New Data on Comparative Cytogenetics of the Mouse-Like Hamsters (Calomyscus Thomas, 1905) from Iran and Turkmenistan

The taxonomy of the genus Calomyscus remains controversial. According to the latest systematics the genus includes eight species with great karyotypic variation. Here, we studied karyotypes of 14 Calomyscus individuals from different regions of Iran and Turkmenistan using a new set of chromosome painting probes from a Calomyscus sp. male (2n = 46, XY; Shahr-e-Kord-Soreshjan-Cheshme Maiak Province). We showed the retention of large syntenic blocks in karyotypes of individuals with identical chromosome numbers. The only rearrangement (fusion 2/21) differentiated C. elburzensis, C. mystax mystax, and Calomyscus sp. from Isfahan Province with 2n = 44 from karyotypes of C. bailwardi, Calomyscus sp. from Shahr-e-Kord, Chahar Mahal and Bakhtiari-Aloni, and Khuzestan-Izeh Provinces with 2n = 46. The individuals from Shahdad tunnel, Kerman Province with 2n = 51–52 demonstrated non-centric fissions of chromosomes 4, 5, and 6 of the 46-chromosomal form with the formation of separate small acrocentrics. A heteromorphic pair of chromosomes in a specimen with 2n = 51 resulted from a fusion of two autosomes. C-banding and chromomycin A3-DAPI staining after G-banding showed extensive heterochromatin variation between individuals.

Chromosomal Instability at Fragile Sites in Blue Foxes, Silver Foxes, and Their Interspecific Hybrids

A cytogenetic assay based on fragile sites (FS) enables the identification of breaks, chromatid gaps, and deletions. In healthy individuals, the number of these instabilities remains low. Genome stability in these species is affected by Robertsonian translocations in the karyotype of the blue fox and by B chromosomes in the silver fox. The aims of the study were to characterise the karyotype of blue foxes, silver foxes, and their hybrids and to identify chromosomal fragile sites used to evaluate genome stability. The diploid number of A chromosomes in blue foxes ranged from 48 to 50, while the number of B chromosomes in silver foxes varied from one to four, with a constant number of A chromosomes (2n = 34). In interspecific hybrids, both types of karyotypic variation were identified, with the diploid number of A chromosomes ranging from 40 to 44 and the number of B chromosomes varying from 0 to 3. The mean frequency of FS in foxes was 4.06 ± 0.19: 4.61 ± 0.37 in blue foxes, 3.46 ± 0.28 in silver foxes, and 4.12 ± 0.22 in hybrids. A relationship was identified between an increased number of A chromosomes in the karyotype of the hybrids and the frequency of chromosomal breaks. The FS assay was used as a biomarker for the evaluation of genomic stability in the animals in the study.

Chromosomal evolution in Raphicerus antelope suggests divergent X chromosomes may drive speciation through females, rather than males, contrary to Haldane's rule

Chromosome structural change has long been considered important in the evolution of post-zygotic reproductive isolation. The premise that karyotypic variation can serve as a possible barrier to gene flow is founded on the expectation that heterozygotes for structurally distinct chromosomal forms would be partially sterile (negatively heterotic) or show reduced recombination. We report the outcome of a detailed comparative molecular cytogenetic study of three antelope species, genus Raphicerus, that have undergone a rapid radiation. The species are largely conserved with respect to their euchromatic regions but the X chromosomes, in marked contrast, show distinct patterns of heterochromatic amplification and localization of repeats that have occurred independently in each lineage. We argue a novel hypothesis that postulates that the expansion of heterochromatic blocks in the homogametic sex can, with certain conditions, contribute to post-zygotic isolation. i.e., female hybrid incompatibility, the converse of Haldane’s rule. This is based on the expectation that hybrids incur a selective disadvantage due to impaired meiosis resulting from the meiotic checkpoint network’s surveillance of the asymmetric expansions of heterochromatic blocks in the homogametic sex. Asynapsis of these heterochromatic regions would result in meiotic silencing of unsynapsed chromatin and, if this persists, germline apoptosis and female infertility.

Multiple Sex Chromosomes and Evolutionary Relationships in Amazonian Catfishes: The Outstanding Model of the Genus Harttia (Siluriformes: Loricariidae)

The armored Harttia catfishes present great species diversity and remarkable cytogenetic variation, including different sex chromosome systems. Here we analyzed three new species, H. duriventris, H. villasboas and H. rondoni, using both conventional and molecular cytogenetic techniques (Giemsa-staining and C-banding), including the mapping of repetitive DNAs using fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) experiments. Both H. duriventris and H. villasboas have 2n = ♀56/♂55 chromosomes, and an X1X1X2X2 /X1X2Y sex chromosome system, while a proto or neo-XY system is proposed for H. rondoni (2n = 54♀♂). Single motifs of 5S and 18S rDNA occur in all three species, with the latter being also mapped in the sex chromosomes. The results confirm the general evolutionary trend that has been noticed for the genus: an extensive variation on their chromosome number, single sites of rDNA sequences and the occurrence of multiple sex chromosomes. Comparative genomic analyses with another congeneric species, H. punctata, reveal that the X1X2Y sex chromosomes of these species share the genomic contents, indicating a probable common origin. The remarkable karyotypic variation, including sex chromosomes systems, makes Harttia a suitable model for evolutionary studies focusing on karyotype differentiation and sex chromosome evolution among lower vertebrates.

Distribution and characterization of Aegilops cylindrica host from Iran

Jointed goatgrass (Aegilops cylindrica Host; 2n = 4x = 28, CcCcDcDc) is a tetraploid remote relative of bread wheat (Triticum aestivum L; 2n=6x=42, AABBDD) with two genomes and 28 chromosomes. The diversity center of this species is in the Fertile Crescent and in central Asia and could also be found in many places in Iran. In this experiment, 359 accessions provided by National Plant Gene Bank of Iran (NPGBI) were used. Based on the geographical distribution, the highest distribution of A. cylindrica is found in North, West and North-West regions of Iran. The data on the distribution of A. cylindrica showed that its distribution centers in Iran are more than those reported in the previous studies. Chromosome counting showed that all A. cylindrica accessions are tetraploid (2n=4x=28). Results of factor analysis for nine morphological chromosome traits showed that karyotypic variation within accessions are related to the length of chromosomes and there is difference between the accessions for their total chromosome length, but the karyotype of different accessions are almost the same for the symmetry. Low coefficient of variation in morphological traits as well as symmetric karyotypes of A. cylindrica species observed in this study could lead us to predict that A. cylindrica could be a recently evolved species among the remote relatives of bread wheat.

Centromere Repeats: Hidden Gems of the Genome

Satellite DNAs are now regarded as powerful and active contributors to genomic and chromosomal evolution. Paired with mobile transposable elements, these repetitive sequences provide a dynamic mechanism through which novel karyotypic modifications and chromosomal rearrangements may occur. In this review, we discuss the regulatory activity of satellite DNA and their neighboring transposable elements in a chromosomal context with a particular emphasis on the integral role of both in centromere function. In addition, we discuss the varied mechanisms by which centromeric repeats have endured evolutionary processes, producing a novel, species-specific centromeric landscape despite sharing a ubiquitously conserved function. Finally, we highlight the role these repetitive elements play in the establishment and functionality of de novo centromeres and chromosomal breakpoints that underpin karyotypic variation. By emphasizing these unique activities of satellite DNAs and transposable elements, we hope to disparage the conventional exemplification of repetitive DNA in the historically-associated context of ‘junk’.

Distribution and characterization of Aegilops cylindrica species from Iran

Jointed goatgrass (Aegilops cylindrica Host; 2n = 4x = 28, CcCcDcDc) is a tetraploid remote relative of bread wheat (Triticum aestivum L; 2n=6x=42, AABBDD) with 2 genomes and 28 chromosomes. The diversity center of this species is in the Fertile Crescent and in central Asia and could also be found in many places in Iran. In this experiment, 359 accessions provided by National Plant Gene Bank of Iran (NPGBI) were used. Based on the geographical distribution, the highest distribution of Ae. cylindrica are from North, West and North West regions of Iran. The distribution data of Ae. cylindrica showed that the distribution centers in Iran are more than those reported in previous studies in Iran. Chromosome counting showed that all Ae. cylindrica accessions are tetraploid (2n=4x=28). Results of factor analysis for 9 morphological chromosome traits showed that karyotypic variation within accessions are related to the length of chromosomes and there is difference between accessions for their total chromosome length, but the karyotype of different accessions were almost the same for the symmetry. Low coefficient of variation in morphological traits as well as symmetric karyotypes of Ae. cylindrica species observed in this study could lead us to more confidently say that Ae. cylindrica could be a recently evolved species among remote relatives of bread wheat.

Karyotypic variation in the long-whiskered catfish Pimelodus blochii Valenciennes, 1840 (Siluriformes, Pimelodidae) from the lower Tapajós, Amazonas and Trombetas Rivers

The genus Pimelodus LaCépède, 1803 comprises 35 formally recognized species distributed along the major neotropical river basins. Despite conservatism in diploid number with 2n=56, an intense variation of chromosomal morphology (karyotypic formula) has been documented in Pimelodus species. In the present study, we analyzed karyotypes of 20 specimens, identified as Pimelodusblochii Valenciennes, 1840 and collected from the lower courses of the Tapajós, Amazonas and Trombetas Rivers. The karyotypes were characterized by Giemsa conventional staining, C-banding, silver staining (Ag-NOR) and fluorescent in situ hybridization (FISH) with 5S and 18S rDNA probes. The karyotypes showed 2n=56 chromosomes in fish from the Tapajós River. In contrast, fish from the Amazonas and Trombetas Rivers had 2n=58. The nucleolus organizing regions were labeled on the short arm of an acrocentric chromosome as demonstrated by silver staining and FISH. Signals for 18S and 5S rDNA were co-localized on one chromosome pair. Our results demonstrate karyotypic divergence between Tapajós and Amazonas-Trombetas populations of P.blochii, interpreted as supporting the existence of a species complex in this taxon.

The worldwide significance of karyotypic variation in Rosa acicularis (Rosaceae)

Evidence is provided which shows that there is a wider variation in karyotypes of Rosa acicularis subspecies seen within their ranges in North America (Rosa acicularis Lindl. subsp. sayi (Schwein.) W.H. Lewis and the Eurasian species (Rosa acicularis Lindl. subsp. acicularis) than was originally presumed. It is proposed that the evolution and distribution of these karyotypes into two distinct subspecies occurred following the closure of the Turgai strait when the climate radically changed within both continents. Their distributions are similar in that they are not only dependent upon the latitudes where they are found but on the ecology and climate that supports their growth. Parallel ethnobotanical and medicinal uses are also evident as is their value in horticulture, where especially in North America their genes for cold hardiness are being exploited. Concerns regarding how global warming may affect these various karyotypes are also discussed.