Random-amplified-polymorphic DNA markers in sorghum

Identification and mapping of a leaf rust resistance gene in barley line Q21861

Barley line Q21861 possesses an incompletely dominant gene (RphQ) for resistance to leaf rust caused by Puccinia hordei. To investigate the allelic and linkage relations between RphQ and other known Rph genes, F2 populations from crosses between Q21861 and donors of Rph1 to Rph14 (except for Rph8) were evaluated for leaf rust reaction at the seedling stage. Results indicate that RphQ is either allelic with or closely linked to the Rph2 locus. A doubled haploid population derived from a cross between Q21861 and SM89010 (a leaf rust susceptible line) was used for molecular mapping of the resistance locus. Bulked segregant analysis was used to identify markers linked to RphQ, using random amplified polymorphic DNAs (RAPDs), restriction fragment length polymorphisms (RFLPs), and sequence tagged sites (STSs). Of 600 decamer primers screened, amplified fragments generated by 9 primers were found to be linked to the RphQ locus; however, only 4 of them were within 10 cM of the target. The RphQ locus was mapped to the centromeric region of chromosome 7, with a linkage distance of 3.5 cM from the RFLP marker CDO749. Rrn2, an RFLP clone from the ribosomal RNA intergenic spacer region, was found to be very closely linked with RphQ, based on bulked segregant analysis. An STS marker, ITS1, derived from Rrn2, was also closely linked (1.6 cM) to RphQ.

DNA markers in plant improvement: an overview

The progress made in DNA marker technology has been tremendous and exciting. DNA markers have provided valuable tools in various analyses ranging from phylogenetic analysis to the positional cloning of genes. The development of high-density molecular maps which has been facilitated by PCR-based markers, have made the mapping and tagging of almost any trait possible. Marker-assisted selection has the potential to deploy favorable gene combinations for disease control. Comparative studies between incompatible species using these markers has resulted in synteny maps which are useful not only in predicting genome organization and evolution but also have practical application in plant breeding. DNA marker technology has found application in fingerprinting genotypes, in determining seed purity, in systematic sampling of germplasm, and in phylogenetic analysis. This review discusses the use of this technology for the genetic improvement of plants.

Comparative Assessment of Variation among Sorghum Germplasm Accessions Using Seed Morphology and RAPD Measurements

The sorghum germplasm collection currently contains over 42 000 accessions, a number that is too large to manage efficiently. The specific objective of this research was to compare clusters developed from agronomic descriptors with phylogenetic groupings based on random amplified polymorphic DNA (RAPD) fingerprinting of selected sorghum [Sorghum bicolor (L.) Moench] races. Our intent was to identify one approach using agronomic descriptors that would most closely approximate the groupings produced by RAPD markers. Ninety-four accessions of sorghum were grouped into four of the five major races. Differences among accessions determined by various clustering procedures based on agronomic characteristics were compared with clusters developed by means of RAPD markers. Each race varied in the degree of similarity between the four clustering approaches taken and the information provided by RAPD fingerprinting. Test 2, standardization of data by Z-scores and cluster analysis using the complete set of data, provided the highest similarity score for the race bicolor, while Test 3, standardization of data by Z-scores and cluster analysis based on a reduced set of variables selected from principle component analysis, provided the highest similarity scores for the races guinea. Test 1, random selection, was highest for the races caudatum and durra. When averaged over all the races, Test 2 provided the highest similarity score. The results of this study indicate that no one approach to develop clusters by means of agronomic descriptors closely approximate the groupings produced by RAPD markers. These results underscore the need for further research in the evaluation of techniques used to develop core collections and their validity.

Application of inter simple sequence repeat (ISSR) markers to plant genetics

Microsatellites or simple sequence repeats (SSRs) are ubiquitous in eukaryotic genomes. Single-locus SSR markers have been developed for a number of species, although there is a major bottleneck in developing SSR markers whereby flanking sequences must be known to design 5'-anchors for polymerase chain reaction (PCR) primers. Inter SSR (ISSR) fingerprinting was developed such that no sequence knowledge was required. Primers based on a repeat sequence, such as (CA)n, can be made with a degenerate 3'-anchor, such as (CA)8RG or (AGC)6TY. The resultant PCR reaction amplifies the sequence between two SSRs, yielding a multilocus marker system useful for fingerprinting, diversity analysis and genome mapping. PCR products are radiolabelled with 32P or 33P via end-labelling or PCR incorporation, and separated on a polyacrylamide sequencing gel prior to autoradiographic visualisation. A typical reaction yields 20-100 bands per lane depending on the species and primer. We have used ISSR fingerprinting in a number of plant species, and report here some results on two important tropical species, sorghum and banana. Previous investigators have demonstrated that ISSR analysis usually detects a higher level of polymorphism than that detected with restriction fragment length polymorphism (RFLP) or random amplified polymorphic DNA (RAPD) analyses. Our data indicate that this is not a result of greater polymorphism genetically, but rather technical reasons related to the detection methodology used for ISSR analysis.