Comparative Gene Mapping Research Articles

Genetic Analysis and Fine Mapping of a Spontaneously Mutated Male Sterility Gene inBrassica rapassp.chinensis

Male sterility has been widely used in hybrid seed production in Brassica, but not in B. rapa ssp. chinensis, and genetic models of male sterility for this subspecies are unclear. We discovered a spontaneous mutant in B. rapa ssp. chinensis. A series of progeny tests indicated that male sterility in B. rapa ssp. chinensis follows a three-allele model with BrMsa, BrMsb, and BrMsc. The male sterility locus has been mapped to chromosome A07 in BC1 and F2 populations through genotyping by sequencing. Fine mapping in a total of 1,590 F2 plants narrowed the male sterility gene BrMs to a 400 kb region, with two SNP markers only 0.3 cM from the gene. Comparative gene mapping shows that the Ms gene in B. rapa ssp. pekinensis is different from the BrMs gene of B. rapa ssp. chinensis, despite that both genes are located on chromosome A07. Interestingly, the DNA sequence orthologous to a male sterile gene in Brassica napus, BnRf, is within 400 kb of the BrMs locus. The BnRf orthologs of B. rapa ssp. chinensis were sequenced, and one KASP marker (BrMs_indel) was developed for genotyping based on a 14 bp indel at intron 4. Cosegregation of male sterility and BrMs_indel genotypes in the F2 population indicated that BnRf from B. napus and BrMs from B. rapa are likely to be orthologs. The BrMs_indel marker developed in this study will be useful in marker-assisted selection for the male sterility trait.

Construction of a bacterial artificial chromosome library of Endoclita excrescens as a tool for comparative gene mapping in Lepidoptera

AbstractKaryotype evolution in one of the most diverse and species‐rich group of insects, moths and butterflies (Lepidoptera), has interesting features that remain to be resolved. Recent studies showed that fluorescence in situ hybridization using bacterial artificial chromosome clones (BAC‐FISH) is an efficient cytogenetic method for identification and gene mapping of lepidopteran chromosomes. Using comparative mapping by BAC‐FISH, extensive synteny of genes was revealed between chromosomes of different lepidopteran species based on Bombyx mori genomic information. However, this comparative mapping has been done only in representatives of advanced groups of Lepidoptera. Here we constructed a BAC library of Endoclita excrescens, which belongs to the primitive lepidopteran family Hepialidae. High molecular weight DNA for the library construction was prepared from the pupae by using a rapid nuclear isolation method known in plants. The BAC clones of E. excrescens contain 66.6 kb inserts on average. The successful application of BAC‐FISH showed that the BAC library of E. excrescens is a useful tool for comparative gene mapping on chromosomes of this species.

A high-density genetic linkage map and QTL mapping for growth and sex of yellow drum (Nibea albiflora)

A high-density genetic linkage map is essential for the studies of comparative genomics and gene mapping, and can facilitate assembly of reference genome. Herein, we constructed a high-density genetic linkage map with 8,094 SNPs selected from 113 sequenced fish of a F1 family. Ultimately, the consensus map spanned 3818.24 cM and covered nearly the whole genome (99.4%) with a resolution of 0.47 cM. 1,457 scaffolds spanning 435.15 Mb were anchored onto 24 linkage groups, accounting for 80.7% of the draft genome assembly of the yellow drum. Comparative genomic analyses with medaka and zebrafish genomes showed superb chromosome-scale synteny between yellow drum and medaka. QTL mapping and association analysis congruously revealed 22 QTLs for growth-related traits and 13 QTLs for sex dimorphism. Some important candidate genes such as PLA2G4A, BRINP3 and P2RY1 were identified from these growth-related QTL regions. A gene family including DMRT1, DMRT2 and DMRT3 was identified from these sex-related QTL regions on the linkage group LG9. We demonstrate that this linkage map can facilitate the ongoing marker-assisted selection and genomic and genetic studies for yellow drum.

In Silico Detection and FISH Analysis to Determine Location of miRNAs in Solea Senegalensis Chromosomes Using BACs

MicroRNAs (miRNAs) are small non-coding RNAs that play a very important role in gene expression through regulation of mRNA cleavage and translation. The Senegalese sole, Solea senegalensis (Kaup 1858), is a flatfish species that shows great potential for marine aquaculture. Nevertheless, the existence of sexual dysfunction of males reared in captivity, high larval mortality and diseases have hampered its production. The integration of sequence information with data on chromosomal physical location is useful for understanding genetic processes and is essential for comparative analysis, QTL mapping and positional cloning of genes. In this study we employed an in silico approach to identify potential miRNAs in a S. senegalensis BAC library. Mature sequences were obtained from miRBase and other sources and a BLAST analysis was carried out against sequences from a S. senegalensis BAC library. Sequences showing similarity were extracted and subjected to an analysis of their secondary structure. This information was combined with BAC-FISH data to assign miRNAs to chromosomes. In addition, a phylogenetic analysis of some miRNAs families was carried out and target genes were predicted against a reference transcriptome. After secondary structure analysis, 45 groups of sequences showed similarity to mature miRNAs. Of these, 19 were assigned to Senegalese Sole chromosomes using BAC-FISH data and thirty-four unique mature sequences targeted 12387 sequences from a S. senegalensis reference transcriptome. Phylogenetic trees showed different levels of support for clustering among types of miRNAs and S. senegalensis sequences usually were closed to other Pleuronectiformes. These results will be helpful to understand how miRNAs affect gene expression in this species and biological processes of interest for its aquaculture. Nevertheless, experimental validation through, for example, RT-PCR, northern blotting or microarrays is required.

Comparative Gene Mapping as a Tool to Understand the Evolution of Pest Crop Insect Chromosomes.

The extent of the conservation of synteny and gene order in aphids has been previously investigated only by comparing a small subset of linkage groups between the pea aphid Acyrthosiphon pisum and a few other aphid species. Here we compared the localization of eight A. pisum scaffolds (covering more than 5 Mb and 83 genes) in respect to the Drosophila melanogaster Muller elements identifying orthologous loci spanning all the four A. pisum chromosomes. Comparison of the genetic maps revealed a conserved synteny across different loci suggesting that the study of the fruit fly Muller elements could favour the identification of chromosomal markers useful for the study of chromosomal rearrangements in aphids. A. pisum is the first aphid species to have its genome sequenced and the finding that there are several chromosomal regions in synteny between Diptera and Hemiptera indicates that the genomic tools developed in A. pisum will be broadly useful not only for the study of other aphids but also for other insect species.

Origin of Amniote Sex Chromosomes: An Ancestral Super-Sex Chromosome, or Common Requirements?

The diversity of sex chromosomes among amniotes is the product of independent evolution of different systems in different lineages, defined by novel sex-determining genes. Convergent evolution is very common, suggesting that some genes are particularly adept at taking on a sex-determining role. Comparative gene mapping, and more recently whole genome sequencing, have now turned up other surprising relationships; different regions of the amniote genome that have become sex determining in some taxa seem to share synteny, or share sequence, in others. Is this, after all, evidence that these regions were once linked in a super-sex chromosome that underwent multiple fission in different ways in different amniote lineages? Or does it signify that special properties of sex chromosomes (paucity of active genes, low recombination, epigenetic regulation to achieve dosage compensation) predispose particular chromosomes to a sex-determining role?

In-silico characterisation and comparative mapping of yellow stripe like transporters in five grass species

Iron (Fe) and Zinc (Zn) are the key elements required for many of the biological process in plants and animals. Transporter proteins are essential for uptake, transport and accumulation for Fe and Zn in plants. The present investigation was undertaken to study and compare the structural and functional diversity and evolutionary significance of the yellow stripe-like (YSL) transporters through in-silico tools in five species (barley, Brachypodium, foxtail millet, maize and rice) of Poaceae. One hundred and two YSL transporters collected from public databases were used in the analysis. All YSL transporters possessed PF03169 domain which belongs to the oligo peptide transporters (OPT) super family. Molecular weight of YSL proteins ranged from 11.10 to 84.70 kDa while pI values ranged from 4.99 to 11.64. Scondary structure analysis identified that, alpha helix and random coils were the most common structures of the YSL proteins. Phylogenetic analysis revealed that the YSL transporters are highly conserved in these five grass species. Comparative mapping of genes of YSL transporters showed maximum synteny between Brachypodium and barley (30%) followed by Brachypodium and rice (25%). Neutrality test has in fact revealed the positive or Darwinian selection on YSL transporters. The results of the present investigation provided a significant understanding of the structural and biological role of YSL transporters as well as the evolutionary pattern in Poaceae family.

Weird mammals provide insights into the evolution of mammalian sex chromosomes and dosage compensation.

The deep divergence of mammalian groups 166 and 190 million years ago (MYA) provide genetic variation to explore the evolution of DNA sequence, gene arrangement and regulation of gene expression in mammals. With encouragement from the founder of the field, Mary Lyon, techniques in cytogenetics and molecular biology were progressively adapted to characterize the sex chromosomes of kangaroos and other marsupials, platypus and echidna-and weird rodent species. Comparative gene mapping reveals the process of sex chromosome evolution from their inception 190 MYA (they are autosomal in platypus) to their inevitable end (the Y has disappeared in two rodent lineages). Our X and Y are relatively young, getting their start with the evolution of the sex-determining SRY gene, which triggered progressive degradation of the Y chromosome. Even more recently, sex chromosomes of placental mammals fused with an autosomal region which now makes up most of the Y. Exploration of gene activity patterns over four decades showed that dosage compensation via X-chromosome inactivation is unique to therian mammals, and that this whole chromosome control process is different in marsupials and absent in monotremes and reptiles, and birds. These differences can be exploited to deduce how mammalian sex chromosomes and epigenetic silencing evolved.

Integrative analysis and expression profiling of secondary cell wall genes in C4 biofuel model Setaria italica reveals targets for lignocellulose bioengineering.

Several underutilized grasses have excellent potential for use as bioenergy feedstock due to their lignocellulosic biomass. Genomic tools have enabled identification of lignocellulose biosynthesis genes in several sequenced plants. However, the non-availability of whole genome sequence of bioenergy grasses hinders the study on bioenergy genomics and their genomics-assisted crop improvement. Foxtail millet (Setaria italica L.; Si) is a model crop for studying systems biology of bioenergy grasses. In the present study, a systematic approach has been used for identification of gene families involved in cellulose (CesA/Csl), callose (Gsl) and monolignol biosynthesis (PAL, C4H, 4CL, HCT, C3H, CCoAOMT, F5H, COMT, CCR, CAD) and construction of physical map of foxtail millet. Sequence alignment and phylogenetic analysis of identified proteins showed that monolignol biosynthesis proteins were highly diverse, whereas CesA/Csl and Gsl proteins were homologous to rice and Arabidopsis. Comparative mapping of foxtail millet lignocellulose biosynthesis genes with other C4 panicoid genomes revealed maximum homology with switchgrass, followed by sorghum and maize. Expression profiling of candidate lignocellulose genes in response to different abiotic stresses and hormone treatments showed their differential expression pattern, with significant higher expression of SiGsl12, SiPAL2, SiHCT1, SiF5H2, and SiCAD6 genes. Further, due to the evolutionary conservation of grass genomes, the insights gained from the present study could be extrapolated for identifying genes involved in lignocellulose biosynthesis in other biofuel species for further characterization.

Global analysis of WRKY transcription factor superfamily in Setaria identifies potential candidates involved in abiotic stress signaling.

Transcription factors (TFs) are major players in stress signaling and constitute an integral part of signaling networks. Among the major TFs, WRKY proteins play pivotal roles in regulation of transcriptional reprogramming associated with stress responses. In view of this, genome- and transcriptome-wide identification of WRKY TF family was performed in the C4model plants, Setaria italica (SiWRKY) and S. viridis (SvWRKY), respectively. The study identified 105 SiWRKY and 44 SvWRKY proteins that were computationally analyzed for their physicochemical properties. Sequence alignment and phylogenetic analysis classified these proteins into three major groups, namely I, II, and III with majority of WRKY proteins belonging to group II (53 SiWRKY and 23 SvWRKY), followed by group III (39 SiWRKY and 11 SvWRKY) and group I (10 SiWRKY and 6 SvWRKY). Group II proteins were further classified into 5 subgroups (IIa to IIe) based on their phylogeny. Domain analysis showed the presence of WRKY motif and zinc finger-like structures in these proteins along with additional domains in a few proteins. All SiWRKY genes were physically mapped on the S. italica genome and their duplication analysis revealed that 10 and 8 gene pairs underwent tandem and segmental duplications, respectively. Comparative mapping of SiWRKY and SvWRKY genes in related C4 panicoid genomes demonstrated the orthologous relationships between these genomes. In silico expression analysis of SiWRKY and SvWRKY genes showed their differential expression patterns in different tissues and stress conditions. Expression profiling of candidate SiWRKY genes in response to stress (dehydration and salinity) and hormone treatments (abscisic acid, salicylic acid, and methyl jasmonate) suggested the putative involvement of SiWRKY066 and SiWRKY082 in stress and hormone signaling. These genes could be potential candidates for further characterization to delineate their functional roles in abiotic stress signaling.

Extraordinary Diversity in the Origins of Sex Chromosomes in Anurans Inferred from Comparative Gene Mapping

Sex determination in frogs (anurans) is genetic and includes both male and female heterogamety. However, the origins of the sex chromosomes and their differentiation processes are poorly known. To investigate diversity in the origins of anuran sex chromosomes, we compared the chromosomal locations of sex-linked genes in 4 species: the African clawed frog (Xenopus laevis), the Western clawed frog (Silurana/X. tropicalis), the Japanese bell-ring frog (Buergeria buergeri), and the Japanese wrinkled frog (Rana rugosa). Comparative mapping data revealed that the sex chromosomes of X. laevis, X. tropicalis and R. rugosa are different chromosome pairs; however, the sex chromosomes of X. tropicalis and B. buergeri are homologous, although this may represent distinct evolutionary origins. We also examined the status of sex chromosomal differentiation in B. buergeri, which possesses heteromorphic ZW sex chromosomes, using comparative genomic hybridization and chromosome painting with DNA probes from the microdissected W chromosome. At least 3 rearrangement events have occurred in the proto-W chromosome: deletion of the nucleolus organizer region and a paracentric inversion followed by amplification of non-W-specific repetitive sequences.

Comparative Chromosome Mapping of U2 snRNA and 5S rRNA Genes in Gymnotus Species (Gymnotiformes, Gymnotidae): Evolutionary Dynamics and Sex Chromosome Linkage in G. pantanal

A comparative mapping of U2 small nuclear RNA (snRNA) and 5S ribosomal RNA (rRNA) genes was performed in 6 Gymnotus species. All species analyzed presented the U2 snDNA organized in conspicuous blocks and not co-located with rRNA genes. In addition, 5 species showed the U2 snDNA located in a single pair of chromosomes, which seems to be a conserved trait in this genus. Conversely, G. pantanal was the only species displaying several terminal signals in different chromosome pairs, including the X₁ sex chromosome but not the Y chromosome. This is the first report of U2 snRNA genes in sex chromosomes of fishes. The absence of sites in the Y chromosome of G. pantanal indicates a possible loss of terminal segments of the chromosomes involved in the Y formation.

Molecular and comparative mapping of genes governing spike compactness from wild emmer wheat

The development and morphology of the wheat spike is important because the spike is where reproduction occurs and it holds the grains until harvest. Therefore, genes that influence spike morphology are of interest from both theoretical and practical stand points. When substituted for the native chromosome 2A in the tetraploid Langdon (LDN) durum wheat background, the Triticum turgidum ssp. dicoccoides chromosome 2A from accession IsraelA confers a short, compact spike with fewer spikelets per spike compared to LDN. Molecular mapping and quantitative trait loci (QTL) analysis of these traits in a homozygous recombinant population derived from LDN×the chromosome 2A substitution line (LDNIsA-2A) indicated that the number of spikelets per spike and spike length were controlled by linked, but different, loci on the long arm of 2A. A QTL explaining most of the variation for spike compactness coincided with the QTL for spike length. Comparative mapping indicated that the QTL for number of spikelets per spike overlapped with a previously mapped QTL for Fusarium head blight susceptibility. The genes governing spike length and compactness were not orthologous to either sog or C, genes known to confer compact spikes in diploid and hexaploid wheat, respectively. Mapping and sequence analysis indicated that the gene governing spike length and compactness derived from wild emmer could be an ortholog of the barley Cly1/Zeo gene, which research indicates is an AP2-like gene pleiotropically affecting cleistogamy, flowering time, and rachis internode length. This work provides researchers with knowledge of new genetic loci and associated markers that may be useful for manipulating spike morphology in durum wheat.

Comparative FISH mapping of BMPR1B, BMP15 and GDF9 fecundity genes on cattle, river buffalo, sheep and goat chromosomes

One of the main goal in livestock genomic projects isthe preparation of extensive genomic maps that identifyloci affecting economically important traits. For this rea-son, the aim of this study was to compare several physi-cal maps using bovine artificial chromosomes (BAC) clonescontaining genes related to fecundity genes. Fluorescent

Genome-Wide Association Study Reveals Genetic Architecture of Eating Behavior in Pigs and Its Implications for Humans Obesity by Comparative Mapping

This study was aimed at identifying genomic regions controlling feeding behavior in Danish Duroc boars and its potential implications for eating behavior in humans. Data regarding individual daily feed intake (DFI), total daily time spent in feeder (TPD), number of daily visits to feeder (NVD), average duration of each visit (TPV), mean feed intake per visit (FPV) and mean feed intake rate (FR) were available for 1130 boars. All boars were genotyped using the Illumina Porcine SNP60 BeadChip. The association analyses were performed using the GenABEL package in the R program. Sixteen SNPs were found to have moderate genome-wide significance (p<5E-05) and 76 SNPs had suggestive (p<5E-04) association with feeding behavior traits. MSI2 gene on chromosome (SSC) 14 was very strongly associated with NVD. Thirty-six SNPs were located in genome regions where QTLs have previously been reported for behavior and/or feed intake traits in pigs. The regions: 64–65 Mb on SSC 1, 124–130 Mb on SSC 8, 63–68 Mb on SSC 11, 32–39 Mb and 59–60 Mb on SSC 12 harbored several signifcant SNPs. Synapse genes (GABRR2, PPP1R9B, SYT1, GABRR1, CADPS2, DLGAP2 and GOPC), dephosphorylation genes (PPM1E, DAPP1, PTPN18, PTPRZ1, PTPN4, MTMR4 and RNGTT) and positive regulation of peptide secretion genes (GHRH, NNAT and TCF7L2) were highly significantly associated with feeding behavior traits. This is the first GWAS to identify genetic variants and biological mechanisms for eating behavior in pigs and these results are important for genetic improvement of pig feed efficiency. We have also conducted pig-human comparative gene mapping to reveal key genomic regions and/or genes on the human genome that may influence eating behavior in human beings and consequently affect the development of obesity and metabolic syndrome. This is the first translational genomics study of its kind to report potential candidate genes for eating behavior in humans.

Reconstruction of the ancestral marsupial karyotype from comparative gene maps

BackgroundThe increasing number of assembled mammalian genomes makes it possible to compare genome organisation across mammalian lineages and reconstruct chromosomes of the ancestral marsupial and therian (marsupial and eutherian) mammals. However, the reconstruction of ancestral genomes requires genome assemblies to be anchored to chromosomes. The recently sequenced tammar wallaby (Macropus eugenii) genome was assembled into over 300,000 contigs. We previously devised an efficient strategy for mapping large evolutionarily conserved blocks in non-model mammals, and applied this to determine the arrangement of conserved blocks on all wallaby chromosomes, thereby permitting comparative maps to be constructed and resolve the long debated issue between a 2n = 14 and 2n = 22 ancestral marsupial karyotype.ResultsWe identified large blocks of genes conserved between human and opossum, and mapped genes corresponding to the ends of these blocks by fluorescence in situ hybridization (FISH). A total of 242 genes was assigned to wallaby chromosomes in the present study, bringing the total number of genes mapped to 554 and making it the most densely cytogenetically mapped marsupial genome. We used these gene assignments to construct comparative maps between wallaby and opossum, which uncovered many intrachromosomal rearrangements, particularly for genes found on wallaby chromosomes X and 3. Expanding comparisons to include chicken and human permitted the putative ancestral marsupial (2n = 14) and therian mammal (2n = 19) karyotypes to be reconstructed.ConclusionsOur physical mapping data for the tammar wallaby has uncovered the events shaping marsupial genomes and enabled us to predict the ancestral marsupial karyotype, supporting a 2n = 14 ancestor. Futhermore, our predicted therian ancestral karyotype has helped to understand the evolution of the ancestral eutherian genome.

Inference of the Protokaryotypes of Amniotes and Tetrapods and the Evolutionary Processes of Microchromosomes from Comparative Gene Mapping

Comparative genome analysis of non-avian reptiles and amphibians provides important clues about the process of genome evolution in tetrapods. However, there is still only limited information available on the genome structures of these organisms. Consequently, the protokaryotypes of amniotes and tetrapods and the evolutionary processes of microchromosomes in tetrapods remain poorly understood. We constructed chromosome maps of functional genes for the Chinese soft-shelled turtle (Pelodiscus sinensis), the Siamese crocodile (Crocodylus siamensis), and the Western clawed frog (Xenopus tropicalis) and compared them with genome and/or chromosome maps of other tetrapod species (salamander, lizard, snake, chicken, and human). This is the first report on the protokaryotypes of amniotes and tetrapods and the evolutionary processes of microchromosomes inferred from comparative genomic analysis of vertebrates, which cover all major non-avian reptilian taxa (Squamata, Crocodilia, Testudines). The eight largest macrochromosomes of the turtle and chicken were equivalent, and 11 linkage groups had also remained intact in the crocodile. Linkage groups of the chicken macrochromosomes were also highly conserved in X. tropicalis, two squamates, and the salamander, but not in human. Chicken microchromosomal linkages were conserved in the squamates, which have fewer microchromosomes than chicken, and also in Xenopus and the salamander, which both lack microchromosomes; in the latter, the chicken microchromosomal segments have been integrated into macrochromosomes. Our present findings open up the possibility that the ancestral amniotes and tetrapods had at least 10 large genetic linkage groups and many microchromosomes, which corresponded to the chicken macro- and microchromosomes, respectively. The turtle and chicken might retain the microchromosomes of the amniote protokaryotype almost intact. The decrease in number and/or disappearance of microchromosomes by repeated chromosomal fusions probably occurred independently in the amphibian, squamate, crocodilian, and mammalian lineages.

Integration of EST-CAPS markers into genetic maps of Eucalyptus urophylla and E. tereticornis and their alignment with E. grandis genome sequence

Abstract A suite of 91 expressed sequence tag (EST) derived cleaved amplified polymorphic sequence (CAPS) markers were developed and used for enriching the genetic maps of Eucalyptus urophylla and E. tereticornis built previously based on random amplified polymorphic DNA (RAPD) markers. The EST-CAPS markers were highly similar to original ESTs, with sequence identity ranging from 92.5% to 100.0%. In linkage analysis, 48 and 42 EST-CAPSs were integrated into the genetic maps of E. urophylla and E. tereticornis, respectively, including 13 shared by both maps, while 14 were unmapped. For E. urophylla, the final map had a total length of 1789.5 cM and a mean interval between markers of 9.7 cM, being 284.9 cM larger and 1.3 cM less than those of the prior RAPD map, respectively. For E. tereticornis, the final map had a length of 1488.1 cM and a mean interval of 10.3 cM, being 452.4 and 0.2 cM more than the prior map, respectively. All the 77 newly mapped EST-CAPSs found each at least one homologue in the E. grandis genome sequence released recently, and conserved synteny and colinearity were observed between E. grandis genome and our linkage groups. The enriched maps would provide a set of useful markers for genome analysis, comparative mapping and fine-mapping of important genes located in conserved regions for the important tree genus Eucalyptus.

Genomic rearrangement between wheat and Thinopyrum elongatum revealed by mapped functional molecular markers

Thinopyrum elongatum serves as an excellent gene pool for wheat improvement. Genes for resistance to many biotic and abiotic stresses have been transferred from Th. elongatum to wheat through chromosome manipulation. For breeding programs, molecular markers enable screening of a large number of genotypes for alien chromosome introgressions. The main objective of the present study was to develop and characterize EST (expressed sequence tags) and PLUG (PCR-based Landmark Unique Gene) markers that can distinguish Th. elongatum chromatin from the wheat genomes. A total of 258 mapped EST primer pairs and 46 PLUG primer pairs were tested on DNA from wheat Chinese Spring (CS) and CS-Th. elongatum addition lines. The results showed that 43 primer pairs could be effectively mapped to specific Th. elongatum chromosomes. Twenty-two of the 43 markers displayed similar homoeologous chromosome locations to hexaploid wheat. Nine markers mapped to different linkage groups between wheat and Th. elongatum, while 12 makers mapped on two or three different Th. elongatum chromosomes. A comparison of molecular marker locations indicated that Th. elongatum genome was closely related to the D genome of wheat, and chromosome rearrangements and duplication had occurred in Th. elongatum and the wheat genomes. The markers will be useful in comparative gene mapping, chromosome evolutionary analysis, and gene introgression for wheat improvement using Th. elongatum accessions as gene donors.

Comparative chromosome mapping of the rRNA genes and telomeric repeats in three Italian pine voles of the Microtus savii s.l. complex (Rodentia, Cricetidae)

The Microtus (Terricola) savii s. l. complex is a group of five species/subspecies of the Italian pine voles, which diverged at different times either with or without chromosomal differentiation. The evidence of chromosomal diversification has so far concerned the shape of the sex chromosomes, especially the X chromosome. Three taxa of the group, Microtus savii savii, Microtus savii nebrodensis, and Microtus savii tolfetanus have identical karyotypes with metacentric X chromosomes. The X chromosomes of Microtus brachycercus and Microtus brachycercus niethammericus are, respectively, subtelocentric and acrocentric in shape. The Microtus savii complex has been long an object of conventional karyological studies, but comparative molecular cytogenetic data were completely missing. Therefore, we conducted a comparative chromosomal mapping of rRNA genes (rDNA) and telomeric repeats in three of the five taxa of the group: Microtus savii savii, Microtus savii nebrodensis, and Microtus brachycercus niethammericus, each of which belongs to a distinct mitochondrial clade.The survey revealed that differentiation of the clades was accompanied by remarkable changes with regard to the number and locations of the rDNA sites. Thus, Microtus savii savii and Microtus savii nebrodensis have especially high numbers of rDNA sites, which are located in the centromeric regions of, correspondingly, 18 and 13 chromosome pairs, whereas Microtus brachycercus niethammericus shows variable (8–10) and heteromorphic rDNA sites on both centromeric and telomeric regions. Interstitial telomeric sites (ITS), which are believed to indicate possible breakpoints of recurring chromosomal rearrangements, are present on the largest biarmed chromosomes and on the metacentric X chromosomes in Microtus savii savii and Microtus savii nebrodensis. These preliminary results are discussed in the context of recent advances in phylogeny of the group, as well as the rDNA genomic organization and X chromosome rearrangements in the genus Microtus.