Metacentric Chromosomes Research Articles

Chromosomal mapping of tandem repeats in the Yesso Scallop, Patinopecten yessoensis (Jay, 1857), utilizing fluorescence in situ hybridization.

Construction of cytogenetic maps can provide important information for chromosome identification, chromosome evolution and genomic research. However, it hasn’t been conducted in many scallop species yet. In the present study, we attempted to map 12 fosmid clones containing tandem repeats by fluorescence in situ hybridization (FISH) in the Yesso scallop Patinopecten yessoensis (Jay, 1857). The results showed 6 fosmid clones were successfully mapped and distributed in 6 different pairs of chromosomes. Three clones were respectively assigned to a pair of metacentric chromosomes, a pair of submetacentric chromosomes and a pair of telocentric chromosomes and the remaining 3 clones showed their loci on three different pairs of subtelocentric chromosomes by co-hybridization. In summary, totally 8 pairs of chromosomes of the Yesso scallop were identified by 6 fosmid clones and two rDNA probes. Furthermore, 6 tandem repeats of 5 clones were sequenced and could be developed as chromosome specific markers for the Yesso scallop. The successful localization of fosmid clones will undoubtedly facilitate the integration of linkage groups with cytogenetic map and genomic research for the Yesso scallop.

Multicolor Spectral Analyses of Mitotic and Meiotic Mouse Chromosomes Involved in Multiple Robertsonian Translocations. I. The CD/Cremona Hybrid Strain

Multicolor spectral analysis (spectral karyotyping) was applied to mitotic and male diakinetic chromosomes of hybrid mice carrying a unique system of 18 autosomal Robertsonian translocation chromosomes with alternating arm homologies. Only the autosomes 19 and the XY sex chromosomes are excluded from these Robertsonian translocations. The translocations, previously identified by conventional banding analyses, could be verified by spectral karyotyping. Besides the Robertsonian translocations, no other interchromosomal rearrangements were detected. In diakineses of male meiosis, the 18 metacentric Robertsonian translocation chromosomes form a very large meiotic ‘superring'. The predictable, specific order of the chromosomes along this ‘superring' was completely confirmed by multicolor spectral analysis. In the majority of diakineses analyzed, the free autosomal bivalent 19 and the XY sex bivalent form a conspicuous complex which tightly associates with the 12;14 Robertsonian translocation chromosome in the ‘superring'.

FISH-mapping of the 5S rDNA locus in chili peppers (Capsicum-Solanaceae).

We present here the physical mapping of the 5S rDNA locus in six wild and five cultivated taxa of Capsicum by means of a genus-specific FISH probe. In all taxa, a single 5S locus per haploid genome that persistently mapped onto the short arm of a unique metacentric chromosome pair at intercalar position, was found. 5S FISH signals of almost the same size and brightness intensity were observed in all the analyzed taxa. This is the first cytological characterization of the 5S in wild taxa of Capsicum by using a genus-derived probe, and the most exhaustive and comprehensive in the chili peppers up to now. The information provided here will aid the cytomolecular characterization of pepper germplasm to evaluate variability and can be instrumental to integrate physical, genetic and genomic maps already generated in the genus.

Karyotype analysis and chromosome banding ofUpeneus moluccensis(Bleeker, 1855) from the north-eastern Mediterranean

Chromosome number, morphology and location of the nucleolus organizer region (NOR) are useful cytological characters for taxonomic and evolutionary studies. The karyotype of Upeneus moluccensis from the north-eastern Mediterranean was investigated through conventional Giemsa staining, Ag-NOR staining, C-banding and GTG-banding techniques. The results showed 2n = 44 (FN = 46), 2m + 2st + 40a chromosomes and four NORs. Two telomeric C-band regions were observed on a metacentric and subtelocentric chromosome pair. On the basis of the chromosome morphology, the GTG and C-banding pattern, it is hypothesized that Upeneus moluccensis is more primitive than the Mediterranean native fish species, Mullus barbatus and Mullus surmuletus. In this study, the cytogenetic characterization of U. moluccensis has been carried out for the first time.

Interstitial telomeric sites and Robertsonian translocations in species of Ipheion and Nothoscordum (Amaryllidaceae).

The genera Nothoscordum and Ipheion (Allioideae, Amaryllidaceae) are cytologically characterized by a dysploid series with variable numbers of metacentric and acrocentric chromosomes typical of karyotypes rearranged by Robertsonian translocations (RT). Since they have large chromosomes, low diploid numbers, and possess two telomeric motifs [the vertebrate-type (TTAGGG) n and the Arabidopsis-type (TTTAGGG) n ] they are suitable for investigating the occurrence and possible role of interstitial telomeric sites (ITS) associated with RT. We analyzed the distributions of telomeric sites in 12 species of Nothoscordum and Ipheion and found that both telomeric probes colocalized in all chromosome termini. Cloning and sequencing PCR products obtained using both telomeric primers simultaneously revealed long stretches of (TTAGGG) n and (TTTAGGG) n sequences together with degenerated telomeric sequences. Most acrocentric chromosomes have a 45S rDNA site at the terminal region of the short arms adjacent to the most distal telomeric sites. Telomeric signals were found at all chromosome ends, but ITS were also detected in a few proximal and subterminal regions in some Nothoscordum species. Although RT are common in this group of plants, our findings suggest that proximal positioning of telomeric motifs are not necessarily related to that kind of rearrangement. Rather, transposition of telomeric sequences followed by amplification, could better explain the presence of (TTAGGG) n and (TTTAGGG) n repeats at those sites. Furthermore, a few small interstitial sites found in some Nothoscordum species indicate that dispersion of these sequences was not restricted to the proximal region.

Evolutionary trends in the family Curimatidae (Characiformes): inferences from chromosome banding

The family Curimatidae is a fish group usually considered chromosomally conserved in their diploid number. However, some studies show small changes in the karyotype microstructure, and the presence of B chromosomes, indicating a chromosomal diversification within the group, even if structural changes in the karyotypes are not visible. Few studies associate this trait with an evolutionary pattern within the family. This study aimed to characterize the karyotype, nucleolus organizer regions (NORs), and heterochromatin distribution of six species of Curimatidae of the genera Cyphocharax Fowler, 1906 and Steindachnerina Fowler, 1906: Cyphocharax voga (Hensel, 1870), Cyphocharax spilotus (Vari, 1987), Cyphocharax saladensis (Meinken, 1933), Cyphocharax modestus (Fernández-Yépez, 1948), Steindachnerina biornata (Braga et Azpelicueta, 1987) and Steindachnerina insculpta (Fernández-Yépez, 1948) and contribute data to a better understanding of the mechanisms involved in the chromosomal evolution of this group of fish. All specimens had 2n=54, m-sm, and B microchromosomes. Five species exhibited single NORs, except for Steindachnerina biornata, which showed a multiple pattern of ribosomal sites. NORs were chromomycin A3 positive (CMA3+) and 4’-6-diamino-2-phenylindole (DAPI-) negative, exhibiting differences in the pair and chromosomal location of each individual of the species. FISH with 5S rDNA probe revealed sites in the pericentrometic position of a pair of chromosomes of five species. However, another site was detected on a metacentric chromosome of Cyphocharax spilotus. Heterochromatin distributed both in the pericentromeric and some terminal regions was revealed to be CMA3+/DAPI-. These data associated with the previously existing ones confirm that, although Curimatidae have a very conservative karyotype macrostructure, NORs and heterochromatin variability are caused by mechanisms of chromosome alterations, such as translocations and/or inversions, leading to the evolution and diversification of this group of fish.

Genomic organization of repetitive DNAs highlights chromosomal evolution in the genus Clarias (Clariidae, Siluriformes).

BackgroundThe genus Clarias (Clariidae, Siluriformes) contains at least 61 species naturally spread over vast regions of Asia, India and Africa. However, Clarias species have also been introduced in many different countries and represent the most widespread catfishes in the world. These fishes are also known as “walking catfishes” due to their ability to move over land. A large degree of chromosomal variation has been previously found in this family, mainly using conventional cytogenetic investigations, with diploid chromosome numbers ranging between 48 and 100. In this study, we analyzed the karyotype structure and distribution of four repetitive DNA sequences (5S and 18S rDNAs and (CA)15 and (GA)15 microsatellites) in three Clarias species (C. batrachus, C. gariepinus, C. macrocephalus), as well as in a probable natural hybrid of the two latter species from different Thailand river basins.ResultsClarias gariepinus and C. macrocephalus had 2n = 56 and 2n = 54, respectively, as well as karyotypes composed mainly by metacentric and submetacentric chromosomes. Their karyotypes differed in the number and location of 5S and 18S rDNA sites and in the degree of microsatellite accumulation. An intermediate chromosomal pattern incorporating those of the parental species was found in the probable hybrid, confirming its interspecific origin. Clarias batrachus had 2n = 104 chromosomes and its karyotype was dominated by mainly acrocentric elements, indicating that unusual multiple centric fissions were involved in its karyotype differentiation. The karyotype of this species presented an unexpected dispersion of ribosomal DNAs, possessing 54 and 12 sites of 5S and 18S rDNAs, respectively, as well as a high accumulation and differential distribution of both microsatellite repeats, representing ‘hot spots’ for chromosomal rearrangement.ConclusionBoth conventional and molecular cytogenetic markers were useful tools for demonstrating remarkable evolutionary dynamism and highlighting multiple chromosomal rearrangements and hybridization events correlated with the notable karyotypic diversity of these walking catfishes.

Karyotype study on 15 populations of Eremostachys laciniata Bunge in Iran

A karyotype study was performed on 15 populations of Eremostachys laciniata Bunge from Iran in order to quantify the extent of cytological variation for use in breeding programmes. Ten mitotic root tip cells at the metaphase stage were prepared from each population by the squash method. The chromosomes of suitable mitotic cells were counted and various parameters, including long and short arm lengths, chromosome lengths, total length of the haploid chromosome complement, arm ratios, and the centromeric index, were measured. All 15 populations were diploid with 2n = 2x = 22 and were differentiated by their karyotypic parameters. All 15 E. laciniata populations occupied Class 1A of Stebbins’ karyotype classification, indicating the presence of a primitive symmetrical karyotype. The size of the mitotic chromosomes was medium, and mean chromosome lengths ranged from 3.84 to 4.77 μm. The shortest chromosome lengths were observed in the Ajabshir population and the longest in the Heydarabad population. The Heydarabad population also had the longest total chromosome length and the Ajabshir population had the shortest total chromosome length. The centromeric indices of the chromosome complements varied from 39.3% to 43.7%. Metacentric chromosomes were the most common in all populations, whereas submetacentric chromosomes were rare. The A1 index varied from 0.215 (Areshtanab) to 0.337 (Til). The A2 index ranged from 0.116 (Til) to 0.143 (Kaleybar), and the karyotype asymmetry index varied between 0.051 and 0.097. The results of a principal component analysis (PCA) showed that the main elements of the first three main principal components accounted for 97% of the total variance. Cluster analysis by Ward’s method indicated that all 15 populations could be classified into three clusters. Finally, the results of the karyotypic parameters showed that the Til, Heydarabad, and Marand populations had higher levels of karyotypic heterogeneity and may be useful when developing breeding programmes.

Caracterização citogenética de Encyclia caximboensis (Orchidaceae) cultivadas in vitro

<em>E</em><em>ncyclia caximboensis</em><span> is an Amazonian species endemic to the Serra do Cachimbo, which is located between the northern of the Mato Grosso state and the southern part of Para state. Studies reporting in vitro cultivation and cytogenetic characterization of this species are still scarce. Therefore, the objective of this work was to determine the cytogenetic characteristics and to identify the nucleolar organizer regions (NORs) of the species </span><em>E. Caximboensis</em><span>, cultivated </span><em>in vitro.</em><span> Seeds of </span><em>E. caximboensis</em><span> were disinfected using a syringe and subsequently cultivated in MS medium without growth regulators. The germination started after 20 days of culture, with the development of protocorm and 40,500 seedlings were obtained after 90 days of culture. To perform the cytogenetic characterization, root tips of 180-day-old seedlings were submitted to blocking treatment using 3 µM trifiuralin and then fixed in methanol-acetic acid solution, 3:1 (v/v). The meristems were submitted to enzymatic digestion, fixed in methanol-acetic acid solution, 3:1 (v/v) and the slides were stained using 5% Giemsa solution. Ag-NOR banding was carried out on 20-day-old slides by incubation in 50% silver nitrate solution (AgNO3) for 19 hours. The results indicated that </span><em>E. caximboensis</em><span> has 2n=2x=24 with all metacentric type chromosomes, ranging from 1.88 to 0.66 pm in length, with simple NORs in small blocks localized in the proximal region of the third chromosome pair. </span>

Chromosomal mapping of H3 histone and 5S rRNA genes in eight species of Astyanax (Pisces, Characiformes) with different diploid numbers: syntenic conservation of repetitive genes.

The genus Astyanax is widely distributed from the southern United States to northern Patagonia, Argentina. While cytogenetic studies have been performed for this genus, little is known about the histone gene families. The aim of this study was to examine the chromosomal relationships among the different species of Astyanax. The chromosomal locations of the 5S rRNA and H3 histone genes were determined in A. abramis, A. asuncionensis, A. altiparanae, A. bockmanni, A. eigenmanniorum, A. mexicanus (all 2n = 50), A. fasciatus (2n = 46), and A. schubarti (2n = 36). All eight species exhibited H3 histone clusters on two chromosome pairs. In six species (A. abramis, A. asuncionensis, A. altiparanae, A. bockmanni, A. eigenmanniorum, and A. fasciatus), syntenic clusters of H3 histone and 5S rDNA were observed on metacentric (m) or submetacentric (sm) chromosomes. In seven species, clusters of 5S rDNA sequences were located on one or two chromosome pairs. In A. mexicanus, 5S rDNA clusters were located on four chromosome pairs. This study demonstrates that H3 histone clusters are conserved on two chromosome pairs in the genus Astyanax, and specific chromosomal features may contribute to the genomic organization of the H3 histone and 5S rRNA genes.

Reassignment of Drosophila willistoni Genome Scaffolds to Chromosome II Arms.

Drosophila willistoni is a geographically widespread Neotropical species. The genome of strain Gd-H4-1 from Guadeloupe Island (Caribbean) was sequenced in 2007 as part of the 12 Drosophila Genomes Project. The assembled scaffolds were joined based on conserved linkage and assigned to polytene chromosomes based on a handful of genetic and physical markers. This paucity of markers was particularly striking in the metacentric chromosome II, comprised two similarly sized arms, IIL and IIR, traditionally considered homologous to Muller elements C and B, respectively. In this paper we present the cytological mapping of 22 new gene markers to increase the number of markers mapped by in situ hybridization and to test the assignment of scaffolds to the polytene chromosome II arms. For this purpose, we generated, by polymerase chain reaction amplification, one or two gene probes from each scaffold assigned to the chromosome II arms and mapped these probes to the Gd-H4-1 strain’s polytene chromosomes by nonfluorescent in situ hybridization. Our findings show that chromosome arms IIL and IIR correspond to Muller elements B and C, respectively, directly contrasting the current homology assignments in D. willistoni and constituting a major reassignment of the scaffolds to chromosome II arms.

Karyotype differentiation in 19 species of river loach fishes (Nemacheilidae, Teleostei): extensive variability associated with rDNA and heterochromatin distribution and its phylogenetic and ecological interpretation.

BackgroundLoaches of the family Nemacheilidae are one of the most speciose elements of Palearctic freshwater ichthyofauna and have undergone rapid ecological adaptations and colonizations. Their cytotaxonomy is largely unexplored; with the impact of cytogenetical changes on this evolutionary diversification still unknown. An extensive cytogenetical survey was performed in 19 nemacheilid species using both conventional (Giemsa staining, C- banding, Ag- and Chromomycin A3/DAPI stainings) and molecular (fluorescence in situ hybridization with 5S rDNA, 45S rDNA, and telomeric (TTAGGG)n probes) methods. A phylogenetic tree of the analysed specimens was constructed based on one mitochondrial (cytochrome b) and two nuclear (RAG1, IRBP) genes.ResultsSeventeen species showed karyotypes composed of 2n = 50 chromosomes but differentiated by fundamental chromosome number (NF = 68–90). Nemachilichthys ruppelli (2n = 38) and Schistura notostigma (2n = 44–48) displayed reduced 2n with an elevated number of large metacentric chromosomes. Only Schistura fasciolata showed morphologically differentiated sex chromosomes with a multiple system of the XY1Y2 type. Chromomycin A3 (CMA3)- fluorescence revealed interspecific heterogeneity in the distribution of GC-rich heterochromatin including its otherwise very rare association with 5S rDNA sites. The 45S rDNA sites were mostly located on a single chromosome pair contrasting markedly with a pattern of two (Barbatula barbatula, Nemacheilus binotatus, N. ruppelli) to 20 sites (Physoschistura sp.) of 5S rDNA. The cytogenetic changes did not follow the phylogenetic relationships between the samples. A high number of 5S rDNA sites was present in species with small effective population sizes.ConclusionDespite a prevailing conservatism of 2n, Nemacheilidae exhibited a remarkable cytogenetic variability on microstructural level. We suggest an important role for pericentric inversions, tandem and centric fusions in nemacheilid karyotype differentiation. Short repetitive sequences, genetic drift, founder effect, as well as the involvement of transposable elements in the dispersion of ribosomal DNA sites, might also have played a role in evolutionary processes such as reproductive isolation. These remarkable dynamics of their genomes qualify river loaches as a model for the study of the cytogenetic background of major evolutionary processes such as radiation, endemism and colonization of a wide range of habitats.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0532-9) contains supplementary material, which is available to authorized users.

Linkage Mapping Reveals Strong Chiasma Interference in Sockeye Salmon: Implications for Interpreting Genomic Data.

Meiotic recombination is fundamental for generating new genetic variation and for securing proper disjunction. Further, recombination plays an essential role during the rediploidization process of polyploid-origin genomes because crossovers between pairs of homeologous chromosomes retain duplicated regions. A better understanding of how recombination affects genome evolution is crucial for interpreting genomic data; unfortunately, current knowledge mainly originates from a few model species. Salmonid fishes provide a valuable system for studying the effects of recombination in nonmodel species. Salmonid females generally produce thousands of embryos, providing large families for conducting inheritance studies. Further, salmonid genomes are currently rediploidizing after a whole genome duplication and can serve as models for studying the role of homeologous crossovers on genome evolution. Here, we present a detailed interrogation of recombination patterns in sockeye salmon (Oncorhynchus nerka). First, we use RAD sequencing of haploid and diploid gynogenetic families to construct a dense linkage map that includes paralogous loci and location of centromeres. We find a nonrandom distribution of paralogs that mainly cluster in extended regions distally located on 11 different chromosomes, consistent with ongoing homeologous recombination in these regions. We also estimate the strength of interference across each chromosome; results reveal strong interference and crossovers are mostly limited to one per arm. Interference was further shown to continue across centromeres, but metacentric chromosomes generally had at least one crossover on each arm. We discuss the relevance of these findings for both mapping and population genomic studies.

Karyotypic characterization of the pike, Esox lucius from the south Caspian Sea basin

The karyotype of pike from Anzali lagoon in the south Caspian Sea basin have been investigated by examining metaphase chromosomes spreads obtained from gill epithelial and kidney cells. The chromosomes spreads were stained with 7% Giemsa solution for 15 min and examined under a light microscope. Appropriate metaphase plates were photographed in order to prepare karyotype. All specimens had a diploid number of 50 chromosomes, with a karyotype constituted by 12 metacentric‏, 24 submetacentric, 14 subtelocentric and fundamental number (NF) of 86. The largest chromosome in this species was a pair of metacentric chromosomes. Based on type of chromosomes, the karyotype of the species differed from the findings of previous studies, which could be attributed to the existence of different populations for this species. Despite the conserved diploid number, the data on the karyotype structure help characterize the cytogenetic profile of this species.

First description of the karyotype of a eucharitid wasp (Hymenoptera, Chalcidoidea, Eucharitidae).

The haploid karyotype of Kapala sp. (Eucharitidae), a parasite of the Neotropical ant Dinoponera lucida Emery, 1901 (Hymenoptera, Formicidae), is reported for the first time. It consists of four metacentric chromosomes. Chromosomes in the family Eucharitidae were unknown so far; therefore, our results confirm that multiple parallel chromosomal fusions have taken place in several lineages within the superfamily Chalcidoidea.

CHARACTERIZATION OF CHROMOSOME OF STRIPPED KEELBACK Xenochrophis vittattus (Linnaeus, 1758) FROM PIYUNGAN, BANTUL, DAERAH ISTIMEWA YOGYAKARTA

<p>Indonesia is a tropical country which has varies of habitat types that are dispersed into thousands islands. The diversity and its tropical climate, makes Indonesia as a suitable place as a natural habitat for herpetofauna, especially Sub Order Serpentes. Xenochrophis vittattus (Linnaeus, 1578), called stripped keelback snakes, is a semi-aquatic snakes spread in Indonesia, especially in Java, Sumatra, Bangka, and Manado, and commonly used as pet. Since there are few reports on genetic studies, especially in the field of karyology, this study aims to characterize the chromosomes of stripped keelback from Piyungan population, Bantul, Yogyakarta. This study used a brief splash of blood cultures method. This has been known as the first stripped keelback cytological study in Indonesia. The results showed that striped keelback has a diploid chromosome (2n) = 34, consists of metacentric chromosome (number 1, 2, 3, 4, 19, 20, 21, 22, 23, 24, 25, and 26); submetacentric chromosome (number 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18). Whereas the chromosome number 27, 28, 29, 30, 31, 32, 33, and 34 were micro chromosomes. Striped keelback karyotype formula was 2n = 2x = 34 = 12 m 14 sm 8 micro chromosomes. The longest chromosome of stripped keelback was 2.9335 ± 0.1772 um, whereas the shortest one was 0.5088 ± 0.013 um, and the R value was 5.9106 ± 1.1265.</p><p><br /><strong>Keywords</strong>: Xenochrophis vittattus, Stripped keelback, chromosome characterization, karyotype</p>

Cytogenetic data on the threatened leafcutter ant Atta robusta Borgmeier, 1939 (Formicidae: Myrmicinae: Attini)

The karyotype of the threatened ant species Atta robusta is described so as to establish the evolutionary relationships of this taxon with other leafcutter ants. Standard Giemsa staining, C-banding, NOR banding, fluorochromes CMA3/DAPI, Hsc-FA technique and Fluorescence in situ Hybridization (FISH) using 18S rDNA probe were conducted on a population from Aracruz, state of Espírito Santo, Brazil, allowing for comparisons with data available on Atta and other fungus-growing ant species. The diploid chromosome number observed for A. robusta was 2n=22, and the karyotypic formula was 18m+2sm+2st. Heterochromatic blocks were observed in the centromeric region of most chromosomes, where one pair of metacentric chromosomes is characterized by a GC-rich heterochromatic band in the interstitial region of its long arm. The detection of 18S rDNA using FISH confirmed the presence of single NOR for A. robusta. This is the first report of rDNA 18S detection using FISH for leafcutter ants. The cytogenetic results of this study confirm the information available for Atta and allow us to confirm the conserved chromosome number, morphology and banding pattern within the genus for the taxa studied to date, which included species from three out of the four groups of Atta indicated by molecular data. The accumulation of cytogenetic data on fungus-growing ants enhances the understanding of the genomic evolutionary patterns of Atta, since it belongs to a group of recent origin between the most well studied ants. Cytogenetic data does not indicate restrictions in relocation or reintroduction in areas where populations were extinct due to the conserved karyotype. This study allows for cytogenetic comparison of A. robusta with other ants of Atta, emphasizing the importance of chromosomal information for species conservation.

Contributions to cytogenetics of Plectranthusbarbatus Andr. (Lamiaceae): a medicinal plant.

Accessions of Plectranthus barbatus (Lamiaceae), a medicinal plant, were investigated using a cytogenetic approach and flow cytometry (FCM). Here, we describe for the first time details of the karyotype including chromosome morphology, physical mapping of GC rich bands (CMA3 banding), as well as the mapping of 45S and 5S rDNA sites. All accessions studied showed karyotypes with 2n = 30 small metacentric and submetacentric chromosomes. The CMA3 banding and fluorescent in situ hybridization techniques revealed coincidence between CMA3 bands and 45S rDNA sites (6 terminal marks) while for the 5S rDNA were observed 4 subterminal marks no coincident with CMA3 marks. For nuclear genome size measurement, the FCM procedure provided histograms with G0/G1 peaks exhibiting CV between 2.0–4.9 and the mean values obtained for the species was 2C = 2.78 pg, with AT% = 61.08 and GC% = 38.92. The cytogenetic data obtained here present new and important information which enables the characterization of Plectranthus barbatus.

Robertsonian translocation in the house mouse: a tale of chromosomal speciation

The subspecies Mus musculus domesticus has a very high chromosome number variability for the presence, in different populations, of different numbers of metacentric chromosomes. These metacentrics are derived by Robertsonian fusion, i.e. the joining of two acro/telocentrics at the centromeres. Since the discovery in 1969 by Alfred Gropp of the first chromosomal variant in an isolated mouse population of the Swiss Poschiavo Valley, more than 100 geographically distinct chromosomal races with metacentrics fixed in homozygosity have been described. The friendship and the common scientific interests among Alfred Gropp, Ernesto Capanna and Maria Gabriella Manfredi Romanini have primed a wealth of studies on the “Robertsonian phenomenon” in this species. These studies have contributed to elucidate i) the molecular bases of metacentric chromosomes formation, ii) the establishment of metacentric races and iii) the impact that chromosome heterozygosities exert on reproductive isolation and speciation.

The ultrastructure of mono- and holocentric plant centromeres: an immunological investigation by structured illumination microscopy and scanning electron microscopy.

The spatial distribution of the three centromere-associated proteins α-tubulin, CENH3, and phosphorylated histone H2A (at threonine 120, H2AThr120ph) was analysed by indirect immunodetection at monocentric cereal chromosomes and at the holocentric chromosomes of Luzula elegans by super-resolution light microscopy and scanning electron microscopy (SEM). Using structured illumination microscopy (SIM) as the super-resolution technique on squashed specimens and SEM on uncoated isolated specimens, the three-dimensional (3D) distribution of the proteins was visualized at the centromeres. Technical aspects of 3D SEM are explained in detail. We show that CENH3 forms curved "pads" mainly around the lateral centromeric region in the primary constriction of metacentric chromosomes. H2AThr120ph is present in both the primary constriction and in the pericentromere. α-tubulin-labeled microtubule bundles attach to CENH3-containing chromatin structures, either in single bundles with a V-shaped attachment to the centromere or in split bundles to bordering pericentromeric flanks. In holocentric L. elegans chromosomes, H2AThr120ph is located predominantly in the centromeric groove of each chromatid as proven by subsequent FIB/FESEM ablation and 3D reconstruction. α-tubulin localizes to the edges of the groove. In both holocentric and monocentric chromosomes, no additional intermediate structures between microtubules and the centromere were observed. We established models of the distribution of CENH3, H2AThr120ph and the attachment sites of microtubules for metacentric and holocentric plant chromosomes.