The chromosomes of two Drosophila races: D. nasuta nasuta and D. n. albomicana

Heterochromatin distribution and differentiation in metaphase chromosomes of two morphologically identical Drosophila races, D. nasuta nasuta and D. n. albomicana, have been studied by C- and N-banding methods. — The total heterochromatin values differ only slightly between these races. However, homologous chromosomes of the two Drosophila forms show striking differences in the size of heterochromatin regions and there is an alternating pattern in D. n. nasuta and D. n. albomicana of chromosomes which contain more, or respectively less heterochromatin than their counterparts in the other race. — Three different N-banding patterns could be obtained depending on the conditions of the method employed: One banding pattern occurs which corresponds to the C-banding pattern. Another pattern is the reverse of the C-band pattern; the euchromatic chromosome regions and the centromeres are stained whereas the pericentric heterochromatin regions remain unstained. In the Y chromosomes of both races and in chromosome 4 of D. n. albomicana, however, the heterochromatin is further differentiated. In the third N-banding pattern only the centromeres are deeply stained. Furthermore, between the races, subtle staining differences in the pericentric heterochromatin regions can be observed as verified in F₁ hybrids. On the basis of C- and N-banding results specific aspects of chromosomal differences between D. n. nasuta and D. n. albomicana are discussed.Dedicated to Prof. W. Beermann on the occasion of his 60th birthday     

The chromosomes of two Drosophila races: D. nasuta nasuta and D. nasuta albomicana

The microchromosomes of the totally cross fertile Drosophila races, D. nasuta nasuta and D. nasuta albomicana have been studied in nietaphase and polytene nuclei. In metaphase the microchromosome of D. n. albomicana is nearly five times longer than the homologous chromosome in D. n. nasuta. As shown by C-banding these length differences are mainly due to a massive addition of heterochromatin to the D. n. albomicana chromosome. In polytene nuclei these striking heterochromatin differences between the microchromosomes of the two Drosophila races cannot be observed. Analysis of the polytene banding pattern shows that the microchromosomes of both races differ by an inversion and by a duplication, present only in D. n, albomicana. The location and orientation of the duplicated regions in D. n. albomicana leads to a specific loop like chromosome configuration. On the basis of these differences within the Drosophila races studied it is assumed that the karyotype of D. n. albomicana is a more recent evolutionary product.     

The chromosomes of two Drosophila races: D. nasuta nasuta and D. nasuta albomicana

Interracial hybridization between Drosophila nasuta nasuta (2n=8) and D. n. albomicana (2n=6) has resulted in the evolution of two new karyotypic strains, called Cytoraces I and II. Males and females of Cytorace I have 2n=7 and 2n=6 respectively. The reconstituted karyotype is totally new in its composition, the chromosomes being drawn from both the parental races. The individuals of Cytorace II have 2n=6. Even though the chromosomes of the parental races are duly represented in the F₁, there is selective retention/elimination of certain chromosomes in the succeeding generations during which repatterning of the karyotype has taken place. Dynamics of each one of the parental chromosomes are presented and its implications re discussed.We dedicate this paper to the memory of the founder of our Department, the late Prof. M.R. Rajasekarasetty on the occasion of the Silver Jubilee of our Department     

Satellite DNA of Drosophila nasuta nasuta and D. n. albomicana: Localization in polytene and metaphase chromosomes

The DNA from the two Drosophila nasuta races, D. n. nasuta and D. n. albomicana was investigated by CsCl density gradient centrifugation. D. n. nasuta has one major AT-rich satellite DNA sequence with a density of 1.664g/cm³, while D. n. albomicana has at least three satellites with densities of 1.674g/cm³, 1.665g/cm³ and 1.661 g/cm³. The isolated satellite sequences hybridize in situ to all heterochromatic regions of all metaphase chromosomes of both races. In polytene chromosomes the satellite sequences hybridize exclusively to the chromocenter. All chromosomal regions hybridizing with the satellites show also bright quinacrine fluorescence.     

Distribution of silver-stained nucleolus-organizing regions in the chromosomes of the Equidae

The distribution of silver-stained nucleolus-organizing regions (NORs) in fibroblast chromosomes from all seven extant species of Equidae are described. There are variations in the number and locations of silver-stained NORs in different species but most of the cells in an individual of any speies had only 2 to 4 silver-stained NORs. In Equus przewalskii and E. caballus NORs were detected on up to three different chromosomes. In E. asinus 11 different chromosomes were observed to possess NOR sites. E. hemionus kulan and E. hemionus onager had NORs on 2 large metacentric pairs and a small acrocentric pair. In E. grevyi and E. burchelli NORs were located on 3 to 4 different pairs. E. zebra hartmannae had silver-staining over the telomere regions of the short arms of the chromosomes 1, 3 and 4. A comparison of G-banded chromosomes and silver-stained NORs has revealed one autosome with conserved G-band patterns and possessing a silver-staining NOR in all the species except E. asinus. Variations in the number and multichromosomal locations of NORs in various species could have evolved by pairing and exchanges between non-homologous chromosomal heterochromatin having similar satellite DNA sequences.     

Comparative cytogenetics in four species of Palinuridae: B chromosomes, ribosomal genes and telomeric sequences

The evolutionary pathway of Palinuridae (Crustacea, Decapoda) is still controversial, uncertain and unexplored, expecially from a karyological point of view. Here we describe the South African spiny lobster Jasus lalandii karyotype: n and 2n values, heterochromatin distribution, nucleolar organizer region (NOR) location and telomeric repeat structure and location. To compare the genomic and chromosomal organization in Palinuridae we located NORs in Panulirus regius, Palinurus gilchristi and Palinurus mauritanicus: all species showed multiple NORs. In J. lalandii NORs were located on three chromosome pairs, with interindividual polymorphism. In P. regius and in the two Palinurus species NORs were located on two chromosome pairs. In the two last species 45S ribosomal gene loci were also found on B chromosomes. In addition, the nature and location of telomeric repeats were investigated by FISH in J. lalandii, P. gilchristi, P. mauritanicus Palinurus elephas, and P. regius (Palinuridae, Achelata), and in Scyllarus arctus (Scyllaridae, Achelata): all these Achelata species showed the (TTAGG)n pentameric repeats. Furthermore, in J. lalandii these repeats occurred in all the telomeres and in some interstitial chromosomal sites, associated with NORs.     

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.     

Cytogenetic studies on Bacillus grandii grandii and B. grandii benazzii (Insecta,Phasmatodea): karyotype description,constitutive heterochromatin and nucleolus organizer regions

The karyotype of the new Sicilian subspecies Bacillus grandii benazzii is described and compared to that of B. g. grandii. Both taxa share constant, very similar chromosome sets with 2n=34, XX, female and 2n=33, XO, male, the sex chromosomes being the third element of the complement. C-banding reveals that a centromeric heterochromatin mass is present on all chromosomes, but no interstitial bands are observed. Differences between karyotypes reside in the satellites which are located on pairs 13 and 16 in B. g. benazzii and only on pair 16 in B. g. grandii, in contracted metaphases larger satellites are positively C-banded. Furthermore, Ag-detected NORs are seen on pair 13 in B. g. benazzii and on pair 16 in B. g. grandii. Relationships between constitutive heterochromatin and NORs, NOR heteromorphism and the position of the 34-chromosome taxa within the genus are discussed.     

Nucleolar organizing chromosomes ofRicinus

Summary Pachytene chromosome morphology was compared in nine races ofRicinus communis L. (2n = 20), using pollen mother cells (PMCs) and light microscopy. Of the ten bivalents, only the two possessing nucleolar organizing regions (NORs), chromosomes 2 and 7, exhibit structural variations among the races. The NORs are located in the short arms of these two chromosomes. Most of the observed structural variations affect these short arms, which are similar morphologically and consist largely of heterochromatic segments. The PMCs contain a single nucleolus and this is associated with the NOR of each of the two chromosomes at a particular frequency in each race. In eight races, a nucleolar constriction (NC) is present in either chromosome 2 or chromosome 7. In these races, the nucleolus is associated with the chromosome possessing an NC at a frequency of 100% and with the chromosome lacking an NC at a frequency ranging between 5.6 and 100%, depending upon the race. No microscopically visible NC is present in the ninth race. In this race, the nucleolus is associated with both chromosomes 2 and 7 at a frequency of 100%. The association of the nucleolus with a chromosome possessing an NC is at the NC and with a chromosome lacking an NC is at the terminal heterochromatic segment of the short arm. Several interpretations are offered to account for the variations in frequency of association between the nucleolus and each of the nucleolar organizing chromosomes. It is suggested that the two non-linked NORs have evolved through some intragenomic changes rather than polyploidy, that this species is highly intolerant to structural variations other than those occurring in or near the NORs, and that structural variations in the nucleolar organizing chromosomes are not associated with racial variations in plant phenotype.Paper of the Journal Series, New Jersey Agricultural Experiment Station     

Chromosome banding studies in an Indian mullet: evidence of structural rearrangements from NOR locations

C-, G- and NOR bands have been studied in the female sex of Rhinomugil corsula. (Mugilidae, Pisces) by deploying the conventional methodologies with suitable modifications of minor nature. The diploid metaphase complements contained 48 acrocentric chromosomes. The localization of C-band heterochromatin was found to be mostly at or near the centromeric regions of the acrocentric chromosomes. The G-type bands were not so well defined, but some of the G-banded chromosomes also contained C-bands. Interestingly, silver-positive NORs were found at the telomeric ends of five acrocentric chromosomes, including one homologous pair having NORs in both chromatids, while one chromosome showed NORs in both of its chromatids and the other two had only one NOR localized at one of its chromatids. This would suggest that one homologue of the second pair of NOR-bearing chromosomes possibly underwent a chromatid exchange with a non-NOR bearing chromosome. This is quite a unique situation not reported earlier in any species of fish., though some other form of NOR-polymorphism/heteromorphism has rarely been reported. Therefore, further exploration in natural populations of this species to examine the other sex and to verify if there also exists other chromosomally polymorphic races (in respect of NOR-polymorphism) of this species, would be rewarding.     

A Drosophila melanogaster cell line tested for the presence of active NORs by silver staining

Silver staining was used to detect active NORs in a Drosophila melanogaster cell line (C1 82) characterized by dimorphic X chromosomes (XX_L), one of the two Xs showing a marked increase in heterochromatin where the nucleolar organizer (NO) is located. The Q-banding technique was used to determine the karyotype characteristics of the line. Ag-positive NORs appeared only on structurally changed X chromosomes (X_L), both in diploid and tetraploid cells, indicating that rRNA genes of X_L are more active or numerous than those on normal homologues. A possible relationship between NOR stainability, the presence of an increased heterochromatic portion and the selective advantage of XX_L cells, recurrent in numerous Drosophila female lines, is discussed.     

Chromosome banding in Amphibia

The mitotic and meiotic chromosomes of the marsupial frog Gastrotheca riobambae were analysed with various banding techniques. The karyotype of this species is distinguished by considerable amounts of constitutive heterochromatin and unusual, heteromorphic XY sex chromosomes. The Y chromosome is considerably larger than the X chromosome and almost completely heterochromatic. The analysis of the banding patterns obtained with GC- and AT-base-pair-specific fluorochromes shows that the constitutive heterochromatin in the Y chromosome consists of at least three different structural categories. The only nucleolus organizer region (NOR) of the karyotype is localized in the short arm of the X chromosome. This causes a sex-specific difference in the number of NOR: female animals have two NORs in diploid cells, male animals one. No cytological indications were found for the inactivation of one of the two X chromosomes in the female cells. In male meiosis, the heteromorphic sex chromosomes form a characteristic sex-bivalent by pairing their telomeres in an end-to-end arrangement. The significance of the XY/XX sex chromosomes of G. riobambae for the study of X-linked genes in Amphibia, the evolution of sex chromosomes and their specific DNA sequences, and the significance of the meiotic process of sex chromosomes are discussed.     

Karyotypic evolution of ribosomal sites in buffalo subspecies and their crossbreed

Domestic buffaloes are divided into two group based on cytogenetic characteristics and habitats: the “river buffaloes” with 2n = 50 and the “swamp buffaloes”, 2n = 48. Nevertheless, their hybrids are viable, fertile and identified by a 2n = 49. In order to have a better characterization of these different cytotypes of buffaloes, and considering that NOR-bearing chromosomes are involved in the rearrangements responsible for the karyotypic differences, we applied silver staining (Ag-NOR) and performed fluorescent in situ hybridization (FISH) experiments using 18S rDNA as probe. Metaphases were obtained through blood lymphocyte culture of 21 individuals, including river, swamp and hybrid cytotypes. Ag-NOR staining revealed active NORs on six chromosome pairs (3p, 4p, 6, 21, 23, 24) in the river buffaloes, whereas the swamp buffaloes presented only five NOR-bearing pairs (4p, 6, 20, 22, 23). The F1 cross-breed had 11 chromosomes with active NORs, indicating expression of both parental chromosomes. FISH analysis confirmed the numerical divergence identified with Ag-NOR. This result is explained by the loss of the NOR located on chromosome 4p in the river buffalo, which is involved in the tandem fusion with chromosome 9 in this subspecies. A comparison with the ancestral cattle karyotype suggests that the NOR found on the 3p of the river buffalo may have originated from a duplication of ribosomal genes, resulting in the formation of new NOR sites in this subspecies.     

Mapping of the nucleolus organizer region on chromosome 4 inArabidopsis thaliana

InArabidopsis thaliana the ribosomal RNA genes (rRNA genes or rDNA) are clustered in tandemly repeated blocks in two nucleolus organizer regions (NORs). Cytogenetic analysis has shown that the NORs are localized on chromosome 2 (NOR 2) and 4 (NOR 4). Recently the map position of NOR 2 was determined using a RFLP which was larger than 100 kb. In the course of a fingerprint analysis of differentArabidopsis ecotypes we have detected four rDNA polymorphisms between the ecotypes Landsberg (La) and Niederzenz (Nd). Mapping of these polymorphisms using established segregating F₂ populations reveals that all polymorphisms detected are dominant. Three of them map to the locus on the second chromosome that has been shown to harbour the NOR 2. The fourth polymorphism can be unambigously assigned to the upper arm of the fourth chromosome. This is the first polymorphism found which originates in the second rDNA cluster ofArabidopsis thaliana. It enables localization of NOR 4 and thus completes the mapping of rDNA genes in the NORs ofArabidopsis.     

Development of silver stained structures during spermatogenesis in different genera of Formicidae

Development of silver stained structures during the spermatogenesis of Tapinoma nigerrlum, Pheidole pallidula, Tetramorium caespitum, Tetramorium semilaeve, Lasius niger and Plagiolepis schmitzii are studied. Nucleolar masses are only observed in early prophase. Obvious NORs are present in metaphase in all genera and species studied. However, there are no nucleolar Ag precipitates after metaphase. A resumption of silver stainability occurs in round spermatids. The majority of these genera present differential activity between the existing NORs. In T. nigerrimum there is primary or secondary NOR activity in all chromosomes of the complement, although there are interpopulation differences in relation to the NOR activity. In the remaining genera only certain chromosomes present NOR activity. Interpopulation genetic differences and environmental factors can cause differential activity of secondary NORs as observed in Tapinoma nigerrimum.     

Nucleolus organizer regions of the canine karyotype

Silver-stained preparations of cultured lymphocytes obtained from 12 pure-bred dogs revealed the presence of nucleolus organizer regions (NORs) on four to seven chromosomes in females and five to eight chromosomes in males. All seven males had a NOR on the Y chromosome. The telomeric location of the NORs on the autosomes suggested by an earlier study was confirmed.     

Position-dependent NOR activity in barley

Two types of intraspecific nucleolar dominance/suppression are described for barley,Hordeum vulgare L. When the nucleolus organizing regions (NORs) originally belonging to chromosomes 6 and 7 are combined by translocation in one chromosome, NOR 6 is dominant over NOR 7. Neither significant loss of rDNA nor its hypermethylation is the reason for the reduced nucleolus forming activity of NOR 7. Intrachromosomal NOR suppression probably does not occur in isochromosome 6s, which has two NORs 6 in one chromosome. Meiotic and somatic pairing of the homologous arms might be the reason for early fusion of their nucleoli and thus for the lower than expected maximum number of interphase nucleoli. Variable suppression of a partial NOR (6³) is described for descendants of crosses between translocation lines with split NORs 6 and 7. In these cases also, the reduced activity of the partial NOR 6³ is not due to deletion of rDNA as shown by in situ hybridization. Unstable methylation of NOR 6³ in heterozygous F₁ individuals is probably the cause of this phenomenon.     

Population differences in the expression of nucleolus organizer regions in the grasshopperEyprepocnemis plorans

Summary Fluorescence in situ hybridization revealed the presence of ribosomal RNA genes in paracentromeric regions of all A chromosomes and in the distal half of B chromosomes in embryonic cells from Moroccan specimens of the grasshopperEyprepocnemis plorans. The expression of these genes was monitored by the presence of nucleoli attached to each chromosome bivalent in diplotene cells from males collected from two different Moroccan populations and was compared to previous data of Spanish populations. Whereas only the nucleolus organizer regions (NORs) on S₉–S₁₁ and X chromosomes were active in the Spanish specimens. Moroccan individuals showed NOR activity in all chromosomes. The rRNA genes on the B chromosome were inactive in both populations. The S₉ and S₁₀ NORs were less active in Moroccan specimens than in Spanish specimen, which might be partly explained by the negative interdependence for expression of the S₁₀ NOR with respect to those on L₂ and X chromosomes. On the other hand, the X NOR was more active in Moroccan specimens than in Spanish specimens, and this might be partly due to the positive effect that the presence of B chromosomes has on the expression of this NOR. The implications of these observations on current models of NOR activity regulation are discussed.Abbreviation NOR nucleolus organizer region