An unusual supernumerary chromosome has been reported for two related avian species, the zebra and Bengalese finches. This large, germline-restricted chromosome (GRC) is eliminated from somatic cells and spermatids and transmitted via oocytes only. Its origin, distribution among avian lineages, and function were mostly unknown so far. Using immunolocalization of key meiotic proteins, we found that GRCs of varying size and genetic content are present in all 16 songbird species investigated and absent from germline genomes of all eight examined bird species from other avian orders. Results of fluorescent in situ hybridization of microdissected GRC probes and their sequencing indicate that GRCs show little homology between songbird species and contain a variety of repetitive elements and unique sequences with paralogs in the somatic genome. Our data suggest that the GRC evolved in the common ancestor of all songbirds and underwent significant changes in the extant descendant lineages.

The Eurasian common shrew (Sorex araneus L.) is characterized by spectacular chromosomal variation, both autosomal variation of the Robertsonian type and an XX/XY 1 Y 2 system of sex determination. It is an important mammalian model of chromosomal and genome evolution as it is one of the few species with a complete genome sequence. Here we generate a high-precision cytological recombination map for the species, the third such map produced in mammals, following those for humans and house mice. We prepared synaptonemal complex (SC) spreads of meiotic chromosomes from 638 spermatocytes of 22 males of nine different Robertsonian karyotypes, identifying each autosome arm by differential DAPI staining. Altogether we mapped 13,983 recombination sites along 7095 individual autosomes, using immunolocalization of MLH1, a mismatch repair protein marking recombination sites. We estimated the total recombination length of the shrew genome as 1145 cM. The majority of bivalents showed a high recombination frequency near the telomeres and a low frequency near the centromeres. The distances between MLH1 foci were consistent with crossover interference both within chromosome arms and across the centromere in metacentric bivalents. The pattern of recombination along a chromosome arm was a function of its length, interference, and centromere and telomere effects. The specific DNA sequence must also be important because chromosome arms of the same length differed substantially in their recombination pattern. These features of recombination show great similarity with humans and mice and suggest generality among mammals. However, contrary to a widespread perception, the metacentric bivalent tu usually lacked an MLH1 focus on one of its chromosome arms, arguing against a minimum requirement of one chiasma per chromosome arm for correct segregation. With regard to autosomal chromosomal variation, the chromosomes showing Robertsonian polymorphism display MLH1 foci that become increasingly distal when comparing acrocentric homozygotes, heterozygotes, and metacentric homozygotes. Within the sex trivalent XY 1 Y 2 , the autosomal part of the complex behaves similarly to other autosomes.

GmbH, Germany). The location of each captured immunostained spread was recorded so that it could be relocated on the slide after FISH. Electron microscopy was carried out using a JEM-1400 electron microscope (JEOL, Tokyo, Japan) at 80 kV. All microscopy studies were carried out at the Center for Microscopic Analysis of Biological Objects of SD RAS (Novosibirsk, Russia). Corel PaintShop Pro X6 (Corel) was used for a correction of image brightness and contrast. Chromosome measurements and generation of recombination maps of GRCs. Centromeres were identified by ACA foci. MLH1 signals were only scored if they were localized on SCs. The length of the SC was measured in micrometers and the positions of MLH1 foci in relation to the centromere were recorded using MicroMeasure 3.3 30. SCs of GRC and macrochromosomes were identified by their relative lengths and centromeric indexes. SC1 is the largest submetacentric. SC2 and SC3 are large subacrocentrics of similar sizes but different centromeric indexes. SC4 is middle size metacentric, SC5 and SC6 are subacrocentrics of the same size, which differ from each other in the centromeric indexes. On bone marrow metaphase chromosome spreads, Z and W are identified as a pair of non-matching macrochromosomes: metacentric and submetacentric (correspondingly). At SC spreads, ZW is identified as macrobivalent with misaligned centromeres and/or asynapsed ends of the axial elements. GRC is identified as the only acrocentric macrobivalent or univalent. To generate recombination maps, we divided the length of the SC into equal intervals approximately equal to 1 µm and plotted the proportion of MLH1 foci located in each interval. STATISTICA 6.0 software package (StatSoft, Tulsa, OK, USA) was used for descriptive statistics. All results were expressed as mean ± SD; p < 0.05 was considered as statistically significant.

In many eutherian mammals, X-Y chromosome pairing and recombination is required for meiotic progression and correct sex chromosome disjunction. Arvicoline rodents present a notable exception to this meiotic rule, with multiple species possessing asynaptic sex chromosomes. Most asynaptic vole species belong to the genus Microtus sensu lato. However, many of the species both inside and outside the genus Microtus display normal X-Y synapsis at meiosis. These observations suggest that the synaptic condition was present in the common ancestor of all voles, but gaps in current taxonomic sampling across the arvicoline phylogeny prevent identification of the lineage(s) along which the asynaptic state arose. In this study, we use electron and immunofluorescent microscopy to assess heterogametic sex chromosome pairing in 12 additional arvicoline species. Our sample includes ten species of the tribe Microtini and two species of the tribe Lagurini. This increased breadth of sampling allowed us to identify asynaptic species in each major Microtine lineage. Evidently, the ability of the sex chromosomes to pair and recombine in male meiosis has been independently lost at least three times during the evolution of Microtine rodents. These results suggest a lack of evolutionary constraint on X-Y synapsis in Microtini, hinting at the presence of alternative molecular mechanisms for sex chromosome segregation in this large mammalian tribe.

We examined A- and B-chromosome pairing and recombination in 12 males from the farm-bred population of the silver fox (2n = 34 + 0-10 Bs) by means of electron and immunofluorescent microscopy. To detect recombination at A and B chromosomes, we used immunolocalisation of MLH1, a mismatch repair protein of mature recombination nodules, at synaptonemal complexes. The mean total number of MLH1 foci at A-autosomes was 29.6 foci per cell. The XY bivalent had one MLH1 focus at the pairing region. Total recombination length of the male fox genome map was estimated as 1,530 centimorgans. We detected single MLH1 foci at 61% of linear synaptic configurations involving B chromosomes. The distribution of the foci along B- and A-bivalents was the same. This may be considered as a first molecular evidence that meiotic recombination does occur in mammalian B chromosomes. There was no correlation between the number of synaptic configurations involving B chromosomes per cell and the recombination rate of the A-genome.

Hybrid sterility is an important step in the speciation process. Hybrids between dwarf hamsters Phodopus sungorus and P. campbelli provide a good model for studies in cytological and genetic mechanisms of hybrid sterility. Previous studies in hybrids detected multiple abnormalities of spermatogenesis and a high frequency of dissociation between the X and Y chromosomes at the meiotic prophase. In this study, we found that the autosomes of the hybrid males and females underwent paring and recombination as normally as their parental forms did. The male hybrids showed a significantly higher frequency of asynapsis and recombination failure between the heterochromatic arms of the X and Y chromosomes than the males of the parental species. Female hybrids as well as the females of the parental species demonstrated a high incidence of centromere misalignment at the XX bivalent and partial asynapsis of the ends of its heterochromatic arms. In all three karyotypes, recombination was completely suppressed in the heterochromatic arm of the X chromosome, where the pseudoautosomal region is located. We propose that this recombination pattern speeds up divergence of the X- and Y-linked pseudoautosomal regions between the parental species and results in their incompatibility in the male hybrids.

Genome-wide association studies have led to a significant progress in identification of genomic loci affecting coronary artery disease (CAD) risk. However, revealing the causal genes responsible for the observed associations is challenging. In the present study, we aimed to prioritize CAD-relevant genes based on cumulative evidence from the published studies and our own study of colocalization between eQTLs and loci associated with CAD using SMR/HEIDI approach. Prior knowledge of candidate genes was extracted from both experimental and in silico studies, employing different prioritization algorithms. Our review systematized information for a total of 51 CAD-associated loci. We pinpointed 37 genes in 36 loci. For 27 genes we infer they are causal for CAD, and for 10 further genes we judge them most likely causal. Colocalization analysis showed that for 18 out of these loci, association with CAD can be explained by changes in gene expression in one or more CAD-relevant tissues. Furthermore, for 8 out of 36 loci, existing evidence suggested additional CAD-associated genes. For the remaining 15 loci, we concluded that evidence for gene prioritization remains inconsistent, insufficient, or absent. Our results provide deeper insights into the genetic etiology of CAD and demonstrate knowledge gaps where further research is warranted. Coronary artery disease (CAD) is the most prevalent cardiovascular disease, the major cause of mortality and morbidity in both developed and developing countries 1. This pathology is the manifestation of atherosclerosis in the coronary arteries. CAD can lead to a variety of complications, including chest pain, myocardial infarction (MI), arrhythmias and heart failure 2. The etiology of CAD is multifactorial and involves a genetic predisposition as well as dietary and other lifestyle risk factors 3. The genetic component to CAD has long been recognized. The Framingham Study demonstrated that positive family history is a strong risk factor for incident CAD 4-6. According to Swedish and Danish twin studies, the narrow-sense heritability of fatal CAD is about 40-60% 7,8. Today, it is widely accepted that much of the genetic component arises from the effect of many common alleles associated with modest increases in CAD risk 3,9. Genome-wide association studies demonstrated that the common variation accounts for 40-50% of heritability of MI/CAD 10,11. Genetic studies of CAD started from family-based linkage studies discovering monogenic drivers of CAD and small candidate-gene studies which often provided controversial results. Development of high-throughput genotyping technologies and new statistical methods opened the era of genome-wide association studies (GWAS) 12,13. MI was among the very first traits studied with use of genome-wide association strategy already in 2002 14 .

Korean field mouse (Apodemus peninsulae) shows a wide variation in the number of B chromosomes composed of constitutive heterochromatin. For this reason, it provides a good model to study the influence of the number of centromeres and amount of heterochromatin on spatial organization of interphase nuclei. We analyzed the three-dimensional organization of fibroblast and spermatocyte nuclei of the field mice carrying a different number of B chromosomes using laser scanning microscopy and 3D fluorescence in situ hybridization. We detected a co-localization of the B chromosomes with constitutive heterochromatin of the chromosomes of the basic set. We showed a non-random distribution of B chromosomes in the spermatocyte nuclei. Unpaired B chromosomes showed a tendency to occur in the compartment formed by the unpaired part of the XY bivalent.