M, Appels R, Craig S. Topographical changes in rye chromosome ultrastructure caused by the C-banding procedure

The changes in the surface features of rye chromosome 1 induced by the C-banding procedure was investigated using the scanning electron microscope. Each step in the procedure produced significant changes in the surface topography of the chromosome and minor differences in its general morphology. The most noticeable alteration, chromosomes flattened and smooth, occurred when they were incubated in phosphate buffer without Leishman's stain. Since chromosomes not treated with barium hydroxide were uniformly stained, it seems probable that this alkali removes chromatin unselectively and that the C-bands produced here are due to a differential affinity of the heterochromatin to the stain.

Key words: rye, heterochromatin, C-banding.

Variability and genetics of spacer DNA sequences between the ribosomal-RNA genes of hexaploid wheat (Triticum aestivum)

Using restriction enzyme digests of genomic DNA extracted from the leaves of 25 hexaploid wheat (Triticum aestivum L. em. Thell.) cultivars and their hybrids, restriction fragment length polymorphisms of the spacer DNA which separates the ribosomal-RNA genes have been examined. (From one to three thousand of these genes are borne on chromosomes 1B and 6B of hexaploid wheat). The data show that there are three distinct alleles of the 1B locus, designated Nor-B1a, Nor-B1b, and Nor-B1c, and at least five allelic variants of the 6B locus, designated Nor-B2a, Nor-B2b, Nor-B2c, Nor-B2d, and Nor-B2e. A further, previously reported allele on 6B has been named Nor-B2f. Chromosome 5D has only one allelic variant, Nor-D3. Whereas the major spacer variants of the 1B alleles apparently differ by the loss or gain of one or two of the 133 bp sub-repeat units within the spacer DNA, the 6B allelic variants show major differences in their compositions and lengths. This may be related to the greater number of rDNA repeat units at this locus. The practical implications of these differences and their application to wheat breeding are discussed.

Controlled introgression to wheat of genes from rye chromosome arm 1RS by induction of allosyndesis : 2. Characterisation of recombinants

Sixteen allosyndetic recombinants involving chromosome arms 1DS and 1RS derived from the wheat-rye translocation 1DL-1RS, and one 1RS recombinant derived from 1BL-1RS were characterised by cytological, biochemical and molecular methods. Analysis of isozyme markers showed that recombination had involved different breakpoints along the 1DS arm. The gene order Tri-D1 - Per-D1 - Gpi-D1 - Gli-D1 was established on chromosome 1DS, and this is consistent with the gene order known to occur on 1BS. The use of three molecular probes for loci on 1RS confirmed one of the recombinants obtained from the 1BL-1RS translocation, and two of the three recombinants obtained from the 1DL-1RS translocation. These two recombinant chromosomes appear to have approximately reciprocal structures and may prove of direct benefit to wheat breeders as they retain the stem rust resistance of the parental translocation but have recovered some of the 1D encoded proteins which are absent in 1DL-1RS lines and which are thought to affect dough quality. A 5S RNA site was discovered on chromosome arm 1DS, probably reflecting the homoeologous relationship of this chromosome with 1BS. Segregational analysis indicated that some of the homoeologous wheat-wheat exchanges probably involved more than a single allosyndetic event.

Alien chromatin in wheat: ribosomal DNA spacer probes for detecting specific nucleolar organizer region loci introduced into wheat

The structure, at the level of restriction endonuclease mapping, of rDNA spacer regions from representatives of the B, R, S, P, N, J1J2, and E genomes within the Triticeae are compared. The results indicate that the evolution of the main spacer region of rDNA units is sufficiently rapid to allow each genome to be clearly identified. The spacer regions can be successfully used to distinguish respective alien rDNA units when they are present in wheat.Key words: Triticeae, alien chromatin, molecular probes, NOR loci.

Rye heterochromatin. I. Studies on clusters of the major repeating sequence and the identification of a new dispersed repetitive sequence element

The structure of appoximately 10 kilobases of DNA originating from the subcloned DNA segments of five lambda clones from rye heterochromatin is described. The sequence data suggested that much of the variation between the 380 base pair (bp) units of the tandem arrays in hetercohromatin must have preceded the amplification events which gave rise to the arrays. Furthermore, evidence for recombination between the tandem arrays of the units (subsequent to the presumed amplification events) was obtained. A single 14-bp region which did not show any variation in sequence was found to be part of a dyad symmetry, characteristic for DNA-binding proteins. The analysis of a molecule which contained a junction between the heterochromatic "350" DNA sequence family and apparently nonheterochromatic DNA is also presented. This nonheterochromatic DNA contains a dispersed repetitive type of sequence ("5.3") which is found adjacent to a wide range of sequences as judged from genomic analyses, in situ hybridization studies and independent isolates of the sequence. The 5.3 sequence is dispersed throughout nonheterochromatic and most heterochromatic regions. However, some heterochromatic regions and the nucleolar organizing region appear not to contain the sequence or else have only very low amounts. The "5.3" element provides a novel probe for further analysis of the rye genome.Key words: rye, heterochromatin, amplification.

Rye heterochromatin. II. Characterization of a derivative from chromosome 1DS.1RL with a reduced amount of the major repeating sequence

Homoeologous recombination between wheat chromosomes and the long arm of chromosome 1R of rye present in a wheat background as the wheat–rye translocation, 1DS.1RL, has been reported. During the course of this study an unexpected plant (308-17), with a recombinant phenotype, arose from a control population where homoeologous pairing was thought to be suppressed. The putative recombinant chromosome in plant 308-17 carried two seed protein loci located on 1RL (namely, the rye Glu-R1 locus and a recently discovered globulin-like marker) but appeared to lack C-banded heterochromatin. Further investigation of this apparent recombinant chromosome in progeny of 308-17 using a cloned probe for the 350 family of rye heterochromatic sequence indicated that some terminal heterochromatin was still present but in a much reduced amount. Because it appears that only the terminal region has been changed, the modified chromosome in 308-17 most likely did not arise from homoeologous recombination but rather from some form of heterochromatin loss event, possibly involving an unequal sister chromatid exchange within the terminal heterochromatin of 1RL. Sister lines to plant 308-17 carrying Glu-R1 contained a normal amount of 1RL heterochromatin.Key words: rye–wheat, heterochromatin, recombinant, unequal sister chromatid exhange.

The structure of DNA from the rye (Secale cereale) NOR R1 locus and its behaviour in wheat backgrounds

The nucleotide sequence of a 4.5-kilobase (kb) segment of rye DNA (in the clone pScR4) containing the major spacer region of ribosomal DNA, 0.25 kb of the 5′ end of the 18S gene and approximately 1.2 kb of the 3′ end of the 26S gene is presented. The 5′ end of the 18S gene, and most of the 3′ end of the 26S gene, could be readily defined by comparisons with previously published sequences. The major spacer region was dominated by the presence of 10 repeating units, with a basic length of 134 base pairs (bp), which showed sequence variability in that deletions of sections of the sequence as well as base-pair changes were common. A portion of the sequence which was common to almost all the spacer repeats and is also found in the corresponding wheat spacer repeats (in pTa250), with only two mismatches (using a consensus sequence of the rye spacer repeats as a comparison), is GCGACATGGAAAACCGGGCAAAACCACGTAC. The occurrence of this conserved sequence between more divergent sequences in both wheat and rye suggests that it may be important in forming the pretranscription complex involving RNA polymerase I and other protein cofactors. Probes were isolated from the spacer region to preferentially assay the rye rDNA in wheat backgrounds. In situ hybridization studies revealed that when 1R was the only NOR-containing chromosome present the rDNA was in a dispersed state. This dispersed state correlates with nucleolus formation and presumably with rDNA transcription. In the presence of chromosomes 1B and (or) 6B of wheat (these chromosomes carry NOR loci) the dispersed state of rye rDNA was generally suppressed, but not completely, suggesting that 1R retains some potential for activity.Key words: rye, NOR R1 locus, nucleotide sequence, transcription.

INVESTIGATION OF HOMOLOGOUS CROSSING OVER AND SISTER CHROMATID EXCHANGE IN THE WHEAT Nor-B2 LOCUS CODING FOR rRNA AND Gli-B2 LOCUS CODING FOR GLIADINS

Recombination was investigated within the Nor-B2 locus of wheat chromosome 6B that contains several thousand of the 18S-5.8S-26S rRNA (rDNA) repeated units. Additionally, recombination was assessed for several chromosome regions, in arm 6Bq between the centromere and the B2 locus (awn suppressor) and in arm 6Bp between the centromere and Nor-B2, between Nor-B2 and a distal C-band and between Nor-B2 and Gli-B2 coding for gliadins. The experimental design permitted the distinction between crossing over between homologous chromosomes and exchange between sister chromatids. No homologous crossing over within the Nor-B2 locus was found in a sample of 446 chromosomes, but one exchange with the attributes of unequal sister chromatid exchange was identified. The molecular characteristics of this presumed sister chromatid exchange indicate that the spacer variants present in the Nor-B2 locus are clustered. No homologous recombination was detected within the distal Gli-B2 locus containing repeated genes coding for gliadin seed-storage proteins. Both arms of chromosome 6B showed low crossing-over frequency in the proximal regions. The distance from the centromere to Nor-B2 was only from 0.3 to 2.2 cM although it accounts for about two-thirds of the metaphase chromosome arm, which shows a great distortion of the metaphase map of the arm. The level of homologous recombination within the Nor-B2 locus is lower than in the chromosome region immediately distal to it. Whether it is comparable to that in the chromosome region proximal to it could not be determined. Recombination frequencies of different pairs of chromosome 6B in all but one interval paralleled the frequencies of their metaphase I pairing: Lower pairing at metaphase I was paralleled by lower crossing-over frequency. This relationship indicated that reduced metaphase I pairing between 6B chromosomes from different populations is due to impaired crossing-over and not due to precocious chiasma terminalization.

Mapping the nucleolus organizer region, seed protein loci and isozyme loci on chromosome 1R in rye

The nucleolus organizer region located on the short arm of chromosome 1R of rye consists of a large cluster of genes that code for ribosomal RNA (designated the Nor-R1 locus). The genes in the cluster are separated by spacer regions which can vary in length in different rye lines. Differences in the spacer regions were scored in two families of F2 progeny. Segregation also occurred, in one or both of the families, at two seed protein loci and at two isozyme loci also located on chromosome 1R. The seed protein loci were identified as the Sec 1 locus controlling ω-secalins located on the short arm of chromosome 1R and the Sec 3 locus controlling high-molecular-weight secalins located on the long arm of 1R. The two isozyme loci were the Gpi-R1 locus controlling glucose-phosphate isomerase isozymes and the Pgd 2 locus controlling phosphogluconate dehydrogenase isozymes. The data indicated linkage between all five loci and map distances were calculated. The results indicate a gene order: Pgd 2 ... Sec 3 ... [centromere] ... Nor-R1 ... Gpi-R1 ... Sec 1. Evidence was obtained that rye possesses a minor 5S RNA locus (chromosome location unknown) in addition to the major 5S RNA locus previously shown to be located on the short arm of chromosome 1R.

The heterochromatin of grasshoppers from the Caledia captiva species complex. I. Sequence evolution and conservation in a highly repeated DNA family

The restriction enzyme TaqI digests 0.2% of the genomic DNA from the grasshopper Caledia captiva to a family of sequences 168 bp in length (length of consensus sequence). The sequence variation of this "Taq family" of repeat units was examined among four races from C. captiva to assay the pattern of evolution within this highly repeated DNA. The Taq-family repeats are located in C-banded heterochromatin on at least one member of each homologous pair of chromosomes; the locations range from centromeric to telomeric. Thirty-nine cloned repeats isolated from two population 1A individuals along with 11 clones from seven populations taken from three of the races demonstrated sequence variation at 72 positions. Pairwise comparisons of the cloned repeats, both within an individual and between different races, indicate that levels of intraspecific divergence, as measured by reproductive incompatibility, do not correlate with sequence divergence among the 168-bp repeats. A number of subsequences within the repeat remain unchanged among all 50 clones; the longest of these is 18 bp. That the same 18-bp subsequence is present in all clones examined is a finding that departs significantly (P less than 0.01) from what would be expected to occur at random. Two other cloned repeats, from a reproductively isolated race of C. captiva, have sequences that show 56% identity with this 18-bp conserved region. An analysis showed that the frequency of occurrence of an RsaI recognition site within the 168-bp repeat in the entire Taq family agreed with that found in the cloned sequences. These data, along with a partial sequence for the entire Taq family obtained by sequencing uncloned repeats, suggest that the consensus sequence from the cloned copies is representative of this highly repeated family and is not a biased sample resulting from the cloning procedure. The 18-bp conserved sequence is part of a 42-bp sequence that possesses dyad symmetry typical of protein-binding sites. We speculate that this may be significant in the evolution of the Taq family of sequences.

Variation at the Nor loci in triticale derived from tissue culture

Plants derived from tissue cultures of six triticale genotypes were the subject of an analysis for changes in the rRNA genes located at the site of nucleolar organizer regions (the Nor loci) on chromosomes 1B, 6B and 1R. In addition whole plant phenotypes and the chromosomal constitutions of their progenies were examined for alterations. Following treatment of DNA with the restriction endonuclease Taq1, it was possible to assign electrophoretic bands representing rDNA spacer sequences to each of the chromosomes known to carry a major Nor locus. In general, the rRNA genes were found to be stable except in one family where a marked reduction in the number of rDNA units was observed. This reduction in 1R rDNA spacer sequences was heritable and correlated with reduced C-banding at the position of Nor-R1 on chromosome 1R. The change was clearly a consequence of tissue culture since six other plants regenerated from the same culture, and the original parent, did not carry the alteration.

Seedling lethality in wheat: a novel phenotype associated with a 2RS/2BL translocation chromosome

When grown at normal temperatures, wheat plants disomic for a 2RS/2BL translocation chromosome substituting for chromosome 2B show seedling lethality. Morphological and physiological studies could not determine the cause of death. However, the seedling lethality can be partly to completely inactivated at higher temperatures and in stressed environments. The lethality can also be completely suppressed if the translocated chromosome is introduced into different wheat cultivars. These wheats must contain genes which suppress the lethal phenotype caused by disomy of 2RS/2BL.Whilst the temperature effect indicates that the seedling lethality is related to the grass clump dwarf phenotype of wheat, our results show that the genes involved in seedling lethality, its suppression and inactivation, are not related to the D genes which cause grass clump dwarfing in wheat.

Chromosome and nucleotide sequence differentiation in genomes of polyploid Triticum species

The nature of genome change during polyploid evolution was studied by analysing selected species within the tribe Triticeae. The levels of genome changes examined included structural alterations (translocations, inversions), heterochromatinization, and nucleotide sequence change in the rDNA regions. These analyses provided data for evaluating models of genome evolution in polyploids in the genus Triticum, postulated on the basis of chromosome pairing at metaphase I in interspecies hybrids.The significance of structural chromosome alterations with respect to reduced MI chromosome pairing in interspecific hybrids was assayed by determining the incidence of heterozygosity for translocations and paracentric inversions in the A and B genomes of T. timopheevii ssp. araraticum (referred to as T. araraticum) represented by two lines, 1760 and 2541, and T. aestivum cv. Chinese Spring. Line 1760 differed from Chinese Spring by translocations in chromosomes 1A, 3A, 4A, 6A, 7A, 3B, 4B, 7B and possibly 2B. Line 2541 differed from Chinese Spring by translocations in chromosomes 3A, 6A, 6B and possibly 2B. Line 1760 also differed from Chinese Spring by paracentric inversions in arms 1AL and 4AL whereas line 2541 differed by inversions in 1BL and 4AL (not all chromosomes arms were assayed). The incidence of structural changes in the A and B genomes did not coincide with the more extensive differentiation of the B genomes relative to the A genomes as reflected by chromosome pairing studies.To assay changing degrees of heterochromatinization among species of the genus Triticum, all the diploid and polyploid species were C-banded. No general agreement was observed between the amount of heterochromatin and the ability of the respective chromosomes to pair with chromosomes of the ancestral species. Marked changes in the amount of heterochromatin were found to have occurred during the evolution of some of the polyploids.The analysis of the rDNA region provided evidence for rapid "fixation" of new repeated sequences at two levels, namely, among the 130 bp repeated sequences of the spacer and at the level of the repeated arrays of the 9 kb rDNA units. These occurred both within a given rDNA region and between rDNA regions on nonhomologous chromosomes. The levels of change in the rDNA regions provided good precedent for expecting extensive nucleotide sequence changes associated with differentiation of Triticum genomes and these processes are argued to be the principal cause of genome differentiation as revealed by chromosome pairing studies.

The wheat ribosomal DNA spacer region: Its structure and variation in populations and among species

The wheat rDNA clone pTA250 was examined in detail to provide a restriction enzyme map and the nucleotide sequence of two of the eleven, 130 bp repeating units found within the spacer region. The 130 bp units showed some sequence heterogeneity. The sequence difference between the two 130 bp units analysed (130.6 and 130.8) was at 7 positions and could be detected as a 4 °C shift in Tm when heterologous and homologous hybrids were compared. This corresponded to a 1.2% change in nucleotide sequence per ΔTm of 1 °C. The sensitivity of the Tm analysis using cloned sequences facilitated the analysis of small sequence variations in the spacer region of different Triticum aestivum cultivars and natural populations of T. turgidum ssp. dicoccoides (referred to as T. dicoccoides). In addition spacer length variation was assayed by restriction enzyme digestion and hybridization with spacer sequence probes.Extensive polymorphism was observed for the spacer region in various cultivars of T. aestivum, although within each cultivar the rDNA clusters were homogeneous and could be assigned to particular chromosomes. Within natural populations of T. dicoccoides polymorphism was also observed but, once again, within any one individual the rDNA clusters appeared to be homogeneous. The polymorphism, at the sequence level (assayed by Tm analysis), was not so great as to prevent the use of spacer sequence variation as a probe for evolutionary relationships. The length variation as assayed by restriction enzyme digestion did not appear to be as useful in this regard, since its range of variation was extensive even within populations of a species.

Relative rates of divergence of spacer and gene sequences within the rDNA region of species in the Triticeae: Implications for the maintenance of homogeneity of a repeated gene family

The relative rates of divergence of 11 regions of the wheat rDNA cloned in pTA250 were estimated by measuring sequence change in 6 Triticum species. The Tm analysis of (32)P probes synthesized from the pTA250 regions and hybridized to DNA from the Triticum species provided an estimate of sequence change relative to T. aestivum. The results revealed a region of 1.2 kb preceding the 18S rRNA gene which was more conserved than the rest of the spacer. In addition the transcribed spacer between the 18S and 26S rRNA genes was shown to be poorly-conserved; the genes, as expected, were highly conserved. A model which proposes RNA as a co-factor in gene conversion is suggested to account for the observations.

Pleiotropic effects associated with the deletion of heterochromatin surrounding rDNA on the X chromosome of Drosophila

In Drosophila melanogaster X chromosome heterochromatin (Xh) constitutes the proximal 40% of the X chromosome DNA and contains a number of genetic elements with homologous sites on the Y chromosome, one of which is well defined, namely, the bobbed locus, the repetitive structural locus for the 18S and 28S rRNAs. This report presents the localisation of specific repeated DNA sequences within Xh and the employment of this sequence map in constructing new chromosomes to analyse the nature of the heterochromatin surrounding the rDNA region. Repeated sequences were located relative to inversion breakpoints which differentiate Xh cytogenetically. When the rDNA region was manipulated to be in a position in the chromosome so that it was without the Xh which normally surrounds it, the following observations were made. (i) The rDNA region of Xh is intrinsically heterochromatic, remaining genetically active and yet possessing major heterochromatic properties even in the absence of the flanking heterochromatin regions. (ii) The size of the deletion removing the portion of Xh normally located distal to the rDNA region affected the dominance relationship between the X and Y nucleolar organizers (activity/endoreduplication assayed in male salivary glands). The X rDNA without any flanking heterochromatin was dominant over Y rDNA while the presence of some Xh allowed both the X and Y rDNA to be utilized. (iii) Enhancement of the position effect variegation on the white locus was demonstrated to occur as a result of the Xh deletions generated. EMS mutagenesis studies argue that the regions of Xh flanking the rDNA region contain no vital loci despite the fact that they strongly effect gene expression in some genotypes. This is consistent with early studies using X-ray mutagenesis (Lindsley et al., 1960). The pleiotropic effects of deleting specific regions of Xh is discussed in relation to the possible influence of heterochromatin on the organisation of the functional interphase nucleus.

Structural variation in the heterochromatin of rye chromosomes in triticales

Although Giemsa C-banding techniques have been used extensively for assaying cereal heterochromatin, a more specific technique for analyzing cereal heterochromatin has been developed recently with the isolation of DNA sequences present in heterochromatin and their employment in in situ hybridization to cereal chromosomes. A number of triticales were examined for the occurrence of modified rye chromosomes using the in situ hybridization technique. With a heterogeneous sequence probe the amount of rye heterochromatin appears to be relatively constant in wheat backgrounds but when a specific sequence probe was employed variation was observed. Whether this variation reflects polymorphism in rye or whether it is a result of adaption of the rye genome to coexistence with the wheat genome in triticales is discussed. - The triticale Rosner was examined in detail and it was established that the rye chromosome 2R had been replaced by the wheat chromosome 2D.

THE INHERITANCE OF RYE CHROMOSOMES IN EARLY GENERATIONS OF TRITICALE × WHEAT HYBRIDS

Triticales (× Triticosecale Wittmack) are being employed as a source of rye (Secale cereale L.) chromatin for the introduction of specific agronomic characters into wheat (Triticum aestivum L. em Thell.). The rye chromosomes present in plants of the first and second generations of a backcrossing program have been identified using a radioactive in situ probe which hybridizes to specific sites on the rye chromosomes. We show that homologous pairs of rye chromosomes are present by the second generation which should thereby ensure their eventual substitution. Furthermore, rye telosomes and a wheat-rye chromosome translocation involving 5RL were also observed as possibly useful modifications of the rye chromosomes in this breeding program.

The cytogenetic boundaries of the rDNA region within heterochromatin in the X chromosome of Drosophila melanogaster and their relation to male meiotic pairing sites

The proximal breakpoints of the inversion chromosomesIn(1)ωm4andIn(1)m51bwere shown, byin situhybridization, to define the boundaries of the ribosomal DNA region located within theXchromosome heterochromatin (Xh). We estimate thatat least95% of the rDNA is located between theIn(1)ωm4andIn(1)ωm51bproximal breakpoints. In contrast only 60–70% of the Type I intervening sequences located inXhare located between these breakpoints. The Type I intervening sequences in the rDNA region occur as inserts in the 28S rRNA sequences while the remainder of the sequences are distal to theIn(1)ωm4breakpoint and not associated with rRNA genes.

The regions of Xh which contain rDNA and Type I intervening sequences were related to regions shown by Cooper (1964) to contribute to meiotic pairing between theXandYchromosomes in male Drosophila. We demonstrate that the rRNA coding region contributes toX/Ypairing. However, no single region of Xh is required for fidelity of male meiotic pairing of the sex chromosomes.

The relationship between heterochromatic homology and meiotic segregation of compound second autosomes during spermatogenesis in Drosophila melanogaster

In this report we examine the meiotic segregation of compound second autosomes sharing varying extents of heterochromatic and euchromatic homology. The second chromosome heterochromatin does not appear to influence the random meiotic segregation of compound second autosomes during spermatogenesis; however, the proximal euchromatin is implicated in male meiotic pairing. We conclude that male autosomal meiotic pairing sites are specific euchromatic chromosomal regions.

The distribution of two highly repeated DNA sequences within Drosophila melanogaster chromosomes

In situ hybridization using 3H-RNA probes has been used to localize the sequences found in two satellites of density 1.705 g/cc and 1.672 g/cc to specific sites within the chromosomal complement. A detailed analysis of the sites on the S chromosome was carried out using the acute series of inversions to relate the heterochromatic breakpoint relative to the location of the sequence on this chromosome. It has also been possible to establish the order of arrangement of 1.705 and 1.672 DNA at the heterochromatic-euchromatic junction on chromosome 3(R). A mitotic map is provided. The Tm of hybrids formed in situ showed that the hybrids were representative of the sequences being analyzed. The two satellites also were traced through a number of purification procedures to show that a covalent linkage may be likely between the 1.705 g/cc and 1.672 g/cc satellite as predicted from in situ hybridization analyses.

Chromosomal distribution of the major insert in Drosophila melanogaster 28S rRNA genes

The major type I insert sequence for the 28S rRNA genes ofDrosophila melanogasterhas been mapped within the chromosomes using a probe synthesized from a cloned sequence containing the entire 5·4 kb segment. The genomic distribution was shown to be complex in that the insert sequence occurred next to many different types of sequences, in addition to occurring as an insert in the 28S rRNA genes of theXchromosome.In situhybridization of mitotic chromosomes showed most of the insert units not contained in the ribosomal genes to be located near the ribosomal gene cluster on theXchromosome. Additional sites were detected in polytene chromosomes in region 102C, 8–12 and in the hetero-chromatin of the autosomes.