Chromosome Evolution in the Genus Partamona (Apidae: Meliponini), with Comments on B Chromosome Origin

The genus <i>Partamona</i> includes 33 species of stingless bees, of which 11 were studied cytogenetically. The main goal of this study was to propose a hypothesis about chromosomal evolution in <i>Partamona</i> by combining molecular and cytogenetic data. Cytogenetic analyses were performed on 3 <i>Partamona</i> species. In addition, the molecular phylogeny included mitochondrial sequences of 11 species. Although the diploid number was constant within the genus, 2n = 34, B chromosomes were reported in 7 species. Cytogenetic data showed karyotypic variations related to chromosome morphology and the amount and distribution of heterochromatin and repetitive DNA. The molecular phylogenetic reconstruction corroborated the monophyly of the genus and separated the 2 clades (A and B). This separation was also observed in the cytogenetic data, in which species within each clade shared most of the cytogenetic characteristics. Furthermore, our data suggested that the B chromosome in the genus <i>Partamona</i> likely originated from a common ancestor of the species that have it in clade B and, through interspecific hybridization, it appeared only in <i>Partamona rustica</i> from clade A. Based on the above, <i>Partamona</i> is an interesting genus for further investigations using molecular mapping of B chromosomes as well as for broadening phylogenetic data.

Chromosome morphology and cytomolecular characteristics of the perennial rye cultivar ‘Kriszta’

The perennial Secale cereanum cultivar ‘Kriszta’ is an artificial hybrid of S. cereale and S. strictum ssp. anatolicum. From the cross between the wheat line Mv9kr1 and ‘Kriszta’, which aimed the transfer of beneficial traits from rye to wheat, numerous translocation lines have been produced. For the identification of the translocated chromosomes, the unambiguous differentiation between chromosome arms of ‘Kriszta’ is essential. The identification of its short chromosome arms using conventional FISH probes is easy, but because of their similar hybridization patterns, its long arms cannot be distinguished. The present study aimed to create the detailed karyotype of ‘Kriszta’, especially that of long arms, by both chromosome measurements and FISH using highly repetitive, as well as subtelomeric tandem repeat, and synthetic microsatellite DNA sequences. Our results indicate that the chromosome complement of ‘Kriszta’ is not a simple combination of the chromosomes of the parental rye species but is composed of rearranged chromosomes. It is also showed that an adequate pair-wise combination of the DNA sequences pSc119.2, pSc200, pSc250, and (AAC)5 makes it possible to identify any of the long arms of S. cereanum cv. Kriszta chromosomes. The future usability of the identified wheat- ‘Kriszta’ translocated chromosomes is also discussed.

Comparison of Karyotypes in Two Hybridizing Passerine Species: Conserved Chromosomal Structure but Divergence in Centromeric Repeats

Changes in chromosomal structure involving chromosomal rearrangements or copy number variation of specific sequences can play an important role in speciation. Here, we explored the chromosomal structure of two hybridizing passerine species; the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia), using conventional cytogenetic approaches, immunostaining of meiotic chromosomes, fluorescence in situ hybridization as well as comparative genomic hybridization (CGH). We found that the two nightingale species show conserved karyotypes with the same diploid chromosome number of 2n = 84. In addition to standard chromosomes, both species possessed a small germline restricted chromosome of similar size as a microchromosome. Just a few subtle changes in chromosome morphology were observed between the species, suggesting that only a limited number of chromosomal rearrangements occurred after the species divergence. The interspecific CGH experiment suggested that the two nightingale species might have diverged in centromeric repetitive sequences in most macro- and microchromosomes. In addition, some chromosomes showed changes in copy number of centromeric repeats between the species. The observation of very similar karyotypes in the two nightingale species is consistent with a generally slow rate of karyotype evolution in birds. The divergence of centromeric sequences between the two species could theoretically cause meiotic drive or reduced fertility in interspecific hybrids. Nevertheless, further studies are needed to evaluate the potential role of chromosomal structural variations in nightingale speciation.

FBXO47 is essential for preventing the synaptonemal complex from premature disassembly in mouse male meiosis

Meiotic prophase is a prolonged G2 phase that ensures the completion of numerous meiosis-specific chromosome events. During meiotic prophase, homologous chromosomes undergo synapsis to facilitate meiotic recombination yielding crossovers. It remains largely elusive how homolog synapsis is temporally maintained and destabilized during meiotic prophase. Here we show that FBXO47 is the stabilizer of synaptonemal complex during male meiotic prophase. Disruption of FBXO47 shows severe impact on homologous chromosome synapsis and DSB repair processes, leading to male infertility. Notably, in the absence of FBXO47, although once homologous chromosomes are synapsed, the synaptonemal complex is precociously disassembled before progressing beyond pachytene. Remarkably, Fbxo47 KO spermatocytes remain in earlier stage of meiotic prophase and lack crossovers, despite apparently exhibiting diplotene-like chromosome morphology. We propose that FBXO47 functions independently of SCF E3 ligase, and plays a crucial role in preventing synaptonemal complex from premature disassembly during cell cycle progression of meiotic prophase.

Chromosomes, Nuclear Genes and the Phylogenetic Placement Within the Reptilia of Sphenodon (Tuatara)

<p>Chromosomes were examined from five populations of Sphenodon (tuatara) using giemsa, Ag-NOR, C-, G- and RE- banding. There were no differences between species, populations or sexes, although one animal had a structural heteromorphism. Chromosome morphology homology to Testudines (turtles), Aves (birds) and to a lesser extent Crocodylia (crocodiles) allowed reconstruction ofa Reptilian proto-karyotype, dated to 300 million years ago. DNA sequence was isolated from the WT1, AMH, DMRT1, FoxG1 and 28S. No variation was present in Sphenodon 28S, FoxG1 or AMH sequence. 28S could be dated to a common ancestor with Testudines, similar to the archaic karyotype. FoxG1 and AMH reflect an Oligocene divergence, WT1 divides north-eastern North Island and Cook Strait, and can be dated to the Pleistocene or the Pliocene, and DMRT1appears a recent post- pliocene divergence. FISH localised DIG-labelled probes of AMH to chromosome 11 and WT1 to chromosome 13 or 14. Human telomeric probes localised to Sphenodon telomeric regions demonstrating the highly conserved nature of telomeric sequences. Comparative genomic hybridisation with chicken chromosomes did not produce any regions of homology, implying significant chromosomal and DNA changes since the Orders shared a common ancestor, although macrochromosome morphology has remained similar. Sphenodon chromosomal and nuclear DNA analyses demonstrate evolutionary decoupling, supporting recent mtDNA work.</p>

Chromosomes, Nuclear Genes and the Phylogenetic Placement Within the Reptilia of Sphenodon (Tuatara)

<p>Chromosomes were examined from five populations of Sphenodon (tuatara) using giemsa, Ag-NOR, C-, G- and RE- banding. There were no differences between species, populations or sexes, although one animal had a structural heteromorphism. Chromosome morphology homology to Testudines (turtles), Aves (birds) and to a lesser extent Crocodylia (crocodiles) allowed reconstruction ofa Reptilian proto-karyotype, dated to 300 million years ago. DNA sequence was isolated from the WT1, AMH, DMRT1, FoxG1 and 28S. No variation was present in Sphenodon 28S, FoxG1 or AMH sequence. 28S could be dated to a common ancestor with Testudines, similar to the archaic karyotype. FoxG1 and AMH reflect an Oligocene divergence, WT1 divides north-eastern North Island and Cook Strait, and can be dated to the Pleistocene or the Pliocene, and DMRT1appears a recent post- pliocene divergence. FISH localised DIG-labelled probes of AMH to chromosome 11 and WT1 to chromosome 13 or 14. Human telomeric probes localised to Sphenodon telomeric regions demonstrating the highly conserved nature of telomeric sequences. Comparative genomic hybridisation with chicken chromosomes did not produce any regions of homology, implying significant chromosomal and DNA changes since the Orders shared a common ancestor, although macrochromosome morphology has remained similar. Sphenodon chromosomal and nuclear DNA analyses demonstrate evolutionary decoupling, supporting recent mtDNA work.</p>

Molecular cytogenetics of Hedysarum gmelinii Ledeb. and H. setigerum Turcz. (Fabaceae) from populations of Southern Siberia

For the first time, a comparative karyotype analysis of closely related species Hedysarum gmelinii andH. setigerum (Hedysarum section Multicaulia) grown in Southern Siberia, has been performed by molecular cytogeneticmarkers. Chromosome numbers in karyotypes of these species were specified – 2n = 4х = 32. In some accessions, additionalB chromosomes were revealed. FISH analyses indicated high similarities in chromosome morphology and also patternsof chromosomal distributions of 45S and 5S rDNA clusters in karyotypes of H. gmelinii and H. setigerum, which confirmsthe close relationship between their genomes.

Karyotype structure and cytogenetic markers of Amoimyrmex bruchi and Amoimyrmex silvestrii: the contribution to understanding leafcutting ant relationships

Leafcutting ants are considered the most important herbivores in terrestrial environments throughout the Neotropics. <i>Amoimyrmex</i> is the sister clade of the remaining leafcutter ants from the genera <i>Atta</i> and <i>Acromyrmex</i>. <i>Amoimyrmex striatus</i> was the only species cytogenetically studied within the genus and shares the same chromosomal number with <i>Atta</i>, bearing 22 chromosomes, whereas <i>Acromyrmex</i> bears 38 chromosomes, with the exception of the social parasite <i>Acromyrmex ameliae</i> (2n=36). Our objective here was to analyze cytogenetically the species of <i>Amoimyrmex bruchi</i> and <i>Amoimyrmex silvestrii</i>, as well as to describe the karyotype of these sister species, by means of an integrative approach using classical and molecular cytogenetics. We aimed to characterize cytogenetic markers that contribute to the systematics and taxonomy of the genus. Our results showed that the karyotypes of these two species are very similar, with an identical chromosome number (2n=22), chromosome morphology (2K=20m+2sm), and location of 18S rDNA and the telomeric repeat TTAGG on the chromosomes. Yet, the microsatellite probe GA(15) showed variation across the species and populations studied. We suggest that both species diverged relatively recently and are unmistakably sisters because of the many shared characteristics, including the highly conserved karyotypes.

Linker histone H1.8 inhibits chromatin-binding of condensins and DNA topoisomerase II to tune chromosome length and individualization

DNA loop extrusion by condensins and decatenation by DNA topoisomerase II (topo II) are thought to drive mitotic chromosome compaction and individualization. Here, we reveal that the linker histone H1.8 antagonizes condensins and topo II to shape mitotic chromosome organization. In vitro chromatin reconstitution experiments demonstrate that H1.8 inhibits binding of condensins and topo II to nucleosome arrays. Accordingly, H1.8 depletion in Xenopus egg extracts increased condensins and topo II levels on mitotic chromatin. Chromosome morphology and Hi-C analyses suggest that H1.8 depletion makes chromosomes thinner and longer through shortening the average loop size and reducing the DNA amount in each layer of mitotic loops. Furthermore, excess loading of condensins and topo II to chromosomes by H1.8 depletion causes hyper-chromosome individualization and dispersion. We propose that condensins and topo II are essential for chromosome individualization, but their functions are tuned by the linker histone to keep chromosomes together until anaphase.

Cytogenetically Elusive Sex Chromosomes in Scincoidean Lizards

The lizards of the species-rich clade Scincoidea including cordylids, gerrhosaurids, skinks, and xantusiids, show an almost cosmopolitan geographical distribution and a remarkable ecological and morphological divergence. However, previous studies revealed limited variability in cytogenetic traits. The sex determination mode was revealed only in a handful of gerrhosaurid, skink, and xantusiid species, which demonstrated either ZZ/ZW or XX/XY sex chromosomes. In this study, we explored the karyotypes of six species of skinks, two species of cordylids, and one gerrhosaurid. We applied conventional and molecular cytogenetic methods, including C-banding, fluorescence in situ hybridization with probes specific for telomeric motifs and rDNA loci, and comparative genomic hybridization. The diploid chromosome numbers are rather conserved among these species, but the chromosome morphology, the presence of interstitial telomeric sequences, and the topology of rDNA loci vary significantly. Notably, XX/XY sex chromosomes were identified only in Tiliqua scincoides, where, in contrast to the X chromosome, the Y chromosome lacks accumulations of rDNA loci. We confirm that within the lizards of the scincoidean clade, sex chromosomes remained in a generally poor stage of differentiation.