Domestication patterns in common bean (Phaseolus vulgaris L.) and the origin of the Mesoamerican and Andean cultivated races

Chloroplast DNA polymorphisms were studied by PCR sequencing and PCR-restriction fragment length polymorphism in 165 accessions of domesticated landraces of common bean from Latin America and the USA, 23 accessions of weedy beans, and 134 accessions of wild beans covering the entire geographic range of wild Phaseolus vulgaris. Fourteen chloroplast haplotypes were identified in wild beans, only five of which occur also in domesticated beans. The chloroplast data agree with those obtained from analyses based on morphology and isozymes and with other DNA polymorphisms in supporting independent domestications of common bean in Mesoamerica and the Andean region and in demonstrating a founder effect associated with domestication in each region. Andean landraces have been classified into three different racial groups, but all share the same chloroplast haplotype. This suggests that common bean was domesticated once only in South America and that the races diverged post-domestication. The haplotype found in Andean domesticated beans is confined to the southern part of the range of wild beans, so Andean beans were probably domesticated somewhere within this area. Mesoamerican landraces have been classified into four racial groups. Our limited samples of Races Jalisco and Guatemala differ from the more widespread and commercially important Races Mesoamerica and Durango in types and/or frequencies of haplotypes. All four Mesoamerican races share their haplotypes with local wild beans in parts of their ranges. Independent domestications of at least some of the races in Mesoamerica and/or conversion of some locally adapted wild beans to cultigens by hybridization with introduced domesticated beans, followed by introgression of the domestication syndrome seem the most plausible explanations of the chloroplast and other molecular data.     

Structure of genetic diversity in the two major gene pools of common bean (Phaseolus vulgaris L., Fabaceae)

Domesticated materials with well-known wild relatives provide an experimental system to reveal how human selection during cultivation affects genetic composition and adaptation to novel environments. In this paper, our goal was to elucidate how two geographically distinct domestication events modified the structure and level of genetic diversity in common bean. Specifically, we analyzed the genome-wide genetic composition at 26, mostly unlinked microsatellite loci in 349 accessions of wild and domesticated common bean from the Andean and Mesoamerican gene pools. Using a model-based approach, implemented in the software STRUCTURE, we identified nine wild or domesticated populations in common bean, including four of Andean and four of Mesoamerican origins. The ninth population was the putative wild ancestor of the species, which was classified as a Mesoamerican population. A neighbor-joining analysis and a principal coordinate analysis confirmed genetic relationships among accessions and populations observed with the STRUCTURE analysis. Geographic and genetic distances in wild populations were congruent with the exception of a few putative hybrids identified in this study, suggesting a predominant effect of isolation by distance. Domesticated common bean populations possessed lower genetic diversity, higher F _ST, and generally higher linkage disequilibrium (LD) than wild populations in both gene pools; their geographic distributions were less correlated with genetic distance, probably reflecting seed-based gene flow after domestication. The LD was reduced when analyzed in separate Andean and Mesoamerican germplasm samples. The Andean domesticated race Nueva Granada had the highest F _ST value and widest geographic distribution compared to other domesticated races, suggesting a very recent origin or a selection event, presumably associated with a determinate growth habit, which predominates in this race. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Patterns of genetic diversity in the Andean gene pool of common bean reveal a candidate domestication gene

Most studies on the genetic diversity of common bean (Phaseolus vulgaris L.) have focussed on accessions from the Mesoamerican gene pool compared to the Andean gene pool. A deeper knowledge of the genetic structure of Argentinian germplasm would enable researchers to determine how the Andean domestication event affected patterns of genetic diversity in domesticated beans and to identify candidates for genes targeted by selection during the evolution of the cultivated common bean. A collection of 116 wild and domesticated accessions representing the diversity of the Andean bean in Argentina was genotyped by means of 114 simple sequence repeat (SSR) markers. Forty-seven Mesoamerican bean accessions and 16 Andean bean accessions representing the diversity of Andean landraces and wild accessions were also included. Using the Bayesian algorithm implemented in the software STRUCTURE we identified five major groups that correspond to Mesoamerican and Argentinian wild accessions and landraces and a group that corresponds to accessions from different Andean and Mesoamerican countries. The neighbour-joining algorithm and principal coordinate clustering analysis confirmed the genetic relationships among accessions observed with the STRUCTURE analysis. Argentinian accessions showed a substantial genetic variation with a considerable number of unique haplotypes and private alleles, suggesting that they may have played an important role in the evolution of the species. The results of statistical analyses aimed at identifying genomic regions with consistent patterns of variation were significant for 35 loci (~20 % of the SSRs used in the Argentinian accessions). One of these loci mapped in or near the genomic region of the glutamate decarboxylase gene. Our data characterize the population structure of the Argentinian germplasm. This information on its diversity will be very valuable for use in introgressing Argentinian genes into commercial varieties because the majority of present-day common bean varieties are of Andean origin.     

Phylogenetic study on wild allies of Lima bean,Phaseolus lunatus (Fabaceae), and implications on its origin

An investigation was made of the phylogenetic relationships among wild accessions of Lima bean (Phaseolus lunatus) and wild allies of Mesoamerican and Andean origins, using electrophoresis of seed storage proteins and isozymes. Mesoamerican wild species are phylogenetically more distant fromP. lunatus than Andean species, and apparently belong to the tertiary gene pool of Lima bean. The Andean wild species, which are investigated for the first time, reveal a high similarity to the Lima bean, and particularly with its Mesoamerican gene pool. These Andean species probably constitute a secondary gene pool of Lima bean, and are thus of considerable interest in the context of genetic improvement of the crop. Based on these observations, an Andean origin is suggested for the Andean wild species and forP. lunatus. These results point out the importance of collecting and conserving AndeanPhaseolus germplasm.     

Diversity in the rhizobia associated with Phaseolus vulgaris L. in Ecuador, and comparisons with Mexican bean rhizobia

Common beans (Phaseolus vulgaris L.) have centers of origin in both Mesoamerica and Andean South America, and have been domesticated in each region for perhaps 5000 years. A third major gene pool may exist in Ecuador and Northern Peru. The diversity of the rhizobia associated with beans has also been studied, but to date with an emphasis on the Mesoamerican center of origin. In this study we compared bean rhizobia from Mexico and Andean South America using both phenotypic and phylogenetic approaches. When differences between the rhizobia of these two regions were shown, we then examined the influence of bean cultivar on the most probable number (MPN) count and biodiversity of rhizobia recovered from different soils. Three clusters of bean rhizobia were distinguished using phenotypic analysis and principal-component analysis of Box AIR-PCR banding patterns. They corresponded principally to isolates from Mexico, and the northern and southern Andean regions, with isolates from southern Ecuador exhibiting significant genetic diversity. Rhizobia from Dalea spp., which are infective and effective on beans, may have contributed to the apparent diversity of rhizobia recovered from the Mesoamerican region, while the rhizobia of wild Phaseolus aborigineus from Argentina showed only limited similarity to the other bean rhizobia tested. Use of P. vulgaris cultivars from the Mesoamerican and Andean Phaseolus gene pools as trap hosts did not significantly affect MPN counts of bean rhizobia from the soils of each region, but did influence the diversity of the rhizobia recovered. Such differences in compatibility of host and Rhizobium could be a factor in the poor reputation for nodulation and N2 fixation in this crop.     

Phosphorus uptake by bean nodules

As part of a breeding program to improve the nitrogen-fixing symbiosis between common bean (Phaseolus vulgaris) and Rhizobium etli, we developed a rapid screen for common bean accessions that preferentially nodulate with KIM5s, a high nitrogen fixing strain of R. etli. We constructed a mutant of KIM5s that did not fix nitrogen (Fix^-) but was otherwise indistinguishable from KIM5s. We screened plants for symptoms of nitrogen deficiency when grown in a Honduran soil containing indigenous common bean-nodulating rhizobia (10⁴ per gram) and KM6001, the Fix^- mutant of KIM5s (10⁴/seedling added 7 days after planting). Leaf color was scored on a scale of 1 to 5, in which 1 was dark green and 5 was bright yellow. Of 820 genetically diverse accessions of P. vulgaris screened, 51 were scored 1, 626 were scored 2 or 3, and 143 were scored 4 or 5. Selfed seed was produced from common bean plants of the accessions scored 1, 4 or 5. Twenty-four accessions that scored 1, and 58 that scored 4 or 5 were screened in soil containing indigenous rhizobia and the wild type KIM5s (Fix₊), and nodule occupancy was determined by antibiotic resistance. On the 24 common bean accessions that were scored 1, KIM5s occupied 0-6% of the nodules, on 26 of the accessions that were scored 4 or 5, KIM5s occupied 90%-100% of the nodules, and on the remaining 34 that scored 4 or 5, there was a distribution of nodule occupancy. Foliar color was highly correlated with nodule occupancy (r = 0.786,p = 0.01). The results indicate that the rapid visual screen using the Fix- mutant accurately identified common bean accessions that preferentially nodulate with the wild-type KIM5s (Fix⁺) strain in soil containing indigenous rhizobia. This screen will facilitate introduction of the preferential nodulation trait into superior cultivars and provides the foundation for studies of the genetic basis of preferential nodulation.     

中国普通菜豆形态性状分析及分类

对129份中国普通菜豆地方品种的形态性状进行分析,结果表明,8个性状共检测到35个变异类型,平均变异类型为4.375个,平均多态信息含量为0.5638。中国普通菜豆包括安第斯和中美两个基因库种质,中美洲基因库资源在参试资源中比重较大,但安第斯基因库资源遗传多样性水平高于中美基因库材料。由中美基因库向安第斯基因库渗透的天然杂交种质可为普通菜豆高产、优质、抗逆育种提供有价值的桥梁品种。     

Isolation and characterization of a new repetitive DNA family recently amplified in the Mesoamerican gene pool of the common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L., Fabaceae)

The common bean (Phaseolus vulgaris) is one of the most important crop plants. About 50% of its genome is composed of repetitive sequences, but only a little fraction was isolated and characterized so far. In this paper, a new repetitive DNA family from the species, named PvMeso, was isolated and characterized in both gene pools of P. vulgaris (Andean and Mesoamerican) and related species. Two fragments, 1.7 and 2.3 kb long, were cloned from BAC 255F18, which has previously shown a repetitive pattern. The subclone PvMeso-31 showed a terminal block in chromosome 7. This subclone contains a 1,705 bp long, AT-rich repeat with small internal repeats and shares a 1.2 kb region with PvMeso-47, derived from the 2.3 kb fragment. The presence of this repetitive block was restricted to Mesoamerican accessions of the common bean. In P. acutifolius, P. leptostachyus and Andean P. vulgaris, only a faint, 2.3 kb fragment was visualized in Southern experiments. Moreover, in Mesoamerican accessions, two other fragments (1.7 kb and 3.4 kb) were strongly labelled as well. Taken together, our results indicate that PvMeso is a recently emerged, repeat family initially duplicated in chromosome 11, on ancestral Mesoamerican accession, and later amplified in chromosome 7, after the split of the two major gene pools of the common bean.     

Mitochondrial restriction fragment length polymorphisms in wild Phaseolus vulgaris L.: insights on the domestication of the common bean

Summary Previous examination of intraspecific mitochondrial DNA (mtDNA) diversity in common bean, Phaseolus vulgaris, showed that five restriction fragment length polymorphisms (RFLPs) distinguish the mitochondrial genomes of the two major gene pools of cultivated beans, the Mesoamerican and the Andean. In the study presented here, mtDNA was used to compare the amount of diversity in cultivated beans to that in collections of wild beans to gain an understanding of how and when the mitochondrial genomes of the gene pools became distinct. The mtDNA of six wild bean accessions from Central and South America were digested with nine restriction endonucleases and analyzed by Southern hybridization. A total of twenty RFLPs were detected demonstrating a significantly higher amount of mtDNA variability in wild beans than in cultivated ones. All of the wild beans had the same mtDNA pattern for four out of the five inter-gene pool RFLPs, indicating that the polymorphism arose soon after domestication: two in the gene pool of the cultivated Mesoamerican beans and two in the gene pool of the cultivated Andean beans. The fifth RFLP must have occurred before domestication since the locus was also polymorphic in the wild beans. Wild beans from the south Andes were distinct and less variable than wild accessions of the north Andes and Mesoamerica. The distribution of mtDNA RFLPs among the wild beans supports the concept of two distinct domestication events for P. vulgaris.     

Nucleotide diversity of a genomic sequence similar to <Emphasis Type="Italic">SHATTERPROOF</Emphasis> (<Emphasis Type="Italic">PvSHP1</Emphasis>) in domesticated and wild common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.)

Evolutionary studies in plant and animal breeding are aimed at understanding the structure and organization of genetic variations of species. We have identified and characterized a genomic sequence in Phaseolus vulgaris of 1,200 bp (PvSHP1) that is homologous to SHATTERPROOF-1 (SHP1), a gene involved in control of fruit shattering in Arabidopsis thaliana. The PvSHP1 fragment was mapped to chromosome Pv06 in P. vulgaris and is linked to the flower and seed color gene V. Amplification of the PvSHP1 sequence from the most agronomically important legume species showed a high degree of interspecies diversity in the introns within the Phaseoleae, while the coding region was conserved across distant taxa. Sequencing of the PvSHP1 sequence in a sample of 91 wild and domesticated genotypes that span the geographic distribution of this species in the centers of origin showed that PvSHP1 is highly polymorphic and, therefore, particularly useful to further investigate the origin and domestication history of P. vulgaris. Our data confirm the gene pool structure seen in P. vulgaris along with independent domestication processes in the Andes and Mesoamerica; they provide additional evidence for a single domestication event in Mesoamerica. Moreover, our results support the Mesoamerican origin of this species. Finally, we have developed three indel-spanning markers that will be very useful for bean germplasm characterization, and particularly to trace the distribution of the domesticated Andean and Mesoamerican gene pools.     

Genetic structure of a Lima bean base collection using allozyme markers

 Genetic diversity and structure within a Lima bean (Phaseolus lunatus L.) base collection have been evaluated using allozyme markers. The results obtained from the analysis of wild and cultivated accessions confirm the existence of Andean and Mesoamerican gene pools characterised by specific alleles. Wild and cultivated accessions of the same gene pool are grouped. The Andean natural populations have a very limited geographic distribution between Ecuador and northern Peru. The Mesoamerican wild form extends from Mexico up to Argentina through the eastern side of the Andes. Andean and Mesoamerican cultivated accessions of pantropical distribution contribute substantially to the genetic diversity of the Lima bean base collection. Population genetic parameters, estimated from allozymes, confirmed the predominant selfing mating system of the Lima bean. The selfing mating system, the occurrence of small populations, and low gene flow lead to an interpopulation gene diversity (D_ST=0.235) higher than the intrapopulation gene diversity (H_S=0.032). On the basis of the results, guidelines are given to preserve and exploit the genetic diversity of this threatened species. The results also confirm the independent domestication of the Lima bean in at least two centres, one of which is located at medium elevation in the western valleys of Ecuador and northern Peru. Received: 3 June 1997 / Accepted: 17 June 1997     

Microsatellite marker diversity in common bean (Phaseolus vulgaris L.)

A diversity survey was used to estimate allelic diversity and heterozygosity of 129 microsatellite markers in a panel of 44 common bean (Phaseolus vulgaris L.) genotypes that have been used as parents of mapping populations. Two types of microsatellites were evaluated, based respectively on gene coding and genomic sequences. Genetic diversity was evaluated by estimating the polymorphism information content (PIC), as well as the distribution and range of alleles sizes. Gene-based microsatellites proved to be less polymorphic than genomic microsatellites in terms of both number of alleles (6.0 vs. 9.2) and PIC values (0.446 vs. 0.594) while greater size differences between the largest and the smallest allele were observed for the genomic microsatellites than for the gene-based microsatellites (31.4 vs. 19.1 bp). Markers that showed a high number of alleles were identified with a maximum of 28 alleles for the marker BMd1. The microsatellites were useful for distinguishing Andean and Mesoamerican genotypes, for uncovering the races within each genepool and for separating wild accessions from cultivars. Greater polymorphism and race structure was found within the Andean gene pool than within the Mesoamerican gene pool and polymorphism rate between genotypes was consistent with genepool and race identity. Comparisons between Andean genotypes had higher polymorphism (53.0%) on average than comparisons among Mesoamerican genotypes (33.4%). Within the Mesoamerican parental combinations, the intra-racial combinations between Mesoamerica and Durango or Jalisco race genotypes showed higher average rates of polymorphism (37.5%) than the within-race combinations between Mesoamerica race genotypes (31.7%). In multiple correspondance analysis we found two principal clusters of genotypes corresponding to the Mesoamerican and Andean gene pools and subgroups representing specific races especially for the Nueva Granada and Peru races of the Andean gene pool. Intra population diversity was higher within the Andean genepool than within the Mesoamerican genepool and this pattern was observed for both gene-based and genomic microsatellites. Furthermore, intra-population diversity within the Andean races (0.356 on average) was higher than within the Mesoamerican races (0.302). Within the Andean gene pool, race Peru had higher diversity compared to race Nueva Granada, while within the Mesoamerican gene pool, the races Durango, Guatemala and Jalisco had comparable levels of diversity which were below that of race Mesoamerica.     

Ability of selected accessions of Phaseolus vulgaris L. to restrict nodulation by particular rhizobia

Plant genotypes that limit nodulation by indigenous rhizobia while nodulating normally with inoculant-strain nodule occupancy in Phaseolus vulgaris. In this study, eight of nine Rhizobium tropici strains and six of 15 Rhizobium etli strains examined, showed limited ability to nodulate and fix nitrogen with the two wild P. vulgaris genotypes G21117 and G10002, but were effective in symbiosis with the cultivated bean genotypes Jamapa and Amarillo Gigante. Five of the R. etli strains restricted in nodulation by G21117 and G10002 produced an alkaline reaction in yeast mannitol medium. In a competition experiment in which restricted strains were tested in 1:1 mixtures with the highly effective R. etli strain CIAT632, the restricted strains produced a low percentage of the nodules formed on G2117, but produced over 40% of the nodules formed on Jamapa. The interaction of the four Rhizobium strains with the two bean genotypes, based on the percentage of nodules formed, was highly significant (P<0.001).     

Genetic Diversity of Phaeoisariopsis griseola in Kenya as Revealed by AFLP and Group-specific Primers

Genetic diversity of 50 Phaeoisariopsis griseola isolates collected from different agroecological zones in Kenya was studied using group‐specific primers and amplified fragment length polymorphism (AFLP) markers. Group‐specific primers differentiated the isolates into Andean and Mesoamerican groups, corresponding to the two common‐bean gene pools. Significant polymorphisms were observed with all the AFLP primer combinations used, reflecting a wide genetic diversity in the P. griseola population. A total of 207 fingerprints was generated, of which 178 were polymorphic. Cluster analysis of the polymorphic bands also separated the isolates into the two groups defined by group‐specific primers. All the isolates examined were grouped into three virulence populations; Andean, Afro‐Andean and Mesoamerican, and their genetic diversity measured. On average, greater diversity (91%) was detected within populations than between populations (9%). The genetic distance between Andean and Mesoamerican populations was higher (D = 0.0269) than between Andean and Afro‐Andean (D = 0.0095). The wide genetic diversity reported here has significant implications in breeding for resistance to angular leaf spot and should be taken into consideration when screening and deploying resistant bean genotypes.     

Multiple origins of the determinate growth habit in domesticated common bean (Phaseolus vulgaris)

Background and Aims

The actual number of domestications of a crop is one of the key questions in domestication studies. Answers to this question have generally been based on relationships between wild progenitors and domesticated descendants determined with anonymous molecular markers. In this study, this question was investigated by determining the number of instances a domestication phenotype had been selected in a crop species. One of the traits that appeared during domestication of common bean (Phaseolus vulgaris) is determinacy, in which stems end with a terminal inflorescence. It has been shown earlier that a homologue of the arabidopsis TFL1 gene – PvTFL1y – controls determinacy in a naturally occurring variation of common bean.

Methods

Sequence variation was analysed for PvTFL1y in a sample of 46 wild and domesticated accessions that included determinate and indeterminate accessions.

Key Results

Indeterminate types – wild and domesticated – showed only synonymous nucleotide substitutions. Determinate types – observed only among domesticated accessions – showed, in addition to synonymous substitutions, non-synonymous substitutions, indels, a putative intron-splicing failure, a retrotransposon insertion and a deletion of the entire locus. The retrotransposon insertion was observed in 70 % of determinate cultivars, in the Americas and elsewhere. Other determinate mutants had a more restricted distribution in the Americas only, either in the Andean or in the Mesoamerican gene pool of common bean.

Conclusions

Although each of the determinacy haplotypes probably does not represent distinct domestication events, they are consistent with the multiple (seven) domestication pattern in the genus Phaseolus. The predominance of determinacy in the Andean gene pool may reflect domestication of common bean prior to maize introduction in the Andes.     

Molecular evidence for an Andean origin and a secondary gene pool for the Lima bean (Phaseolus lunatus L.) using chloroplast DNA

 Chloroplast DNA (cpDNA) diversity has been examined using PCR-RFLP and RFLP strategies for phylogenetic studies in the genus Phaseolus. Twenty-two species, including 4 of the 5 cultivated species (P. lunatus L., the Lima bean; P. vulgaris L., the common bean; P. coccineus L., the runner bean and P. polyanthus Greenman, the year-bean), represented by 86 accessions were included in the study. Six PCR primers designed from cpDNA and a total cpDNA probe were used for generating markers. Phylogenetic reconstruction using both Wagner parsimony and the neighbor-joining method was applied to the restriction fragment data obtained from each of the molecular approaches. P. vulgaris L. was shown to separate with several species of largely Mesoamerican distribution, including P. coccineus L. and P. polyanthus Greenman, whereas P. lunatus L. forms a complex with 3 Andean species (P. pachyrrhizoides Harms, P. augusti Harms and P. bolivianus Piper) co-evolving with a set of companion species with a Mesoamerican distribution. Andean forms of the Lima bean are found to be more closely related to the 3 Andean wild species than its Mesoamerican forms. An Andean origin of the Lima bean and a double derivative process during the evolution of P. lunatus are suggested. The 3 Andean species are proposed to constitute the secondary gene pool of P. lunatus, while its companion allies of Mesoamerican distribution can be considered as members of its tertiary gene pool. On the basis of these data, an overview on the evolution of the genus Phaseolus is also discussed. Received: 1 May 1998 / Accepted: 13 July 1998     

Sources of Resistance to Anthracnose in Traditional Common Bean Cultivars from Paraná, Brazil

Pathogenicity of physiologically distinct races of Colletotrichum lindemuthianum originating from Andean (races 7, 19 and 55) and Mesoamerican (races 9, 31, 65, 69, 73, 81, 89, 95 and 453) locations of the new world were evaluated on 26 landrace genotypes of common bean (Phaseolus vulgaris L.) from Paraná State, Brazil. Races 7 (Andean), 65, 73 and 89 (Mesoamerican) were the most pathogenic, while race 31 (Mesoamerican) was the least pathogenic. Most of the landrace genotypes evaluated (88%) were resistant to race 31, except Carioca 3, Preto 1 and Preto 2. In addition, about 50% of the landrace genotypes had resistance to races 9, 19, 55 and 453; and about 30% to races 7, 65, 69, 73, 81, 89 and 95. The resistance index, which measured the pathogenicity response averaged across all the physiologically distinct Andean and Mesoamerican races of C. lindemuthianum, of the landrace genotypes ranged from 8% to 83%. The most resistant cultivars were Carioca Pintado 1, Carioca Pintado 2, Jalo Vermelho and Jalo de Listras Pretas. In contrast, the most susceptible cultivars were Jalo Pardo, Jalo Pintado 1 and Bolinha that showed resistance only to the least pathogenic race 31. These results indicated that many of the common bean landrace cultivars evaluated have genes that could be useful in breeding programmes to enhance resistance to Andean and Mesoamerican races of C. lindemuthianum.     

Selection of Competitive Strains of Rhizobium Nodulating Phaseolus Vulgaris and Adapted to Environmental Conditions in Egypt,Using the Gus-Reporter Gene Technique

Two Rhizobium etli strains, EBRI 2 and EBRI 26, isolated from Egypt were tested for nodulation competitiveness on beans using Rhizobium tropici CIAT 899G as the competing strain. The insertion of the gus-reporter transposon mTn5ssgusA30 did not alter the nodulation or nitrogen fixation capacity of mutant strain CIAT 899G compared to the wild type. At neutral pH, R. etli strains EBRI 2 and EBRI 26 were more competitive than CIAT 899G with the bean cultivar Saxa. These two strains gave nodule occupancies of 52.1 and 61.1% competing with equal cell numbers of CIAT 899G. Nodule occupancies from these two native strains increased with the bean cultivar Giza 6 from Egypt to 66 and 67.5%. Based on these results, cultivar Giza 6 was used to select the most competitive strains under stress of salinity or alkalinity as a major problem for a large part of Egyptian soils. Under stress of salinity (0.2% NaCl or 34.2 mM NaCl), the salt-sensitive strain EBRI 2 was more competitive than the salt-resistant strain EBRI 26. Strain EBRI 2 gave 87.4% but strain EBRI 26 gave 63.7% nodule occupancy against CIAT 899G. The same trend of results was observed under stress of alkalinity (pH 8). Strain EBRI 2 occupied 83% while Strain EBRI 26 occupied 53.2%.     

Symbiotic performance of common bean and soybean co-inoculated with rhizobia and <Emphasis Type="Italic">Chryseobacterium balustinum</Emphasis> Aur9 under moderate saline conditions

The effect of co-inoculating beans and soybeans with rhizobia and Chryseobacterium, a plant growth promoting bacteria (PGPR), was studied under conditions of mild saline stress. Chryseobacterium balustinum Aur9 was used with Rhizobium tropici CIAT899 or R. etli ISP42 to inoculate common bean (Phaseolus vulgaris L.), or jointly with Ensifer (Sinorhizobium) fredii SMH12 and HH103 to inoculate soybean (Glycine max (L.) Merrill). The effect of co-inoculation was studied by following nodule primordia initiation, nodulation kinetics and symbiotic performance in plants grown under moderate saline conditions (25 mM NaCl). In common bean, co-inoculation improved nodule primordia formation when compared with single inoculation (R. tropici CIAT899). However, co-inoculation did not provide benefits in the development of nodule primordia in soybean with E. fredii SMH12. The kinetic of nodulation in bean was also favored by double inocula resulting in a higher number of nodules. Long-term effects of co-inoculation on beans and soybeans depended on the rhizobial species used. In both, control and saline conditions, co-inoculation of R. tropici CIAT899 and C. balustinum Aur9 improved bean growth when compared with the single inoculation (CIAT899). However, the positive effect of double inocula on plant growth did not occur when using R. etli ISP42. Soybean plants receiving double inoculation (E. fredii SMH12 and C. balustinum Aur9) showed better symbiotic performance, mostly under saline stress, than with a single inoculation. The results indicate that co-inoculation with C. balustinum and rhizobia under mild saline conditions partially relieves the salt-stress effects, although do not always result advantageous for symbiotic N₂ fixation in legume plants.     

Race structure within the Mesoamerican gene pool of common bean (Phaseolus vulgaris L.) as determined by microsatellite markers

Common bean (Phaseolus vulgaris L.) cultivars are distinguished morphologically, agronomically and ecologically into specific races within each of the two gene pools found for the species (Andean and Mesoamerican). The objective of this study was to describe the race structure of the Mesoamerican gene pool using microsatellite markers. A total of 60 genotypes previously described as pertaining to specific Mesoamerican races as well as two Andean control genotypes were analyzed with 52 markers. A total of 267 bands were generated with an average of 5.1 alleles per marker and 0.297 heterozygosity across all microsatellites. Correspondence analysis identified two major groups equivalent to the Mesoamerica race and a group containing both Durango and Jalisco race genotypes. Two outlying individuals were classified as potentially of the Guatemala race although this race does not have a defined structure and previously classified members of this race were classified with other races. Population structure analysis with K = 1–4 agreed with this classification. The genetic diversity based on Nei’s index for the entire set of genotypes was 0.468 while this was highest for the Durango–Jalisco group (0.414), intermediate for race Mesoamerica (0.340) and low for race Guatemala (0.262). Genetic differentiation (G _ST) between the Mesoamerican races was 0.27 while genetic distance and identity showed race Durango and Jalisco individuals to be closely related with high gene flow (N _m) both between these two races (1.67) and between races Durango and Mesoamerica (1.58). Observed heterozygosity was low in all the races as would be expected for an inbreeding species. The analysis with microsatellite markers identified subgroups, which agreed well with commercial class divisions, and seed size was the main distinguishing factor between the two major groups identified.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.