Assembling a puzzle of dispersed retrotransposable sequences in the genome of chickpea (Cicer arietinum L.)

Several repetitive elements are known to be present in the genome of chickpea (Cicer arietinum L.) including satellite DNA and En/Spm transposons as well as two dispersed, highly repetitive elements, CaRep1 and CaRep2. PCR was used to prove that CaRep1, CaRep2, and previously isolated CaRep3 of C. arietinum represent different segments of a highly repetitive Ty3-gypsy-like retrotransposon (Metaviridae) designated CaRep that makes up large parts of the intercalary heterochromatin. The full sequence of this element including the LTRs and untranslated internal regions was isolated by selective amplification. The restriction pattern of CaRep was different within the annual species of the genus Cicer, suggesting its rearrangement during the evolution of the genus during the last 100 000 years. In addition to CaRep, another LTR and a non-LTR retrotransposon family were isolated, and their restriction patterns and physical localization in the chickpea genome were characterized. The LINE-like element CaLin is only of comparatively low abundance and reveals a considerable heterogeneity. The Ty1-copia-like element (Pseudoviridae) CaTy is located in the distal parts of the intercalary heterochromatin and adjacent euchromatic regions, but it is absent from the centromeric regions. These results together with earlier findings allow to depict the distribution of retroelements on chickpea chromosomes, which extensively resembles the retroelement landscape of the genome of the model legume Medicago truncatula Gaertn.

Molecular markers closely linked to fusarium resistance genes in chickpea show significant alignments to pathogenesis-related genes located on Arabidopsis chromosomes 1 and 5

A population of 131 recombinant inbred lines from a wide cross between chickpea ( Cicer arietinum L., resistant parent) and Cicer reticulatum (susceptible parent) segregating for the closely linked resistances against Fusarium oxysporum f.sp. ciceri races 4 and 5 was used to develop DNA amplification fingerprinting markers linked to both resistance loci. Bulked segregant analysis revealed 19 new markers on linkage group 2 of the genetic map on which the resistance genes are located. Closest linkage (2.0 cM) was observed between marker R-2609-1 and the race 4 resistance locus. Seven other markers flanked this locus in a range from 4.1 to 9.0 cM. These are the most closely linked markers available for this locus up to date. The sequences of the linked markers were highly similar to genes encoding proteins involved in plant pathogen response, such as a PR-5 thaumatin-like protein and an important regulator of the phytoalexin pathway, anthranilate N-hydroxycinnamoyl-benzoyltransferase. Others showed significant alignments to genes encoding housekeeping enzymes such as the MutS2 DNA-mismatch repair protein. In the Arabidopsis genome, similar genes are located on short segments of chromosome 1 and 5, respectively, suggesting synteny between the fusarium resistance gene cluster of chickpea and the corresponding regions in the Arabidopsis genome. Three marker sequences were similar to retrotransposon-derived and/or satellite DNA sequences. The markers developed here provide a starting point for physical mapping and map-based cloning of the fusarium resistance genes and exploration of synteny in this highly interesting region of the chickpea genome.

A PCR-based assay to detect En/Spm-like transposon sequences in plants

Degenerate primers deduced from the TPase region of plant En/Spm-like transposons allowed the amplification of similar sequences from various plant species including sugar beet, wheat and pea. These primers are efficient tools for the detection of this family of transposons in many plant genomes irrespective of sequence knowledge or phenotypic pecularities. An efficient PCR assay was therefore developed for these class II transposons, similar to assays already available for Ty1-copia-, Ty3-gypsy- or LINEs. This approach allowed us not only to show the widespread almost-ubiquitous presence of En/Spm-elements in plant genomes, but also to characterize their genomic organization and chromosomal distribution in the genome of chickpea (Cicer arietinum L.) and its abundance in related Cicer species. This approach can be used for the detection and characterization of endogenous DNA transposable elements in plant species, their complete isolation and evaluation of their use for genome analysis.

Molecular structure and chromosomal localization of major repetitive DNA families in the chickpea (Cicer arietinum L.) genome

Three major repetitive DNA sequences were isolated from a genomic library of chickpea (Cicer arietinum L.) and characterized with respect to their genomic organization and chromosomal localization. All repetitive elements are genus-specific and mostly located in the AT-rich pericentric heterochromatin. Two families are organized as satellite DNAs with repeat lengths of 162-168 bp (CaSat1) and 100 bp (CaSat2). CaSat1 is mainly located adjacent to the 18S rDNA clusters on chromosomes A and B, whereas CaSat2 is a major component of the pericentric heterochromatin on all chromosomes. The high abundance of these sequences in closely related species of the genus Cicer as well as their variation in structure and copy number among the annual species provide useful tools for taxonomic studies. The retrotransposon-like sequences of the third family (CaRep) display a more complex organization and are represented by two independent sets of clones (CaRep1 and CaRep2) with homology to different regions of Ty3-gypsy-like retrotransposons. They are distributed over the pericentric heterochromatin block on all chromosomes with extensions into euchromatic regions. Conserved structures within different crossability groups of related Cicer species suggest independent amplification or transposition events during the evolution of the annual species of the genus.