Three RFLP maps, as well as several RAPD maps have been developed in common bean (Phaseolus vulgaris L.). In order to align these maps, a core linkage map was established in the recombinant inbred population BAT93;Jalo EEP558 (BJ). This map has a total length of 1226 cM and comprises 563 markers, including some 120 RFLP and 430 RAPD markers, in addition to a few isozyme and phenotypic marker loci. Among the RFLPs mapped were markers from the University of California, Davis (established in the F of the BJ cross), University of Paris-Orsay, and University of Florida maps. These shared markers allowed us to Communicated by P. M. A. Tigerstedt

Honduras, Nicaragua, and Costa Rica, a convention that is accepted within this region. "Middle America" refers More than 60% of common bean production worldwide is derived to the combined regions of Central America and from cultivars of Middle American origin. Understanding the diversity of these will facilitate their use in genetic improvement. The objective Mexico.of this study was to analyze a collection of 269 landraces of common Singh et al. (1991a) proposed that within each gene bean (Phaseolus vulgaris L.) by correspondence analysis of random pool three races could be distinguished on the basis of amplified polymorphic DNA (RAPD) data to determine the genetic differences in plant and seed morphology and adaptastructure of the Middle American gene pool of cultivated bean. One tion regimes. Growth habit is an important distinguishhundred eighty landraces originating in Mexico, the remainder in ing criterion, and is classed as Type 1 (determinate Central America and secondary centers of diversity within the Ameribush), Type 2 (indeterminate upright bush), Type 3 cas, and two checks were studied. DNA was extracted, RAPD reac-(indeterminate semi-viney prostrate), and Type 4 (indetions carried out, and polymorphic bands were scored as present or terminate climbing) (CIAT, 1987). Within the Middle absent on the basis of 39 primers. Groups were formed which in American gene pool race Mesoamerica (M) is common part corresponded to races defined previously by morphological and agroecological criteria. However, tropical small-seeded Race M was to both Mexico and Central America, and is charactercomposed of two groups: one largely Mexican that included most ized by relatively small seed and warm lowland adaptasmall-seeded black beans of upright plant habit; and one Central tion. Most Race M landraces have habits of Type 2 or American with landraces of various seed colors. Most non-black small-3, although some have Type 4 habit. Commercial classes seeded germplasm of Race M phenotype from secondary centers within Race M include small black, small Central Amerigrouped with the Central American landraces, except for creamcan red and navy beans. Race Durango (D) is composed seeded and purple-seeded accessions from Brazil. Races D and J principally of growth habit Type 3 genotypes with small could be distinguished and within races D and J further divisions leaves, medium size seed, and adaptation to dry highcould be recognized which were related to geographic origin. The land areas of Mexico. Commercial Race D classes inmore commercial Race D landraces formed a genetic group that was clude pinto, great northern, and small red Mexican predominant in the western part of the Mexican highland plateau. Another Race D group was concentrated at the eastern extreme of beans. Race Jalisco (J) is found in the more humid the neovolcanic axis and was differentiated morphologically as well.

A culture-independent survey of the soil microbial diversity in a clover-grass pasture in southern Wisconsin was conducted by sequence analysis of a universal clone library of genes coding for small-subunit rRNA (rDNA). A rapid and efficient method for extraction of DNA from soils which resulted in highly purified DNA with minimal shearing was developed. Universal small-subunit-rRNA primers were used to amplify DNA extracted from the pasture soil. The PCR products were cloned into pGEM-T, and either hypervariable or conserved regions were sequenced. The relationships of 124 sequences to those of cultured organisms of known phylogeny were determined. Of the 124 clones sequenced, 98.4% were from the domain Bacteria. Two of the rDNA sequences were derived from eukaryotic organelles. Two of the 124 sequences were of nuclear origin, one being fungal and the other a plant sequence. No sequences of the domain Archaea were found. Within the domain Bacteria, three kingdoms were highly represented: the Proteobacteria (16.1%), the Cytophaga-Flexibacter-Bacteroides group (21.8%), and the low-G؉C-content gram-positive group (21.8%). Some kingdoms, such as the Thermotogales, the green nonsulfur group, the Fusobacteria, and the Spirochaetes, were absent. A large number of the sequences (39.4%) were distributed among several clades that are not among the major taxa described by Olsen et al. (G.

Anthracnose, one of the most important diseases of common bean (Phaseolus vulgaris), is caused by the fungus Colletotrichum lindemuthianum. A "candidate gene" approach was used to map anthracnose resistance quantitative trait loci (QTL). Candidate genes included genes for both pathogen recognition (resistance genes and resistance gene analogs [RGAs]) and general plant defense (defense response genes). Two strains of C. lindemuthianum, identified in a world collection of 177 strains, displayed a reproducible and differential aggressiveness toward BAT93 and JaloEEP558, two parental lines of P. vulgaris representing the two major gene pools of this crop. A reliable test was developed to score partial resistance in aerial organs of the plant (stem, leaf, petiole) under controlled growth chamber conditions. BAT93 was more resistant than JaloEEP558 regardless of the organ or strain tested. With a recombinant inbred line (RIL) population derived from a cross between these two parental lines, 10 QTL were located on a genetic map harboring 143 markers, including known defense response genes, anthracnose-specific resistance genes, and RGAs. Eight of the QTL displayed isolate specificity. Two were co-localized with known defense genes (phenylalanine ammonia-lyase and hydroxyproline-rich glycoprotein) and three with anthracnose-specific resistance genes and/or RGAs. Interestingly, two QTL, with different allelic contribution, mapped on linkage group B4 in a 5.0 cM interval containing Andean and Mesoamerican specific resistance genes against C. lindemuthianum and 11 polymorphic fragments revealed with a RGA probe. The possible relationship between genes underlying specific and partial resistance is discussed.

White mold (WM), incited by Sclerotinia sclerotiorum (Lib.) de Bary, is a serious disease of common bean (Phaseolus vulgaris L.). However, plant breeders have had very limited success in developing resistant (R) cultivars. Molecular markers linked to genes for R to WM may improve selection for R. The objective was to identify random amplified polymorphic DNA (RAPD) markers linked to quantitative trait loci (QTL) for partial physiological resistance (PPR), partial field resistance (PFR), porosity over the furrow (POF), and plant height (PH) in a linkage map by means of recombinant inbred lines (RILs) from the cross ‘PC‐50’ (R) × XAN‐159 (susceptible). The parents and RILs were inoculated in two separate greenhouse experiments for each isolate and were also infected naturally in the field. Significant correlations (0.39, 0.47) were found for the WM reactions in the greenhouse and field. Nine candidate QTL were found affecting PPR isolate 152 (comparison‐wise P < 0.05) with strong evidence (genome‐wise P < 0.01) for three QTL on linkage groups (LGs) 4, 7, and 8, based on composite interval mapping analysis. Candidate QTL affecting PPR to isolate 279 were found on LGs 2, 3, 4, 7, and 8 with very strong evidence (genome‐wise P < 0.001) for a QTL linked to the C locus for seedcoat pattern. Seven candidate QTL for PFR were observed on LGs 4, 7, 8, and 11. Six of the seven candidate QTL for PFR were found in the same locations as QTL for PPR. However, two of the seven genomic regions were associated with PFR and POF that may contribute to disease avoidance.