J. P. Murphy

A comparison of RAPD and isozyme analyses for determining the genetic relationships among Avena sterilis L. accessions

Isozyme analysis is a valuable tool for determining genetic relationships among breeding lines and populations. The recently developed DNA technologies which can assay a greater proportion of the plant genome are providing a plentiful array of additional genomic markers. The objective of this research was to compare random amplified polymorphic DNA (RAPD) versus isozyme-based estimation of relationships among 24 accessions of a hexaploid wild oat, Avena sterilis L. The accessions were evaluated for variation in 23 enzyme systems and by 21 10-mer primers. A total of 77 polymorphic isozyme bands and 115 polymorphic RAPD bands were observed. Two matrices of genetic distances were estimated based on band presence/ absence. These matrices were subsequently utilized in cluster analysis and principal coordinate analysis. Both isozymes and RAPDs were proficient at distinguishing between the 24 accessions. The correspondence between the elements of both distance matrices was moderate (r=0.36**). Nevertheless, the overall representation of relationships among accessions by cluster analysis and ordination was in considerable agreement. The two techniques contrasted most notably in pair-by-pair comparisons of relationships. RAPD analysis resulted in a more definitive separation of clusters of accessions. The most significant impact of the DNA-based markers probably will be the more accurate determination of relationships between accessions that are too close to be accurately differentiated by isozymes.

Pm34 : a new powdery mildew resistance gene transferred from Aegilops tauschii Coss. to common wheat ( Triticum aestivum L . )

Powdery mildew is a major fungal disease in wheat growing areas worldwide. A novel source of resistance to wheat powdery mildew present in the germplasm line NC97BGTD7 was genetically characterized as a monogenic trait in greenhouse and field trials using F2 derived lines from a NC97BGTD7 X Saluda cross. Microsatellite markers were used to map and tag this resistance gene, now designated Pm34. Three co-dominant microsatellite markers linked to Pm34 were identified and their most likely order was established as: Xbarc177-5D, 5.4cM, Pm34, 2.6cM, Xbarc144-5D, 14cM, Xgwm272-5D. These microsatellite markers were previously mapped to the long arm of the 5D chromosome and their positions were confirmed using Chinese Spring nullitetrasomic Nulli5D-tetra5A and ditelosomic Dt5DL lines. Pm2, the only other known Pm gene on chromosome 5D, has been mapped to the short arm and its specificity is different from that of Pm34.

The effect of parental divergence on F2 heterosis in winter wheat crosses.

SummaryIn winter wheat (Triticum aestivum L.), the development of a methodology to estimate genetic divergence between parental lines, when combined with knowledge of parental performance, could be beneficial in the prediction of bulk progeny performance. The objective of this study was to relate F2 heterosis for grain yield and its components in 116 crosses to two independent estimates of genetic divergence among 28 parental genotypes of diverse origins. Genetic divergence between parents was estimated from (a) pedigree relationships (coefficients of kinship) determined without experimentation, and (b) quantitative traits measured in two years of field experimentation in Kansas and North Carolina, USA. These distances, designated (1 -r) and G, respectively, provided ample differentiation among the parents. The 116 F2 bulks were evaluated at four locations in Kansas and North Carolina in one year. Significant rank correlations of 0.46 (P = 0.01) and 0.44 (P = 0.01) were observed between G and grain yield and kernel number heterosis, respectively. Although (1 -r) was poorly associated with grain yield heterosis, G and midparent performance combined to account for 50% of the variation in F2 yields among crosses when (1 -r) was above the median value, whereas they accounted for only 9% of the variation among crosses when (1-r) was below the median. Midparent and (1 -r) had equal effects on F2 grain yield (R2= 0.40) when G was greater than the median value. A breeding strategy is proposed whereby parents are first selected on the basis of performance per se and, subsequently, crosses are made between genetically divergent parents that have both large quantitative (G) and pedigree divergence (1 -r).

Variation of isozyme patterns among Arachis species

The genus Arachis contains a large number of species and undescribed taxa with patterns of genetic variation that are little understood. The objectives of this investigation were to estimate genetic diversity among species of Arachis by utilizing electrophoretic techniques and to establish the potential for use of isozymes as markers for germplasm introgression. One-hundred-and-thirteen accessions representing six of the seven sections of the genus were analyzed for isozyme variation of 17 enzymes. Section Rhizomatosae species were not included because they produce very few seeds. Seeds were macerated and the crude extract was used for starch-gel electrophoretic analyses. Although the cultivated species has few polymorphic isozymes, the diploid species are highly variable and two-to-six bands were observed for each isozyme among accessions. Because of the large number of isozyme differences between A. hypogaea and A. batizocoi (the presumed donor of the B genome), this species can no longer be considered as a progenitor of the cultivated peanut. Seed-to-seed polymorphisms within many accessions were also observed which indicate that germplasm should be maintained as bulk seed lots, representative of many individuals, or as lines from individual plants from original field collections. The area of greatest interspecific genetic diversity was in Mato Grosso, Brazil; however, the probability of finding unique alleles from those observed in A. hypogaea was greatest in north, north-central, south and southeast Brazil. The large number of polymorphic loci should be useful as genetic markers for interspecific hybridization studies.

Isozyme variation in germplasm accessions of the wild oat Avena sterilis L.

SummaryOptimal exploitation of crop genetic resources requires a knowledge of the range and structure of the variation present in the gene pool of interest. Avena sterilis L., the cultivated oat progenitor, contains a store of genetic diversity that is readily accessible to the oat breeder. The objectives of the present paper were: (1) to evaluate isozyme polymorphisms in a sample of A. sterilis accessions from the U.S. National Small Grains Collection, (2) to analyze the distribution of isozyme diversity across the geographic range of the accessions, (3) to classify the accessions into groups based on isozyme variation, and (4) to suggest strategies for efficient sampling of this germplasm collection. One thousand and five accessions from 23 countries and 679 collection sites were screened for variation using 23 enzyme systems. Due to limited information about the genetic relationship among individual members of families of isozymes in hexaploid oat species, data were recorded solely for band presence. The frequencies of bands in accessions from the various countries were used to calculate the probability of genotypic identity (Ix.y), the probability of a unique genotype (Ux.y), and an adjusted polymorphic index (Hx). Accessions from Turkey and Lebanon had the largest polymorphic index values, Turkish and Moroccan accessions displayed the greatest numbers of bands. Accessions from Iran, Turkey, Iraq, and Lebanon had the largest mean probabilities of containing unique genotypes. Based on isozyme data, Turkey appeared to represent the center of diversity in this germplasm collection. Band frequencies calculated among countries were used in a principal component analysis. Accessions from Israel and Morocco clustered together; accessions from Iran, Iraq, Turkey, and Ethiopia formed another group; and Algerian accessions formed an outlying group. Several isozyme bands had a regional distribution. These results suggested that choosing accessions from countries based on their groupings in the principal component analysis should secure a greater range of diversity than sampling from the collection at random. Cluster analyses based on Jaccards distances calculated for all pairwise combinations of the 1005 accessions revealed six broad genetic groups of accessions. Groups 1 and 6 contained accessions from many countries and encompassed half of all accessions. Groups 2 and 4 were heavily populated by accessions from Israel and Morocco. Groups 3 and 5 were composed almost exclusively of accessions from Iran, Iraq, and Turkey. By selecting representative accessions from these six groups, oat breeders could most effectively sample the range of genetic variation in this A. sterilis collection.