Allelic complementation between mutants in the fertility factyors of the Y chromosome in Drosophila melanogaster

Summary Complementation maps of the seven male fertility factors in the Y chromosome of D. melanogaster have been constructed and are linearly consistent in all cases. These observations are further evidence that genetic loci in heterochromatic chromosomes share many characteristics with loci in euchromatic regions of chromosomes. These functional maps are consistent with the hypothesis that the genetic material of the male fertility factors in the Y chromosome is made up of single-copy sequences which become amplified in the primary spermatocyte.     

Specific recognition and differential affinity of meiotic X-Y pairing sites in Lucilia cuprina males (Diptera: Calliphoridae)

Meiotic pairing of X and Y chromosomes in male Lucilia cuprina was studied by cytological observation of normal, rearranged and deficient sex chromosome karyotypes in spermatogenesis. Two X-Y pairing regions located distally in each arm of the X and Y chromosomes were defined. Contrasting with findings in Drosophila melanogaster, these pairing regions show specific recognition of their partners. By studying rearranged sex chromosomes short arm pairing was localised to their distal ends, closely associated with secondary constrictions containing nucleolar organisers in both sex chromosomes. Short arm pairing is very tight and not greatly disrupted by chromosome rearrangement, deficiency for the Y chromosome long arm or the presence of supernumerary X chromosomes. The pairing region of the long arms could not be precisely localised but probably also occurs at their distal ends. Pairing between the long arm sites is much weaker and is easily disrupted by chromosome rearrangement, failing completely in flies deficient for the Y chromosome short arm. No cytologically visible pairing was seen between X chromosomes and the remainder of the Y. In males with an extra X chromosome, the ends of both X chromosomes pair to form multivalents with normal and rearranged Y chromosomes provided the Y short arm is present, otherwise an independent X chromosome bivalent is formed. The mechanism of pairing in male Lucilia sex chromosomes thus seems to depend on specific loci of distinctive structure within the X and Y heterochromatin. Comparison of cytological and genetic data shows that increasing cytological pairing failure is matched by higher genetic X-Y nondisjunction but that the former occurs at much higher levels. In some karyotypes cytologically observed X-Y pairing failure is not matched by high frequencies of nondisjunction presumably because weak pairing associations are disrupted during slide preparation.     

Characterization of a new aberration of the human Y chromosome by banding methods and DNA restriction endonuclease analysis

Summary Comparative cytogenetic analyses were performed with ten different banding methods on a previously undescribed, inherited structural aberration of a Y chromosome, and the results compared with those of normal Y chromosomes occurring in the same family. The value of the individual staining techniques in investigations of Y chromosomal aberrations is emphasized. The aberrant Y chromosome analyzed can be formally derived from an isodicentric Y chromosome for the short arm with a very terminal long-arm breakpoint, in which the centromere, an entire short arm, and the proximal region on one long arm was lost. This interpretation was confirmed by determining the amount of the two Y-specific DNA sequences (2.1 and 3.4 kb in length) by means of HaeIII restriction endonuclease analysis. The karyotype-phenotype correlations in the men with this aberrant Y chromosome, especially the fertility dysfunctions (oligoasthenoteratozoospermia, cryptozoospermia), are discussed. The possibility of the existence of fertility factors involved in the control of spermatogenesis within the quinacrine-bright heterochromatic region of the Y long arm is presented.     

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.     

The Drosophila simulans Y chromosome interacts with the autosomes to influence male fitness

The Y chromosome should degenerate because it cannot recombine. However, male‐limited transmission increases selection efficiency for male‐benefit alleles on the Y, and therefore, Y chromosomes should contribute significantly to variation in male fitness. This means that although the Drosophila Y chromosome is small and gene‐poor, Y‐linked genes are vital for male fertility in Drosophila melanogaster and the Y chromosome has large male fitness effects. It is unclear whether the same pattern is seen in the closely related Drosophila simulans. We backcrossed Y chromosomes from three geographic locations into five genetic backgrounds and found strong Y and genetic background effects on male fertility. There was a significant Y‐background interaction, indicating substantial epistasis between the Y and autosomal genes affecting male fertility. This supports accumulating evidence that interactions between the Y chromosome and the autosomes are key determinants of male fitness.     

Cytological identification of two X-chromosome types in the wood lemming (Myopus schisticolor)

In the wood lemming (Myopus schisticolor) three genetic types of sex chromosome constitution in females are postulated: XX, X^*X and X^*Y (X^*=X with a mutation inactivating the male determining effect of the Y chromosome). Males are all XY. It is shown in the present paper that the two types of X chromosomes, X and X^*, exhibit differences in the G-band patterns of their short arms. In addition, it was demonstrated in unbanded chromosomes that the short arm in X^* is shorter than in X. The origin of these differences is still obscure; but they allow to identify and to distinguish the individual types of sex chromosome constitution, as of XX versus X^*X females and of X^*Y females versus XY males, on the basis of G-banded chromosome preparations from somatic cells.     

Y-Linked male sterile mutations induced by P element in Drosophila melanogaster.

The Y chromosome in Drosophila melanogaster is composed of highly repetitive sequences and is essential only in the male germ line. We employed P-element insertional mutagenesis to induce male sterile mutations in the Y chromosome. By using a combination of two modifiers of position effect variegation, adding an extra Y chromosome and increasing temperature, we isolated 61 P(ry+) elements in the Y chromosome. Six of these Y-linked insertions (approximately 10%) induced male sterile mutations that are mapped to two genes on the long and one on the short arms of the Y chromosome. These mutations are revertible to the wild type in a cell-autonomous and germ-line-dependent manner, consistent with previously defined Y-linked gene functions. Phenotypes associated with these P-induced mutations are similar to those resulting from deletions of the Y chromosome regions corresponding to the male fertility genes. Three alleles of the kl-3 gene on the Y long arm result in loss of the axonemal outer dynein arms in the spermatid tail, while three ks-2 alleles on the Y short arm induce defects at early postmeiotic stages. The recovery of the ms(Y) mutations induced by single P-element insertions will facilitate our effort to understand the structural and functional properties of the Y chromosome.     

MAMMALIAN CHROMOSOMES IN VITRO : XVIII. DNA Replication Sequence in the Chinese Hamster

The complete DNA replication sequence of the entire complement of chromosomes in the Chinese hamster may be studied by using the method of continuous H³-thymidine labeling and the method of 5-fluorodeoxyuridine block with H³-thymidine pulse labeling as relief. Many chromosomes start DNA synthesis simultaneously at multiple sites, but the sex chromosomes (the Y and the long arm of the X) begin DNA replication approximately 4.5 hours later and are the last members of the complement to finish replication. Generally, chromosomes or segments of chromosomes that begin replication early complete it early, and those which begin late, complete it late. Many chromosomes bear characteristically late replicating regions. During the last hour of the S phase, the entire Y, the long arm of the X, and chromosomes 10 and 11 are heavily labeled. The short arm of chromosome 1, long arm of chromosome 2, distal portion of chromosome 6, and short arms of chromosomes 7, 8, and 9 are moderately labeled. The long arm of chromosome 1 and the short arm of chromosome 2 also have late replicating zones or bands. The centromeres of chromosomes 4 and 5, and occasionally a band on the short arm of the X are lightly labeled.     

Molecular and cytological characterization of an extra acrocentric chromosome that restores male fertility of wheat in the msH1 CMS system

A new CMS system designated as ‘msH1’ has been reported in bread wheat using the cytoplasm of H. chilense. While testing this system in different wheat backgrounds, a highly fertile line with chromosome number 42 plus an extra acrocentric chromosome was obtained. The extra chromosome did not pair with any wheat chromosome at meiosis, and progeny from this line which lack the acrocentric chromosome showed pollen abortion and male sterility. In order to establish the origin of this chromosome, FISH using H. chilense genomic DNA as probe was used and showed that it had originated from H. chilense chromosome(s). The novel chromosome did not possess sequences similar to wheat rDNA; however, the probe pSc119.2 from S. cereale containing the 120 bp family was found to occur at the end of its long arm. Data obtained from FISH and EST molecular markers confirm that the long arm of the acrocentric chromosome is indeed, the short arm of chromosome 1H^ch from H. chilense. We suggest that the novel chromosome originated from a deletion of the distal part of the long arm of chromosome 1H^ch. Neither the 1H^chS short arm, nor the whole chromosome 1H^ch restores pollen fertility of the alloplasmic wheat. Therefore, the restorer gene on the acrocentric chromosome must be located on the retained segment from the hypothetical 1H^chL, while some pollen fertility inhibitor could be present on the deleted 1H^chL distal segment. Disomic addition of the acrocentric chromosome was obtained and this line resulted fully stable and fertile.     

Genetic linkage between C-bands and storage protein genes in chromosome 1B of tetraploid wheat

Summary Genetic mapping of polymorphic C-bands allows direct comparisons between genetic and physical maps. Eleven C-bands and two seed storage protein genes on chromosome 1B, polymorphic between Langdon durum and four accessions of T. dicoccoides, were used to study the distribution of recombination along the entire length of the chromosome. Recombination in the short arm was almost completely restricted to the satellite, two-thirds of the arm's length from the centromere; the Gli-B1 gene was found to be tightly linked to the telomeric C-band. In the long arm, the distal 51.4% of the arm accounted for 88% of recombination; the proximal half of the arm accounted for the remaining 12%. While the amount of crossing-over differed significantly between the four T. dicoccoides 1B chromosomes, there were no significant differences in the relative distributions of crossing-over along the chromosome. Consequently, the genetic maps obtained from the four individual T. dicoccoides chromosomes were combined to yield a consensus map of 14 markers (including the centromere) for the chromosome.     

Cytogenetic mapping of the male-determining region of Lucilia cuprina (Diptera: Calliphoridae)

The Y chromosome of Lucilia cuprina was cytogenetically dissected by recovering adjacent segregation products from crosses with appropriate autosomal and Y-autosome translocations. By these means Y chromosomes lacking most of the short, long, or both arms were isolated. Only the centromeric portion of the Y chromosome was necessary for male determination and fertility, the bulk of the short and long arms having no role in sex determination. Additionally, it was shown that most of the short arm can be passed into the female line with no marked effect. These results, together with evidence from other studies, indicate that male determination in L. cuprina is centred in a discrete region near the Y chromosome centromere.     

Chromosomal localization of zein genes by in situ hybridization in Zea mays

Summary In order to localize the genes coding for zein, the major storage protein of maize endosperm, zein ¹²⁵I-mRNA and ³H-cDNA labelled at high specific activity were used for in situ hybridization on heterozygous interchanges and paracentric inversions of the KYS strain of Zea mays. The analysis of the diplotene-metaphase I microsporocytes indicated the presence of zein structural genes on the long arm of chromosomes 4 and 5, the short arm of chromosome 7 and the distal segment of the long arm of chromosome 10. The two hybridization sites on chromosomes 7 and 10 are found near opaque-2 and opaque-7 loci which are known to regulate zein synthesis. The present data are discussed in relation to results obtained by other authors using genetical mapping of zein genes.     

Relationship between Chromosome 9 of Maize and Wheat Homeologous Group 7 Chromosomes

Comparison of the genetic map of maize chromosome 9 with maps of wheat chromosomes has revealed a high degree of colinearity between maize chromosome 9 and the group 4 and 7 chromosomes of wheat. The order of DNA markers on the short arm and a proximal region of the long arm of the genetic map of maize chromosome 9 is highly conserved with the marker order on the short arm and proximal region of the long arm of the genetic maps of the wheat homeologous group 7 chromosomes. A major part of the long arm of the genetic map of maize chromosome 9 is homeologous with a short segment in the proximal region of the long arm of the genetic map of the wheat group 4 chromosomes. Evidence is also presented that maize chromosome 9 has diverged from the wheat group 7 chromosomes by both a pericentric and a paracentric inversion. The paracentric inversion is probably unique to maize among the major cereal genomes.     

Synaptonemal complex of the sex-autosome trivalent in a male Indian muntjac

Bright-field microscopy of silver-stained pachytene spermatocytes of a male Indian muntjac, Muntiacus muntjak revealed that (a) the synapsis between the autosomal homologs, including the long arm of the X and Y₂, was normal, (b) the nucleolus organizer regions were present in both the No. 1 bivalent and the long arm of the X and Y₂, (c) the accessory structures of the X chromosome short arm in the forms of light and dark thickenings and the hairpin-like bend were present despite the X-autosome translocation, (d) a short synaptonemal complex was present between the Y₁ (real Y) and the short arm of the X chromosome, and (e) the centromeric orientation of the Y₁ and Y₂ chromosomes was in Cis configuration as opposed to the X chromosome.     

Comparative genetic maps reveal extreme crossover localization in the Aegilops speltoides chromosomes

A total of 137 loci were mapped in Aegilops speltoides, the closest extant relative of the wheat B genome, using two F₂ mapping populations and a set of wheat-Ae. speltoides disomic addition (DA) lines. Comparisons of Ae. speltoides genetic maps with those of Triticum monococcum indicated that Ae. speltoides conserved the gross chromosome structure observed across the tribe Triticeae. A putative inversion involving the short arm of chromosome 2 was detected in Ae. speltoides. A translocation between chromosomes 2 and 6, present in the wheat B genome, was absent. The ligustica/aucheri spike dimorphism behaved as allelic variation at a single locus, which was mapped in the centromeric region of chromosome 3. The genetic length of each chromosome arm was about 50 cM, irrespective of its physical length. Compared to T. monococcum genetic maps, recombination was virtually eliminated from the proximal 50–100 cM and was localized in short distal regions, which were often expanded compared to the T. monococcum maps. The wheat B genome and the genome of Ae. longissima, a close relative of Ae. speltoides, do not show the extreme localization of crossovers observed in Ae. speltoides.     

The Location of the nucleolus organizer regions in Drosophila hydei

The positions of the nucleolus organizer regions in metaphase chromosomes of Drosophila hydei were detected by in situ hybridization experiments. In agreement with earlier conclusions the nucleolus of the X chromosome was found to originate in a terminal region of the heterochromatic arm. The Y chromosome contains two nucleolus organizers, one in a terminal position of the long arm, and the other in the short arm. The implications with respect to the evolution of the Y chromosome are discussed.     

Two recessive genes controlling thermophotoperiod-sensitive male sterility in wheat

Male sterility of wheat-breeding line 337S (Triticum aestivum L.) is sensitive to both short day-length/low temperature and long day-length/high temperature. 337S was crossed with the common wheat variety, Huamai No. 8 and the F₁ was highly fertile. The F₂ population segregated in a 15:1 ratio for fertility/sterility in 243 individuals under long day-length/high-temperature. The two thermophotoperiod-responsive male sterile genes were mapped to chromosomes 5B and 2B using Simple Sequence Repeat (SSR) markers and bulked segregant analysis. Partial linkage maps around the sterility loci of chromosomes 2B and 5B were constructed using the 243 individuals in the F₂ population. One gene (wptms1) for male sterility was flanked by the SSR markers Xgwm335 and Xgwm371 at a genetic distance in chromosome 5B of 4.1 and 24.4 cM, respectively. The second gene (wptms2) was mapped between markers Xgwm374 and Xgwm120 at a genetic distance of 6.6 and 20.9 cM, respectively. The closest linked markers Xgwm335 (wptms1) and Xgwm374 (wptms2) explained 53 and 38% of phenotypic variation for the fertility. The SSR markers provide a useful tool to transfer the male sterile genes into elite wheat germplasm.     

Common Mechanisms of Y Chromosome Evolution

Y chromosome evolution is characterized by the expansion of genetic inertness along the Y chromosome and changes in the chromosome structure, especially the tendency of becoming heterochromatic. It is generally assumed that the sex chromosome pair has developed from a pair of homologues. In an evolutionary process the proto-Y-chromosome, with a very short differential segment, develops in its final stage into a completely heterochromatic and to a great extends genetically eroded Y chromosome. The constraints evolving the Y chromosome have been the objects of speculation since the discovery of sex chromosomes. Several models have been suggested. We use the exceptional situation of the in Drosophila mirandato analyze the molecular process in progress involved in Y chromosome evolution. We suggest that the first steps in the switch from a euchromatic proto-Y-chromosome into a completely heterochromatic Y chromosome are driven by the accumulation of transposable elements, especially retrotransposons inserted along the evolving nonrecombining part of the Y chromosome. In this evolutionary process trapping and accumulation of retrotransposons on the proto-Y-chromosome should lead to conformational changes that are responsible for successive silencing of euchromatic genes, both intact or already mutated ones and eventually transform functionally euchromatic domains into genetically inert heterochromatin. Accumulation of further mutations, deletions, and duplications followed by the evolution and expansion of tandem repetitive sequence motifs of high copy number (satellite sequences) together with a few vital genes for male fertility will then represent the final state of the degenerated Y chromosome. This revised version was published online in July 2006 with corrections to the Cover Date.     

Linkage, physical mapping, and DNA sequence analysis of pseudoautosomal loci on the human X and Y chromosomes

The pseudoautosomal region of the human X and Y chromosomes is subject to frequent X-Y recombination during male meiosis. We report the finding of two pseudoautosomal loci, DXYS20 and DXYS28, characterized by highly informative restriction fragment length polymorphisms (RFLPs). The pseudoautosomal character of DXYS20 and DXYS28 was formally demonstrated by comparing their transmission to 45,X and to normal individuals. Studies of the inheritance of these loci reveal that the pseudoautosomal region, though highly recombinogenic, is subject to marked recombinational interference in male meiosis; no double recombinants were observed in 143 triply informative meioses, and the coefficient of coincidence is likely less than 0.45. In female meiosis, linkage of these pseudoautosomal RFLPs to strictly sex-linked RFLPs on the short arm of the X is readily detected; the genetic length of the pseudoautosomal region in female meiosis is at least 4 cM but not more than 18 cM. The genetic map of the human X chromosome is now defined from near the short-arm telomere to band q28 on the long arm. Locus DXYS20, which maps near the X and Y short-arm telomeres, is composed of long tandem arrays of 61-bp repeats. Occasional, seemingly random base-pair substitutions within these arrays of 61-bp repeats, in combination with marked variation in the size of the array, generate the high degree of DNA polymorphism at DXYS20.     

Distribution of genes and recombination on wheat homoeologous group <Emphasis Type="Italic">6</Emphasis> chromosomes: a synthesis of available information

Presence of genes in gene-rich regions on wheat chromosomes has been widely reported. However, there is a lack of information on the precise characterization of these regions with respect to the distribution of genes and recombination. We attempted to critically analyze the available data to characterize gene-rich regions and to study the distribution of genes and recombination on wheat homoeologous group 6 chromosomes which are a reservoir of several useful genes controlling traits of economic importance. Consensus physical and genetic linkage maps were constructed for homoeologous group 6 using physical and genetic mapping data. Five major gene-rich regions were identified on homoeologous group 6 chromosomes, with two on the short and three on long arm. More than 90% of marker or gene loci were present in these five gene-rich regions, which comprise about 30% of the total physical chromosomal length. The gene-rich regions were mainly present in the distal 60% regions of the chromosomes. About 61% of the total loci map in the most distal regions which span only about 4% of the physical length of the chromosome. A range of sub-microscopic regions within each gene-rich region were also identified. Comparisons of the consensus physical and genetic linkage maps revealed that recombination occurred mainly in the gene-rich regions. Seventy percent of the total recombination occurred in the two most distally located regions that span only 4% of the physical length of the chromosomes. The relationship of recombination to the gene-rich region is not linear with distance from the centromere, especially on the long arm. The kb/cM estimates for group 6 chromosomes ranged from 146 kb in the gene-rich to about 10 Mb in the gene-poor region. The information obtained here is vital in understanding wheat genome structure and organization, which may lead in developing better strategies for positional cloning in wheat and related cereals.This revised version was pubished online in April 2005 with corrections to the page numbering.