4×10^-3) exchanges of the duplication occur between different Y chromosomes (T(X; Y) and free Y) or between the X and the Y chromosome. About 60% of the T(X; Y)-Y exchanges induce mutations in the Y chromosomal male fertility genes of the recipient Y chromosome. From the mutational spectrum generated by the T(X; Y)-Y transpositions and from the variable efficiency as acceptor of different X-Y translocations it can be concluded that the exchanges show a remarkable site specificity: distal positions in the long arm of the Y chromosome are occupied preferentially. More proximal positions in the long arm of insertions into the short arm of the Y chromosome are found only with a lower frequency. No transpositions to the autosomes have been recovered. Duplications are lost with highly differing frequencies. The losses are not linked with insertions of the w ^m Co element into a new position and are more frequent than transpositions. Therefore, we regard the w ^m Co element as a giant transposon.     

Multiple Ty-mediated chromosomal translocations lead to karyotype changes in a wine strain of Saccharomyces cerevisiae

Enological strains of Saccharomyces cerevisiae display a high level of chromosome length polymorphism, but the molecular basis of this phenomenon has not yet been clearly defined. In order to gain further insight into the molecular mechanisms responsible for the karyotypic variability, we examined the chromosomal constitution of a strain known to possess aberrant chromosomes. Our data revealed that the strain carries four rearranged chromosomes resulting from two reciprocal translocations between chromosomes III and I, and chromosomes III and VII. The sizes of the chromosomal fragments exchanged through translocation range from 40 to 150?kb. Characterization of the breakpoints indicated that the translocations involved the RAHS of chromosome III, a transposition hot-spot on the right arm of chromosome I and a region on the left arm of chromosome VII. An analysis of the junctions showed that in all cases Ty elements were present and suggested that the translocations result from recombination between transposable Ty elements. The evidence for multiple translocations mediated by Ty elements in a single strain suggests that spontaneous Ty-driven rearrangement could be quite common and may play a major role in the alteration of karyotypes in natural and industrial yeasts.     

Variation in the relative length of chromosomes in mammalian karyotypes. Hypothesis of equalizing selection

A hypothesis on the selective neutrality of relative lengths of karyotype chromosomes was tested. Idiograms expected based on an assumption of selective neutrality of chromosome lengths were compared with actual idiograms in more than a hundred mammalian species. The observed idiograms differed from those expected in a similar manner: in the observed idiograms, the longest chromosomes were shorter, and the shortest were longer than expected. It is suggested that karyotype chromosome variation is limited by selection against chromosome rearrangements that produce very long or very short chromosomes. An analysis of reciprocal translocations in the mouse and Drosophila showed that translocations generating chromosomes of extreme lengths were more deleterious than those generating normal-sized chromosomes. A working hypothesis was advanced stating that within-karyotype variation of chromosome lengths is accounted for by two factors: chromosome rearrangements and natural selection. Chromosome rearrangements tend to randomize relative chromosome lengths in a karyotype, whereas natural selection acts to equalize them.     

Identification of the Sex-Determining Region of the Ceratitis Capitata Y Chromosome by Deletion Mapping

In the medfly Ceratitis capitata, the Y chromosome is responsible for determining the male sex. We have mapped the region containing the relevant factor through the analysis of Y-autosome translocations using fluorescence in situ hybridization with two different probes. One probe, the clone pY114, contains repetitive, Y-specific DNA sequences from C. capitata, while the second clone, pDh2-H8, consists of ribosomal DNA sequences from Drosophila hydei. Clone pY114 labeled most of the long arm and pDh2-H8 hybridizes to the short arm and the centromeric region of the long arm. In 12 of the analyzed 19 Y-autosome translocation strains, adjacent-1 segregation products survive to the late pupal or even adult stage and can, therefore, be sexed. This was correlated with the length of the Y fragment still present in these aberrant individuals and allowed us to map the male-determining factor to a region of the long arm representing ~15% of the entire Y chromosome. No additional factors, affecting for example fertility, were detected outside the male-determining region.     

Rearrangements between germ-line Limited and somatic chromosomes in Smittia parthenogenetica (Chironomidae,Diptera)

Among more than 700 chromosome rearrangements induced by X-rays in oocytes of newly hatched females of Smittia parthenogenetica two cases of insertion of heterochromatin into an S chromosome have been obtained. As the S chromosomes do not contain such heterochromatic sections, the insertions must be derived from K chromosomes. Whether all K chromosomes are completely heterochromatic or whether all or some contain euchromatic sections remains open, but for the two latter possibilities no proof has been obtained. Euchromatic insertions or translocations with a banding pattern non-homologous with S chromosome sections have not been observed with certainty. Homologous duplications could all be interpreted as being derived from S chromosomes. From the K chromosome cycle it can be inferred that the K complement consists of more or less identical elements and that genetic isolation has led to their heterochromatinization.To Sally Hughes-Schrader with affection and admiration.     

Reassessment of presumed Y/22 and Y/15 translocations in man using a new technique.

A new chromosome banding technique, distamycin A plus DAPI, has been used to reexamine cases of presumed Y/autosome translocations. In contrast with the results obtained with quinacrine fluorescence (Q-banding), the satellites of acrocentric chromosomes do not fluoresce brightly with this new (DA-DAPI) method, making it more specific for the long arm of the Y chromosome. Previous cases with intensely Q-fluorescent and abnormally long short arms on a chromosome 22 were considered as presumptive 22/Y translocations: The new technique clearly shows that, in these cases, the additional material on 22p is not derived from Yq. In contrast, in other cases the Yq nature of additional material on 15p, in conjunction with the presence of an extra Y-body in interphase nuclei and the presence of a male-specific DNA, supports the previous diagnosis of a presumptive 15/Y translocation.     

Discordant Phylogeographic Patterns between the Y Chromosome and Mitochondrial DNA in the House Mouse: Selection on the Y Chromosome?

We have compared patterns of geographic variation and molecular divergence of mitochondrial DNA (mtDNA) and Y chromosome over the range of the different subspecies of Mus musculus. MtDNA was typed for 305 nucleotides in the control region, the Y chromosome for 834 base pairs (bp) in Zfy introns and 242 bp in Sry, a Zfy2 18-bp deletion, and two microsatellites. Apparent discrepancies exist between the distributions of the lineages of mtDNA and of the two major Y-chromosome lineages thus defined: some subspecies share the same mtDNA lineage but have different Y-chromosome lineages or vice versa. One microsatellite reveals a geographically clustered variation inside the distribution of each Y-chromosome lineage, showing that new Y-chromosome variants can rapidly spread locally. The two major Y-chromosome lineages have a divergence time only about one fourth of that between mtDNA lineages. Although this recent coalescence of the Y chromosomes between subspecies could partly be due to a lower ancestral polymorphism of the Y chromosome, it suggests that secondary introgression after the radiation of the subspecies might have occurred. There is evidence that the differentiation of the Y-chromosome lineages contributes to partial reproductive isolation between subspecies, and patterns of molecular evolution suggest that selection has played a role in the rapid spread across subspecies.