Evolutionary plasticity of acipenseriform genomes.

Acipenseriformes is an order of ray-finned fishes, comprising 27 extant species of sturgeons and paddlefishes inhabiting waters of the Northern Hemisphere. The order has a basal position within Actinopteri (ray-finned fish minus polypterids) and is characterized by many specific morphological and genomic features, including high diploid chromosome numbers, various levels of ploidy between species, unclear sex determination, and propensity to interspecific hybridization. Recent advances in molecular genetics, genomics, and comparative cytogenetics produced novel data on different aspects of acipenseriform biology, including improved phylogenetic reconstructions and better understanding of genome structure. Here, we discuss the cytogenetic and genomic traits of acipenseriforms and their connection with polyploidization and tolerance to interspecific hybridization.

Comparative Chromosome Map and Heterochromatin Features of the Gray Whale Karyotype (Cetacea).

Cetacean karyotypes possess exceptionally stable diploid numbers and highly conserved chromosomes. To date, only toothed whales (Odontoceti) have been analyzed by comparative chromosome painting. Here, we studied the karyotype of a representative of baleen whales, the gray whale (Eschrichtius robustus, Mysticeti), by Zoo-FISH with dromedary camel and human chromosome-specific probes. We confirmed a high degree of karyotype conservation and found an identical order of syntenic segments in both branches of cetaceans. Yet, whale chromosomes harbor variable heterochromatic regions constituting up to a third of the genome due to the presence of several types of repeats. To investigate the cause of this variability, several classes of repeated DNA sequences were mapped onto chromosomes of whale species from both Mysticeti and Odontoceti. We uncovered extensive intrapopulation variability in the size of heterochromatic blocks present in homologous chromosomes among 3 individuals of the gray whale by 2-step differential chromosome staining. We show that some of the heteromorphisms observed in the gray whale karyotype are due to distinct amplification of a complex of common cetacean repeat and heavy satellite repeat on homologous autosomes. Furthermore, we demonstrate localization of the telomeric repeat in the heterochromatin of both gray and pilot whale (Globicephala melas, Odontoceti). Heterochromatic blocks in the pilot whale represent a composite of telomeric and common repeats, while heavy satellite repeat is lacking in the toothed whale consistent with previous studies.

Segmental paleotetraploidy revealed in sterlet (Acipenser ruthenus) genome by chromosome painting.

BACKGROUND: Acipenseriformes take a basal position among Actinopteri and demonstrate a striking ploidy variation among species. The sterlet (Acipenser ruthenus, Linnaeus, 1758; ARUT) is a diploid 120-chromosomal sturgeon distributed in Eurasian rivers from Danube to Enisey. Despite a high commercial value and a rapid population decline in the wild, many genomic characteristics of sterlet (as well as many other sturgeon species) have not been studied. RESULTS: Cell lines from different tissues of 12 sterlet specimens from Siberian populations were established following an optimized protocol. Conventional cytogenetic studies supplemented with molecular cytogenetic investigations on obtained fibroblast cell lines allowed a detailed description of sterlet karyotype and a precise localization of 18S/28S and 5S ribosomal clusters. Localization of sturgeon specific HindIII repetitive elements revealed an increased concentration in the pericentromeric region of the acrocentric ARUT14, while the total sterlet repetitive DNA fraction (C0t30) produced bright signals on subtelomeric segments of small chromosomal elements. Chromosome and region specific probes ARUT1p, 5, 6, 7, 8 as well as 14 anonymous small sized chromosomes (probes A-N) generated by microdissection were applied in chromosome painting experiments. According to hybridization patterns all painting probes were classified into two major groups: the first group (ARUT5, 6, 8 as well as microchromosome specific probes C, E, F, G, H, and I) painted only a single region each on sterlet metaphases, while probes of the second group (ARUT1p, 7 as well as microchromosome derived probes A, B, D, J, K, M, and N) marked two genomic segments each on different chromosomes. Similar results were obtained on male and female metaphases. CONCLUSIONS: The sterlet genome represents a complex mosaic structure and consists of diploid and tetraploid chromosome segments. This may be regarded as a transition stage from paleotetraploid (functional diploid) to diploid genome condition. Molecular cytogenetic and genomic studies of other 120- and 240-chromosomal sturgeons are needed to reconstruct genome evolution of this vertebrate group.

Chromosomal evolution of Arvicolinae (Cricetidae, Rodentia). III. Karyotype relationships of ten Microtus species.

The genus Microtus consists of 65 extant species, making it one of the rodentia genera with the highest number of species. The extreme karyotype diversification in Microtus has made them an ideal species group for comparative cytogenetics and cytotaxonomy. Conventional comparative cytogenetic studies in Microtus have been based mainly on chromosomal banding patterns; the number of Microtus species examined by molecular cytogenetics-cross-species chromosome painting-is limited. In this study, we used whole chromosome painting probes of the field vole Microtus agrestis to detect regions of homology in the karyotypes of eight Microtus species. For almost all investigated species, species-specific associations of conserved chromosomal segments were revealed. Analysis of data obtained here and previously published data allowed us to propose that the ancestral Microtus species had a 2n = 54 karyotype, including two associations of field vole chromosomal segments (MAG 1/17 and 2/8). Further mapping of the chromosome rearrangements onto a molecular phylogenetic tree allows the reconstruction of a karyotype evolution pathway in the Microtus genus.