Development of a D genome specific marker resource for diploid and hexaploid wheat

Development of molecular markers based on LTR retrotransposon in the Cleistogenes songorica genome

Long terminal repeat retrotransposons (LTR-RTs) contribute a large fraction of many sequenced plant genomes and play important roles in genomic diversity and phenotypic variations. LTR-RTs are abundantly distributed in plant genomes, facilitating the development of markers based on LTR-RTs for a variety of genotyping purposes. Whole-genome analysis of LTR-RTs was performed in Cleistogenes songorica. A total of 299,079 LTR-RTs were identified and classified as Gypsy type, Copia type, or other type. LTR-RTs were widely distributed in the genome, enriched in the heterochromatic region of the chromosome, and negatively correlated with gene distribution. However, approximately one-fifth of genes were still interrupted by LTR-RTs, and these genes are annotated. Furthermore, four types of primer pairs (PPs) were designed, namely, retrotransposon-based insertion polymorphisms, inter-retrotransposon amplified polymorphisms, insertion site-based polymorphisms, and retrotransposon-microsatellite amplified polymorphisms. A total of 350 PPs were screened in 23 accessions of the genus Cleistogenes, of which 80 PPs showed polymorphism, and 72 PPs showed transferability among Gramineae and non-Gramineae species. In addition, a comparative analysis of homologous LTR-RTs was performed with other related grasses. Taken together, the study will serve as a valuable resource for genotyping applications for C. songorica and related grasses.

St2-80: a new FISH marker for St genome and genome analysis in Triticeae

The St genome is one of the most fundamental genomes in Triticeae. Repetitive sequences are widely used to distinguish different genomes or species. The primary objectives of this study were to (i) screen a new sequence that could easily distinguish the chromosome of the St genome from those of other genomes by fluorescence in situ hybridization (FISH) and (ii) investigate the genome constitution of some species that remain uncertain and controversial. We used degenerated oligonucleotide primer PCR (Dop-PCR), Dot-blot, and FISH to screen for a new marker of the St genome and to test the efficiency of this marker in the detection of the St chromosome at different ploidy levels. Signals produced by a new FISH marker (denoted St₂-80) were present on the entire arm of chromosomes of the St genome, except in the centromeric region. On the contrary, St₂-80 signals were present in the terminal region of chromosomes of the E, H, P, and Y genomes. No signal was detected in the A and B genomes, and only weak signals were detected in the terminal region of chromosomes of the D genome. St₂-80 signals were obvious and stable in chromosomes of different genomes, whether diploid or polyploid. Therefore, St₂-80 is a potential and useful FISH marker that can be used to distinguish the St genome from those of other genomes in Triticeae.

A High Resolution Radiation Hybrid Map of Wheat Chromosome 4A

Bread wheat has a large and complex allohexaploid genome with low recombination level at chromosome centromeric and peri-centromeric regions. This significantly hampers ordering of markers, contigs of physical maps and sequence scaffolds and impedes obtaining of high-quality reference genome sequence. Here we report on the construction of high-density and high-resolution radiation hybrid (RH) map of chromosome 4A supported by high-density chromosome deletion map. A total of 119 endosperm-based RH lines of two RH panels and 15 chromosome deletion bin lines were genotyped with 90K iSelect single nucleotide polymorphism (SNP) array. A total of 2316 and 2695 markers were successfully mapped to the 4A RH and deletion maps, respectively. The chromosome deletion map was ordered in 19 bins and allowed precise identification of centromeric region and verification of the RH panel reliability. The 4A-specific RH map comprises 1080 mapping bins and spans 6550.9 cR with a resolution of 0.13 Mb/cR. Significantly higher mapping resolution in the centromeric region was observed as compared to recombination maps. Relatively even distribution of deletion frequency along the chromosome in the RH panel was observed and putative functional centromere was delimited within a region characterized by two SNP markers.

High Transferability of Homoeolog-Specific Markers between Bread Wheat and Newly Synthesized Hexaploid Wheat Lines

Bread wheat (Triticum aestivum, 2n = 6x = 42, AABBDD) has a complex allohexaploid genome, which makes it difficult to differentiate between the homoeologous sequences and assign them to the chromosome A, B, or D subgenomes. The chromosome-based draft genome sequence of the ‘Chinese Spring’ common wheat cultivar enables the large-scale development of polymerase chain reaction (PCR)-based markers specific for homoeologs. Based on high-confidence ‘Chinese Spring’ genes with known functions, we developed 183 putative homoeolog-specific markers for chromosomes 4B and 7B. These markers were used in PCR assays for the 4B and 7B nullisomes and their euploid synthetic hexaploid wheat (SHW) line that was newly generated from a hybridization between Triticum turgidum (AABB) and the wild diploid species Aegilops tauschii (DD). Up to 64% of the markers for chromosomes 4B or 7B in the SHW background were confirmed to be homoeolog-specific. Thus, these markers were highly transferable between the ‘Chinese Spring’ bread wheat and SHW lines. Homoeolog-specific markers designed using genes with known functions may be useful for genetic investigations involving homoeologous chromosome tracking and homoeolog expression and interaction analyses.