A comparative study of the recombination pattern in three species of Platyrrhini monkeys (primates)

Homologous chromosomes exchange genetic information through recombination during meiotic synapsis, a process that increases genetic diversity and is fundamental to sexual reproduction. Meiotic studies in mammalian species are scarce and mainly focused on human and mouse. Here, the meiotic recombination events were determined in three species of Platyrrhini monkeys (Cebus libidinosus, Cebus nigritus and Alouatta caraya) by analysing the distribution of MLH1 foci at the stage of pachytene. Moreover, the combination of immunofluorescence and fluorescent in situ hybridisation has enabled us to construct recombination maps of primate chromosomes that are homologous to human chromosomes 13 and 21. Our results show that (a) the overall number of MLH1 foci varies among all three species, (b) the presence of heterochromatin blocks does not have a major influence on the distribution of MLH1 foci and (c) the distribution of crossovers in the homologous chromosomes to human chromosomes 13 and 21 are conserved between species of the same genus (C. libidinosus and C. nigritus) but are significantly different between Cebus and Alouatta. This heterogeneity in recombination behaviour among Ceboidea species may reflect differences in genetic diversity and genome composition.     

Location and variation of the constitutive heterochromatin in Petunia hybrida

The location of heterochromatin in the chromosomes of Petunia hybrida (2n=14) is presented. C-banded mitotic metaphase chromosomes and carmine-stained pachytene bivalents have been studied. It is shown that the heterochromatin is predominantly located near the centromeres and at the secondary constrictions of the satellite chromosomes. The distribution of chromomeres in pachytene bivalents also reveals that heterochromatin is not restricted to distinct blocks, as is the case in tomato, but occurs in smaller chromomeres which gradually decrease in size towards the ends. Conspicuous telomeres have not been observed. Both C-banding technique and pachytene analysis demonstrate large variation of heterochromatin between different lines of Petunia. The study of pachytene morphology has been hampered by a high degree of non-specific stickiness of the bivalents. Both techniques prove to be unsuitable tools for large-scale chromosome identification of Petunia lines.     

Chromosome plasticity in Ctenomys (Rodentia Octodontidae): chromosome 1 evolution and heterochromatin variation

A chromosome 1 (Cr1) pericentric inversion is described in six of seven species in the genus Ctenomys (tuco-tucos) from Uruguay. The inversion was inferred from G-band analyses of subtelocentric Cr1 hypothesised to be derived from the ancestral metacentric condition. Cr1 varies across species in heterochromatin amount and localisation including a metacentric chromosome without positive C-bands in C. torquatus, a subtelocentric chromosome with heterochromatic short arms in C. rionegrensis, and a subtelocentric chromosome negative after C-banding in five of the species analysed here. Pachytene chromosomes from C. rionegrensis, a species with the highest heterochromatin content, and C. torquatus, one of the species with the lowest heterochromatin content, were analysed in order to assess possible mechanisms of heterochromatin evolution. This analysis revealed the presence of three heterochromatic chromocenters in C. rionegrensis where bivalents converge, while in C. torquatus only one chromocenter was observed. In both species, highly repetitive DNA was observed, localised in chromocenters after “in situ” hybridisation. Heterochromatin associated protein M31 was localised in chromocenters of both species after immuno-detection. The spread of heterochromatin in Ctenomys chromosomes could be produced by chromatin exchanges at the chromocenter level. We propose the exchange of this DNA associated proteins between non-homologous chromosomes in pachytene to be the responsible for the spread of heterochromatin through the karyotypes of species like C. rionegrensis     

The fine structure of chickpea (Cicer arietinum L.) chromosomes as revealed by pachytene analysis

A standard pachytene karyotype of chickpea (Cicer arietinum L.) is presented for the first time. Individual pachytene chromosomes were identified and described in detail. An idiogram was prepared on the basis of chromosome length, arm ratio, and distribution of heterochromatin and euchromatin. Chickpea pachytene chromosomes belong to the differentiated type with darker staining heterochromatin proximal to and lighter staining euchromatin distal to the centromeres. Chromosomes were numbered from 1 to 8 following a descending order of length. The total length of the chromosome complement at pachytene was 335.33 , and chromosome size ranged from 58.05 to 30.53 .     

Constitutive heterochromatin, 5S and 18S rDNA genes in Apareiodon sp. (Characiformes, Parodontidae) with a ZZ/ZW sex chromosome system

Karyotype, sex chromosome system and cytogenetics characteristics of an unidentified species of the genus Apareiodon originating from Piquiri River (Paraná State, Brazil) were investigated using differential staining techniques (C-banding and Ag-staining) and fluorescent in situ hybridization (FISH) with 5S and 18S rDNA probes. The diploid chromosome number was 2n = 54 with 25 pairs of meta- (m) to submetacentric (sm) and 2 pairs of subtelocentric (st) chromosomes. The major ribosomal rDNA sites as revealed by Ag-staining and FISH with 18S rDNA probe were found in distal region of longer arm of st chromosome pair 26, while minor 5S sites were observed in the interstitial sites on chromosome pairs 2 (smaller cluster) and 7 (larger one). The C-positive heterochromatin had pericentromeric and telomeric distribution. The heteromorphic sex chromosome system consisted of male ZZ (pair 21) and female middle-sized m/st Z/W chromosomes. The pericentric inversion of heterochromatinized short arm of ancestral Z followed by multiplication of heterochromatin segments is hypothesized for origin of W chromosome. The observed karyotype and chromosomal markers corresponded to those found in other species of the genus.     

Characterisation of a very complex constitutive heterochromatin in two Gerbillus species (Rodentia)

A large amount of heterochromatin is observed in two species of the genus Gerbillus, G. nigeriae and G. hesperinus. The C-band material represents about one-half of the total karyotype length in the former species, and about one-third in the latter. Several banding techniques and various 5-bromodeoxyuridine (BrdU) treatments were used to characterise these heterochromatic segments. After applying the R-banding technique, three different staining responses of the heterochromatin can be distinguished. In G. nigeriae, strongly stained segments (R-band positive) appear in most chromosomes and, in particular, constitute the short arms of all the larger chromosomes. Palely staining heterochromatic segments (R-band negative) are less abundant in G. nigeriae but predominate in G. hesperinus. In addition, in both species an intermediate staining of heterochromatin is observed near the centromere or in the heterochromatic short arms of some acrocentric and small submetacentric chromosomes. Very short BrdU treatment during the end of the last cell cycle results in asymmetrical staining of chromatids in heterochromatic segments after applying the acridine orange or FPG (fluorescence plus Giemsa) technique. The alternating location of strongly staining segments in one or the other chromatid simulates sister chromatid exchanges (pseudo-SCE). This pattern persists after longer BrdU treatment during different stages of the last cell cycle and is independent of the R-staining properties of the heterochromatin. The lateral asymmetric appearance of the large heterochromatic segments in Gerbillus is interpreted as reflecting an uneven distribution of adenine and thymidine between the two strands of DNA.     

CHROMOSOME NUMBERS IN THE HYPOCREALES 1. NUCLEAR DIVISION IN THE ASCUS OF NECTRIA PEZIZA

El -Ani , Arif S. (Columbia U., New York, N. Y.) Chromosome numbers in the Hypocreales. 1. Nuclear division in the ascus of Nectria peziza. Amer. Jour. Bot. 46(6): 412–417. Illus. 1959.—The 4 nuclear divisions in the ascus of Nectria peziza were studied with the aid of acetoorcein and aceto-carmine techniques. The ascus was found to arise by crozier formation. Synapsis takes place while the chromosomes are still contracted and elongation of chromosomes continues throughout the pachytene phase. The haploid complement was found to consist of 5 chromosomes, the second of which is the nucleolus-organizing chromosome. This chromosome number which had never been reported in the Hypocreales was also found in several other species of this group of fungi. Chromosome numbers in the Hypocreales are discussed with regard to the species concept in the imperfect genus Fusarium.     

Heterogeneity of heterochromatin in six species ofCtenomys (Rodentia: Octodontoidea: Ctenomyidae) from Argentina revealed by a combined analysis of C- and RE-banding

Exceptional chromosomal variability makesCtenomys an excellent model for evolutionary cytogenetic analysis. Six species belonging to three evolutionary lineages were studied by means of restriction endonuclease and C-chromosome banding. The resulting banding patterns were used for comparative analysis of heterochromatin distribution on chromosomes. This combined analysis allowed intra- and inter-specific heterochromatin variability to be detected, groups of species belonging to different lineages to be characterized, and phylogenetic relationships hypothesized from other data to be supported. The “ancestral group”,Ctenomys pundti andC. talarum, share three types of heterochromatin, the most abundant of which was also found in C. aff.C. opimus, suggesting that the latter species also belongs to the “ancestral group”. Additionally, within the subspeciesC. t. talarum, putative chromosomal rearrangements distinguishing two of the three chromosomal races were identified. Two species belong to an “eastern lineage”,C. osvaldoreigi andC. rosendopascuali, and share only one type of heterochromatin homogeneously distributed across their karyotypes.C. latro, the only analyzed species from the “chacoan” lineage, showed three types of heterochromatin, one of them being that which characterizes the “eastern lineage”.C. aff.C. opimus, because of its low heterochromatin content, is the most primitive karyotype of the genus yet described. The heterochromatin variability showed by these species, reflecting the evolutionary divergence toward different heterochromatin types, may have diverged since the origin of the genus. Heterochromatin amplification is proposed as a trend withinCtenomys, occurring independently of chromosomal change in diploid numbers.     

Pachytene variations in Ricinus

The pachytene complement in microsporocytes was compared among ten races of Ricinus communis L., the castor plant (2n=20). Four kinds of pachytene variations were observed: (1) variation in degree of spreading of the chromosomes; (2) variation in degree of attenuation of heterochromatin; (3) variation in morphology, which appeared to be restricted to the two nucleolar organizing bivalents; (4) variation in frequency of association of each of these two bivalents with the nucleolus. It is suggested that variations in the pachytene chromosomes occur ubiquitously in this species.     

Restrictive distribution of asymmetric C-bands in the chromosome complement of the rhesus (Macaca mulatto)

Lymphocyte chromosomes from a cercopithecoid species, Macaca mulatta, were studied for the occurrence of lateral asymmetry in constitutive heterochromatin. The technique consisted of growing the lymphocytes for one cell cycle in BrdUrd, staining with 33258 Hoechst, exposing them to UV light, treating them with 2 SSC and staining with Giemsa. This procedure revealed asymmetric staining in the region of constitutive heterochromatin of the nucleolar organizer marker chromosome (no. 13 of the complement). In these chromosomes, the darkly staining region was confined at any given point to a single chromatid, while the corresponding region on the sister chromatid was lightly stained. This pattern of asymmetric staining in the constitutive heterochromatic region was not observed in any other chromosome of Macaca mulatta. The lateral asymmetry of constitutive heterochromatin in this species is presumed to reflect the strand bias in the distribution of thymine in the alphoid DNA fractions.     

Karyomorphological parameters and C-band distribution suggest phyletic relationships within the subtribeLimodorinae (Orchidaceae)

Karyotype attributes and heterochromatin distribution were used to characterize fourteen taxa of the subtribeLimodorinae (Orchidaceae). The karyotypes were established using morphometrical parameters following Feulgen staining and C-banding. No significant differences in heterochromatin content were found between specimens collected from various sites. Four species of theEpipactis helleborine group possess some chromosome pairs with quite similar heterochromatin patterns; some differences were found inE. distans with respect to other species of this group.Epipactis palustris differed significantly from otherEpipactis species in its different karyotype and its numerous terminal C-bands. The largest differences from the other genera were shown inLimodorum as far as karyomorphology and heterochromatin patterns were concerned. C-band distribution indicated similarity among non-homologous chromosomes, supporting a possible palaeo-polyploid origin for theCephalanthera andEpipactis karyotypes.     

New occurrence of microchromosomes B in Moenkhausia sanctaefilomenae (Pisces, Characidae) from the Paraná River of Brazil: analysis of the synaptonemal complex

The Moenkhausia sanctaefilomenae specimens showed a karyotype consisting of 2n = 50 chromosomes with 12 metacentrics, 36 submetacentrics and two subtelocentrics. In addition to the basic karyotype, all the males specimens have cells ranging from zero to two B microchromosomes in mitotic metaphases. These chromosomes were not observed in the female specimens. C-band analysis showed a distribution pattern of characteristic heterochromatin with interstitial and centromeric blocks. However, the B chromosomes were faintly stained with C-banding and were not fluorescent with CMA₃ staining. The meiotic studies showed the formation of bivalents in metaphase I and in pachytene under an optical microscope. Through synaptonemal complex analysis with an electron microscope, the pachytene showed 25 bivalents completely paired and a small bivalent corresponding to the B chromosomes. In the same preparation, one of the B chromosomes was observed in a univalent form. On the basis of pairing behavior and morphology it is assumed that B chromosomes of M. sanctaefilomenae show homology between them and their evolutionary aspects are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Differential staining of plant chromosomes with Giemsa

Simple Giemsa staining techniques for revealing banding patterns in somatic chromosomes of plants are described. The value of the methods in the recognition of heterochromatin was demonstrated using five monocotyledonous and two dicotyledonous species. In Trillium grandiflorum the stronger Giemsa stained chromosome segments were shown to be identical with the heterochromatic regions (H-segments) revealed by cold treatment. Preferential staining of H-segments was also observed in chromosomes from three species of Fritillaria and in Scilla sibirica. Under suitable conditions the chromosomes of Vicia faba displayed a characteristic banding pattern and the bands were identified as heterochromatin. The Giemsa techniques proved to be more sensitive than Quinacrine fluorescence in revealing a longitudinal differentiation of the chromosomes of Crepis capillaris, where plants with and without B-chromosomes were examined. Again all chromosome types had their characteristic bands but there was no difference in Giemsa staining properties between the B-chromosomes and those of the standard complement.     

Satellite DNA probes of Alstroemeria longistaminea (Alstroemeriaceae) paint the heterochromatin and the B chromosome,reveal a G-like banding pattern,and point to a strong structural karyotype conservation

Alstroemeria species present a well-conserved and asymmetric karyotype. The genus is divided into a Chilean clade, rich in heterochromatin, and a Brazilian clade, poor in heterochromatin. We investigated the distribution of the main repetitive sequences in the chromosomes of the Brazilian species A. longistaminea (2n = 16 + 0-6B) aiming to evaluate the role played by these sequences on the structural organization of the karyotype. In situ hybridization of the three most abundant retrotransposons, corresponding to ~ 45% of the genome, was uniformly distributed. Three satellite DNA sequences, representing near half of the whole satellite fraction (1.93% of the genome), were mainly concentrated on the heterochromatin and one of them painted the whole B chromosome. Noteworthy, some satellites were located on euchromatin, either dispersed or concentrated in clusters along the chromosomes, revealing a G-band-like pattern. The two satellites that presented more C-band- and G-band-like labeling were also hybridized in situ in two other Alstroemeria species. They revealed astonishing similar patterns of distribution, indicating an unusually structural karyotype conservation among Brazilian species.

     

Tritiated uridine induced chromosome aberrations in relation to heterochromatin and nucleolar organisation in Microtus agrestis L.

Microtus agrestis is characterised by long sex chromosomes, most of which are constitutively heterochromatic, and thus supposedly, genetically inactive. A method to assess the template activity of the chromosomes is to study the distribution of chromatid aberrations produced by H³UdR, among and within the chromosomes. In such a study, in female Microtus agrestis cells in culture, it was found that, a large number of localised chromatid aberrations was induced in the constitutively heterochromatic regions of both X chromosomes. The frequency distribution and types of aberrations were found to be cell cycle dependent. With differential staining it has been possible to demonstrate that the constitutive heterochromatin of the sex chromosomes are involved in the nucleolar organisation in this species, thus containing the ribosomal RNA cistrons.     

Chromatin structure and physical mapping of chromosome 6 of potato and comparative analyses with tomato

Potato (Solanum tuberosum) has the densest genetic linkage map and one of the earliest established cytogenetic maps among all plant species. However, there has been limited effort to integrate these maps. Here, we report fluorescence in situ hybridization (FISH) mapping of 30 genetic marker-anchored bacterial artificial chromosome (BAC) clones on the pachytene chromosome 6 of potato. The FISH mapping results allowed us to define the genetic positions of the centromere and the pericentromeric heterochromatin and to relate chromatin structure to the distribution of recombination along the chromosome. A drastic reduction of recombination was associated with the pericentromeric heterochromatin that accounts for ~28% of the physical length of the pachytene chromosome. The pachytene chromosomes 6 of potato and tomato (S. lycopersicum) share a similar morphology. However, distinct differences of heterochromatin distribution were observed between the two chromosomes. FISH mapping of several potato BACs on tomato pachytene chromosome 6 revealed an overall colinearity between the two chromosomes. A chromosome inversion was observed in the euchromatic region of the short arms. These results show that the potato and tomato genomes contain more chromosomal rearrangements than those reported previously on the basis of comparative genetic linkage mapping.     

Chromosome evolution in Australian rodents

The chromosome complements of 188 specimens of 29 species of Australian murid rodents belonging to the subfamilies Pseudomyinae and Hydromyinae and the Uromys/Melomys group have been compared. At least one specimen of 18 different species was successfully C-banded. — The autosomal complements of many (9) diverse Pseudomyinae, one species of Melomys and one Hydromyinae proved to be identical, comprising 48 elements in the diploid set, the two smallest autosomal pairs of which are metacentric. No other karyotype is common to more than one species. From this we conclude that these three groups have been derived from a common ancestor which also possessed such a karyotype. The genus Zyzomys is exceptional since it possesses only 44 elements and lacks the two smallest metacentrics. — Karyotypic evolution within this apparently single phyletic line has been remarkably conservative, only three rearrangements being required to derive the most divergent karyotype. Moreover most of the observed rearrangements involve pericentric inversions and only one example of a fusion was found. Considerable differences in heterochromatin content, as determined by C-banding, occur between species however. Two species proved exceptional in this respect, namely Notomys cervinus and Uromys caudimaculatus. N. cervinus possesses numerous heterochromatic short arms. In U. caudimaculatus, there is a striking difference between northern and southern populations; in the former heterochromatin is present principally in the telomeric areas of the conventional A-chromosomes whereas in the latter it is found as separate supernumerary chromosomes. — In contrast to the autosomes, the X and Y chromosomes show high inter- and intra-specific variability in both size and morphology. All of this variability can be explained in terms of variation in heterochromatin content. Moreover the amount of heterochromatin in the X and Y chromosomes is highly correlated both within and between species. The Y chromosome of Uromys caudimaculatus is, however, distinctive in that it lacks C-banding.     

The meiotic behaviour of autosomal heterochromatic segments in hedgehogs

Male meiosis in the two species of hedgehogs Erinaceus europaeus and Aethechinus algirus, possessing respectively three and two pairs of autosomes with large blocks of heterochromatin, has been studied. The heterochromatic segments pair homologously till the end of pachytene, but separate during diplotene, owing to lack of chiasmata in these regions. They also organize the nucleolus in both species. The sex chromosomes (sex vesicle) are not associated with the nucleolus. The lack of chiasmata in the heterochromatic segments is interpreted as possible mechanism for the conservation of vital genes, such as ribosomal cistrons.     

Relationships within the Melanogaster subgroup species of the genus Drosophila (Sophophora)

Five members of the melanogaster species subgroup of the subgenus Sophophora have been studied cytologically, their mitotic chromosomes analysed after Giemsa, C-banding and quinacrine staining. In all five species (D. yakuba, D. teissieri, D. erecta, D. orena and D. mauritiana) n=4 and all of the species except D. orena have a typical melanogaster like mitotic karyotype though there are clear differences between species in the distribution of both C⁺ and Q⁺ material. D. orena has large metacentric X and Y chromosomes due to the accumulation of intensively fluorescing material on these elements with respect to their homologues in melanogaster. This extra heterochromatin of D. orena correlates with a very high proportion of satellite DNA in its nuclear genome (S. Barnes, unpublished). The polytene chromosomes of these species were studied after quinacrine staining and Q⁺ material found to be restricted to the polytene fourth chromosomes, with the exception of D. orena which possesses considerable Q⁺ material in its chromocentre. These findings are discussed in the light of other studies of karyotype evolution in the genus Drosophila.     

Distribution of constitutive heterochromatin in mammalian chromosomes

Using a special staining technique, a survey of the chromosomes of many mammalian species showed that constitutive heterochromatin is present in all cases and that the heterochromatin pattern appears to be specific and consistent or each chromosome and each taxon. Usually heavy heterochromatin is found in the centromeric areas, but terminal heterochromatin is not uncommon. Occasionally interstitial heterochromatin bands occur. In some species, such as the Syrian hamster and Peromyscus, many chromosome arms are completely heterochromatic.Supported in part by Research Grant GB-13661 from the National Science Foundation.