Morris Levy

DNA Fingerprinting with a Dispersed Repeated Sequence Resolves Pathotype Diversity in the Rice Blast Fungus.

The poor definition of pathotype variation in the rice blast fungus has historically handicapped strategies for reducing blast disease damage to the worlds rice crop. We have employed a probe for a dispersed repeated DNA sequence called MGR [Hamer et al. (1989). Proc. Natl. Acad. Sci. USA 86, 9981-9985] to construct genotype-specific, EcoRl restriction fragment length profiles (MGR-DNA fingerprints) from United States field isolates of this fungus. By using a blind-test design, we demonstrated that MGR-DNA fingerprints distinguished the major pathotypes in the United States, accurately identified the pathotypes of isolates collected over a 30-year period, and defined the organization of clonal lineages within and among pathotype groups. These results resolved a lingering controversy regarding rice blast pathotype stability and illustrated new opportunities for tracking the population dynamics and evolution of this important crop pathogen.

Characterisation of the European pathogen population of Magnaporthe grisea by DNA fingerprinting and pathotype analysis

The genetic variability among 41 isolates of the blast pathogen (Magnaporthe grisea) from five European rice growing countries was studied. The genealogy of the isolates was investigated by DNA fingerprinting and the results compared to the degree of similarity for (a)virulence factors. Fingerprinting grouped the isolates into five discrete lineages, that typically showed less than 65% band similarity. Within each lineage, two or more haplotypes were detected with a band similarity of 80% or higher. Each lineage showed a characteristic virulence pattern. All isolates of lineage ‘E5’ belonged to the same pathotype. The other lineages were composed of clusters of closely related pathotypes that showed variation for virulence to cultivars with certain known resistance genes, while remaining invariably (a)virulent to others. In most cases, lineage classification of an isolate could be easily inferred by its pathotype. Certain resistance genes and certain lineage-excluding resistance gene combinations appear to provide protection against all of the virulence factors sampled.

Pedigree assessment using RAPD-DGGE in cereal crop species

SummaryThe introduction of molecular biology methodologies to plant improvement programs offers an invaluable opportunity for extensive germplasm characterization. However, the detection of adequate DNA polymorphism in self-pollinating species remains on obstacle. We have optimized a denaturing-gradient-gel electrophoresis (DGGE) system which, when used in combination with random amplified polymorphic DNA (RAPD) analysis, greatly facilitates the detection of reproducible DNA polymorphism among closely related plant lines. We have used this approach to estimate pedigree relationships among a spectrum of plant materials in wheat, barley and oat. Based on analysis with one or two primers, we were able to distinguish soft from hard winter wheat, and 2-rowed from 6-rowed barley. Further analysis with additional primers allowed resolution of polymorpisms even among closely related lines in highly selected populations. We placed 17 cultivars of oat into two distinct clusters that differed significantly from previous oat pedigree assessments. We believe that DGGE-RAPD is a superior method for detecting DNA polymorphism when compared to RFLP, agarose-RAPD, or polyacrylamide-RAPD methods.

Sibling Species of Cercospora Associated with Gray Leaf Spot of Maize

ABSTRACT Monoconidial isolates of the fungus causing gray leaf spot of maize were obtained from diseased leaves collected throughout the United States and analyzed for genetic variability at 111 amplified fragment length polymorphism (AFLP) loci. Cluster analysis revealed two very distinct groups of Cercospora zeae-maydis isolates. Both groups were found to be relatively uniform internally with an average genetic similarity among isolates of approximately 93 and 94%, respectively. The groups were separated from each other by a genetic distance of approximately 80%, a distance greater than that separating each group from the sorghum pathogen, C. sorghi (67 to 70%). Characteristics and dimensions of conidia and conid-iophores produced on infected plants or nutrient media were unreliable criteria for taxonomic differentiation of isolates composing the two groups of C. zeae-maydis. Nucleotide sequences of 5.8S ribosomal DNA (rDNA) and the internal transcribed spacer (ITS) regions were identical within each group but different between the two groups and different from C. sorghi. Restriction fragment length polymorphisms generated by digestion of the 5.8S rDNA and ITS regions with TaqI readily distinguished each group and C. sorghi. Isolates in one group were generally distributed throughout maize-producing regions of the United States; isolates in the other group were localized in the eastern third of the country. Both types were present in the same fields at some locations. The genetic distance based on AFLP profiles and different ITS nucleotide sequences between the two morphologically indistinguishable groups indicate that they are sibling species. Although it is unlikely that breeding for resistance to gray leaf spot will be confounded by local or regional variation in the pathogen, a vigilant approach is warranted, because two pathogenic species exist with unknown abilities to evolve new pathotypes.

Genetic Relatedness of African and United States Populations of Cercospora zeae-maydis

Two taxonomically identical but genetically distinct sibling species, designated groups I and II, of Cercospora zeae-maydis cause gray leaf spot of maize in the United States. Isolates of the gray leaf spot pathogen from Africa were compared with isolates from the United States by amplified fragment length polymorphism (AFLP) analysis and restriction digests of internal transcribed spacer (ITS) regions and 5.8S ribosomal DNA (rDNA), as well as by morphological and cultural characteristics. The isolates from Africa were morphologically indistinguishable from the U.S. isolates in both groups, but like isolates of group II, they grew more slowly and failed to produce detectable amounts of cercosporin in culture. Analysis of restriction fragments from the ITS and rDNA regions digested with five endonucleases indicated that all of the African isolates shared the profile of the C. zeae-maydis group II population from the eastern United States and, thus, are distinct from the group I population, which is more prevalent in the United States and other parts of the world. Cluster analysis of 85 AFLP loci confirmed that the African and U.S. group II populations were conspecific (greater than 97% average similarity) with limited variability. Among all group II isolates, only 8 of 57 AFLP loci were polymorphic, and none was specific to either population. Thus, although gray leaf spot was reported in the United States several decades prior to the first record in Africa, the relative age of the two populations on their respective continents could not be ascertained with confidence. The absence of C. zeae-maydis group I in our samples from four countries in the major maize-producing region of Africa as well as the greater AFLP haplotype diversity found in the African group II population, however, suggest that Africa was the source of C. zeae-maydis group II in the United States. The overall paucity of AFLP variation in this sibling species further suggests that its origin is recent or that the ancestral population experienced a severe bottleneck prior to secondary migration.

Intraspecies differentiation in the powdery mildew Erysiphe cichoracearum determined with rDNA RFLPs

The powdery mildew species Erysiphe cichoracearum has a described host range of over 300 plant species from among several families. Host‐range testing indicates host‐specialized subdivision within this taxonomic species. However, the extent of subdivision remains largely undetermined among host‐limited forms. We have characterized diversity among field collections of E. cichoracearum from a variety of hosts, and from other powdery mildew species, with RFLPs from a PCR amplified ribosomal DNA (rDNA) segment The E. cichoracearum samples expressed six distinct RFLP haplotypes. Each haplotype was specific to either a single host or to a set of related host species. These haplotypes formed a continuum of divergence ranging from about 18–35% average pairwise distance from one another, while those from other mildew species clustered at consistently higher average pairwise distances from E. cichoracearum and from each other. Our findings support earlier suggestions, based on host‐range and morphological characterizations, that E. cichoracearum is a complex of morphologically similar, but host‐limited forms. Also, comparisons of rDNA haplotype distance between E. cichoracearum and Blumeria (Erysiphe) graminis were consistently greater than between E. cichoracearum and Sphaerotheca fulginea. This result supports earlier questions concerning the monophyletic nature of Erysiphe.

Alcohol dehydrogenase activity in diploid and autotetraploid Phlox

The effect of polyploidy on the activity of alcohol dehydrogenase was examined in a series of diploid and synthetic autotetraploid Phlox drummondii. In most cases autotetraploids had about twice as much activity as the corresponding diploids, in two cases autotetraploids had about 1.5 times more activity, and in one wild seed pair the activity of the tetraploid was somewhat lower.

Allozyme genetics in permanent translocation heterozygotes of the Oenothera biennis complex.

Allozyme inheritance and transmission genetics of 11 enzyme systems were determined in the permanent translocation heterozygotes Oenothera biennis, Oe. strigosa, and Oe. parviflora. Electrophoretic variation was examined first among 164 strains of structural heterozygotes. Allelic configurations were then judged from inheritance patterns in reciprocal F1 hybrids between each of 22 ring-forming strains and tester strains of the related bivalent-formers, Oe. hookeri and Oe. grandiflora. Allozymes are inherited as codominant markers, and, as dictated by the genetic system, within a strain individual allelic variants are generally transmitted through only one germ line. Of the 20 loci resolved, only eight are polymorphic in any species, and, within species, generally only two alleles are present at each polymorphic locus. Despite the relatively meager allelic array, each of the 22 strains whose chromosome complexes were characterized is genotypically unique. Generally, within taxa, α (egg) and β (sperm) complexes differ in allele frequency at several polymorphic loci. Such variability is correlated with differences in the phylogenetic origins of complexes and not with differences in segmental arrangement within a group of related complexes.

Geographic Variation of Flavonoids in Phlox carolina

Non-anthocyanin flavonoid expression was determined among 64 continuous range and nine disjunct range populations of Phlox carolina. The species contains four flavone nuclei and a total of 25 derivatives, most of which are C-glycosides. Nearly all populations express unique flavone patterns. Using a qualitative similarity index of minimum biosynthetic step distance (MBSD), the average population contains only ca. 50% of the overall substitutional diversity. The separate ranges are distinctive in the % occurrence of most compounds but only weakly divergent in comparative mean MBSD. Proximity and MBSD are significantly correlated overall but local variability is extensive and no discrete racism is apparent. Within the continuous range 18 compounds are distributed in independent but overlapping gradients and 13 of these extend consistently through the disjunct range. The patch-work continuum of flavone variation is the apparent consequence of dissection and contraction of a historically more widespread distribution. Other general aspects of the variation patterns are discussed.

Secondary intergradation and genome incompatibility in Phlox pilosa (polemoniaceae)

Phlox pilosa ssp.pilosa andP. pilosa ssp.fulgida form a zone of secondary intergradation extending from northern Illinois and Indiana along the prairie-forest border to southwestern Missouri. The structure of the zone was analyzed in Illinois and Indiana. Trichome morphology offered the only diagnostic characteristic. Character gradients in the zone are highly irregular and steeply sloping. Closely adjacent populations often have disparate phenotype distributions. Most populations are strongly skewed toward the parental ends of the morphological spectrum. The properties of the zone are attributed to genome incompatibility, which is seen in the crossability barriers between the two subspecies and between the subspecies and hybrids, a reduction in pollen fertility, anther size and fecundity in hybrids, and the formation of multiple nucleoli in the hybrids. Subspeciesfulgida appears to be the primary recipient of alien gene flow. However, the position of the zone ostensibly is stabilized by the presence of an ecological barrier.