F2 Bulks Research Articles

IS MICRO-ENVIRONMENT A HURDLE IN GENETIC ENHANCEMENT OF YIELD IN PIGEONPEA?

Yield stagnation in pigeonpea remains a serious concern. Since most cultivars in the past were bread using pedigree method, it seems that this method was ineffective in increasing the crop productivity. To get insight into this failure, 10 highly inbred parents and their F2 bulks were examined by comparing their single plant yield variances. Surprisingly, the inbred parents exhibited high inter-plant variability; and their F2s were less variable than their one or both the parents. These unusual observations were attributed to the interaction of individual plants with unspecied micro-environmental factors prevailing in their vicinity, both below and above the ground. Under such circumstances, the selections had low heritability and unpredictable performance in the subsequent generation. Since these factors cannot be targeted through breeding, it is suggested that the breeders should try other breeding approaches such as bulk, composites, and single pod descent. The pedigree method of breeding, however, will continue to have its utility in the genetic enhancement of qualitative traits like yield.Yield stagnation in pigeonpea remains a serious concern. Since most cultivars in the past were bread using pedigree method, it seems that this method was ineffective in increasing the crop productivity. To get insight into this failure, 10 highly inbred parents and their F2 bulks were examined by comparing their single plant yield variances. Surprisingly, the inbred parents exhibited high inter-plant variability; and their F2s were less variable than their one or both the parents. These unusual observations were attributed to the interaction of individual plants with unspecied micro-environmental factors prevailing in their vicinity, both below and above the ground. Under such circumstances, the selections had low heritability and unpredictable performance in the subsequent generation. Since these factors cannot be targeted through breeding, it is suggested that the breeders should try other breeding approaches such as bulk, composites, and single pod descent. The pedigree method of breeding, however, will continue to have its utility in the genetic enhancement of qualitative traits like yield.

The bHLH transcription factor PPLS1 regulates the color of pulvinus and leaf sheath in foxtail millet (Setaria italica).

The bHLH transcription factor, PPLS1, interacts with SiMYB85 to control the color of pulvinus and leaf sheath by regulating anthocyanin biosynthesis in foxtail millet (Setaria italica). Foxtail millet (Setaria italica), a self-pollinated crop with numerous small florets, is difficult for cross-pollination. The color of pulvinus and leaf sheath with purple being dominant to green is an indicative character and often used for screening authentic hybrids in foxtail millet crossing. Deciphering molecular mechanism controlling this trait would greatly facilitate genetic improvement of cultivars in foxtail millet. Here, using the F2 bulk specific-locus amplified fragment sequencing approach, we mapped the putative causal gene for the purple color of pulvinus and leaf sheath (PPLS) trait to a 100Kb region on chromosome 7. Expression analyses of the 15 genes in this region revealed that Seita.7G195400 (renamed here as PPLS1) was differentially expressed between purple and green cultivars. PPLS1 encodes a bHLH transcription factor and is localized in the nucleus with a transactivation activity. Furthermore, we observed that expression of a MYB transcription factor gene, SiMYB85 (Seita.4G086300) involved in anthocyanin biosynthesis, shows a totally positive association with that of PPLS1. Heterologous co-expression of both PPLS1 and SiMYB85 in tobacco leaves led to elevated anthocyanin accumulation and expression of some anthocyanin-related genes. Furthermore, PPLS1 physically interacts with SiMYB85. Taken together, our results suggest that PPLS1 interacts with SiMYB85 to control the color of pulvinus and leaf sheath by regulating anthocyanin biosynthesis in foxtail millet.

Association analysis between constructed SNPLDBs and GCA effects of 9 quality-related traits in parents of hybrid rice (Oryza sativa L.)

BackgroundThe general combining ability (GCA) of parents in hybrid rice affects not only heterotic level of grain yield and other important agronomic traits, but also performance of grain quality traits of F2 bulk population which is the commodity consumed by humans. In order to make GCA improvement for quality traits in parents of hybrid rice by molecular marker assisted selection feasible, genome-wide GCA loci for quality traits in parents were detected through association analysis between the effects of GCA and constructed single nucleotide polymorphism linkage disequilibrium blocks (SNPLDBs), by using unhusked rice grains harvested from F1 plants of 48 crosses of Indica rice and 78 crosses of Japonica rice. GCA-SNPLDBs association analysis.ResultsAmong the 8 CMS and 6 restorer lines of indica rice subspecies, CMS lines Zhenpin A, Zhenshan97 A, and 257A, and restorers Kanghui98, Minghui63 and Yanhui559 were recognized as good general combiners based on their GCA effect values for the 9 quality traits (brown rice rate, milled rice rate, head rice rate, percentage of chalky grains, chalky area size, chalkiness degree, gelatinization temperature, gel consistency and amylose content). Among the 13 CMS and 6 restorer lines of japonica rice subspecies, CMS 863A, 6427A and Xu 2A, and restorers C418, Ninghui8hao and Yunhui4hao showed elite GCA effect values for the 9 traits. GCA-SNPLDB association analysis revealed 39 significant SNPLDB loci associated with the GCA of the 9 quality-related traits, and the numbers of SNPLDB loci located on chromosome 1, 2, 3, 4, 5, 8, 9, 11 and 12 were 1, 4, 3, 9, 6, 5, 5, 4 and 2, respectively. Number of superior GCA alleles for the 9 traits among the 33 parents ranged from 1 to 26.ConclusionsThirty-nine significant SNPLDBs loci were identified associated with the GCA of 9 quality-related traits, and the superior SNPLDB alleles could be used to improve the GCA of parents for the traits in the future by molecular marker assisted selection. The genetic basis of trait GCA in parents is different from that of trait itself.

Genetic mapping of green curd gene Gr in cauliflower.

Gr5.1 is the major locus for cauliflower green curd color and mapped to an interval of 236 Kbp with four most likely candidate genes. Cauliflower with colored curd enhances not only the visual appeal but also the nutritional value of the crop. Green cauliflower results from ectopic development of chloroplasts in the normal white curd. However, the underlying genetic basis is unknown. In this study, we employed QTL-seq analysis to identify the loci that were associated with green curd phenotype in cauliflower. A F2 population was generated following a cross between a white curd (Stovepipe) and a green curd (ACX800) cauliflower plants. By whole-genome resequencing and SNP analysis of green and white F2 bulks, two QTLs were detected on chromosomes 5 (Gr5.1) and 7 (Gr7.1). Validation by traditional genetic mapping with CAPS markers suggested that Gr5.1 represented a major QTL, whereas Gr7.1 had a minor effect. Subsequent high-resolution mapping of Gr5.1 in the second large F2 population with additional CAPS markers narrowed down the target region to a genetic and physical distance of 0.3cM and 236Kbp, respectively. This region contained 35 genes with four of them representing the best candidates for the green curd phenotype in cauliflower. They are LOC106295953, LOC106343833, LOC106345143, and LOC106295954, which encode UMP kinase, DEAD-box RNA helicase 51-like, glutathione S-transferase T3-like, and protein MKS1, respectively. These findings lay a solid foundation for the isolation of the Gr gene and provide a potential for marker-assisted selection of the green curd trait in cauliflower breeding. The eventual isolation of Gr will also facilitate better understanding of chloroplast biogenesis and development in plants.

Validation of molecular markers for pod shattering tolerance in soybean [Glycine max (L.) Merrill

Tagging of pod shattering/dehiscence tolerance trait in soybean (qPDH loci) with molecular markers was undertaken in a segregating population of cross cultivar Kalitur (shattering of 12.02% at field level; 77.71% on oven drying) x DS-9712 (shattering of 1.63% at field level; 10.94% on oven drying) by bulked segregant analysis (BSA). Pod shattering was partially dominant over the tolerance in the cross. Inhibitory epistasis of two major genes was evidenced from F2 ratio (13:3) (3.0-16.0% shattering field level and 5-85% on oven drying), with chi-square test indicating goodness of fit which was confirmed by test cross. Only two of the five shortlisted microsatellite markers from previous study viz., Satt674 and SRM1generated polymorphism between shattering tolerant and susceptible parents and their corresponding F2 bulks. Satt674 marker alleles were closely placed (223 bp vrs. 228 bp) hampering clear resolution and hence this primer was not used forfurther validation. SRM1 primer yielded distinct polymorphic markers (237bp vrs. 225 bp) between shattering-tolerant and susceptible parents and bulks. SRM1 marker was validated in 60 individual F2 plants which were contrasting for pod dehiscence trait. Forty-seven susceptible plants yielded either only 237 bp bands or were heterozygous (having both 237 bp and 225 bp bands), while 13 shattering tolerant F2 plants amplified only 225 bp marker. Hence, in future this SRM1-225bp marker linked to a major pod shattering tolerance loci qPDH1, could be utilized for screening the parental lines, segregating generations, breeding lines and varieties of soybean.

Use of F2 Bulks in Training Sets for Genomic Prediction of Combining Ability and Hybrid Performance.

Developing training sets for genomic prediction in hybrid crops requires producing hybrid seed for a large number of entries. In autogamous crop species (e.g., wheat, rice, rapeseed, cotton) this requires elaborate hybridization systems to prevent self-pollination and presents a significant impediment to the implementation of hybrid breeding in general and genomic selection in particular. An alternative to F1 hybrids are bulks of F2 seed from selfed F1 plants (F1:2). Seed production for F1:2 bulks requires no hybridization system because the number of F1 plants needed for producing enough F1:2 seed for multi-environment testing can be generated by hand-pollination. This study evaluated the suitability of F1:2 bulks for use in training sets for genomic prediction of F1 level general combining ability and hybrid performance, under different degrees of divergence between heterotic groups and modes of gene action, using quantitative genetic theory and simulation of a genomic prediction experiment. The simulation, backed by theory, showed that F1:2 training sets are expected to have a lower prediction accuracy relative to F1 training sets, particularly when heterotic groups have strongly diverged. The accuracy penalty, however, was only modest and mostly because of a lower heritability, rather than because of a difference in F1 and F1:2 genetic values. It is concluded that resorting to F1:2 bulks is, in theory at least, a promising approach to remove the significant complication of a hybridization system from the breeding process.

Relative potential of seed yield component traits as selection criteria in the segregating generations of a desi × kabuli cross of chickpea

To assess the response to early generation selection in chickpea, a total of 150 F4 progenies derived from selection for total branches per plant, pods per plant, seeds per pod, seed yield per plant and 100-seed weight as independent selection criteria in F2 population of Pusa 362 (desi) × PG 0515 (kabuli) cross were evaluated along with their parents and unselected F2 bulk. The F2 population revealed high variability for all the yield components and transgressive segregation for all traits except 100-seed weight. Mean of the F4 families relative to the corresponding F2 plants was high, indicating effectiveness of early generation selection for all characters studied except branch number per plant. Significant correlated response for seed yield was also observed in F4. Based on realized response to selection as percentage of mean, realized heritability and realized generalized response values, we suggest utilization of pods per plant, seed yield per se and 100-seed weight as selection criteria in desi-kabuli introgression breeding for higher genetic gains.

Molecular Genetics of Salt Tolerance in Tomato F2 Segregating Population with the Aid of RAPD Markers

Salt tolerance of segregating progenies of a cross between a domesticated salt sensitive tomato cultivar (CA4) and a natural salt-tolerant wild-type tomato species (LA1606) was characterized. The F1 plants from this cross were selfed and 120 F2 segregating progenies from the resulting population along with parental CA4 and LA1606 plants were evaluated for salt tolerance. These plants were irrigated everyday with 185 mM NaCl for 82 days and quantitative traits were quantified including number of flowers, fruit number, fruit weight, fruit length, fruit width, fruit set percentage, and total yield. The two parental lines were evaluated for the presence of 27 seven independent RAPD markers and 7 markers were found to be polymorphic for the two genotypes. Bulk Segregant (BSA) analyses consisting of pooling 10 “most tolerant” and 10 “most sensitive” F2 segregating plants showed association of two RAPD polymorphic markers with higher salt tolerance. Two DNA markers that exhibit co-segregation with salt tolerance were identified and characterized. RAPD marker OPX-17 and MRTOMR-022 exhibited 2 positive molecule markers (polymorphism) which were found only in the resistant parent (LA1606) and resistant F2 bulk.

COMPARISON OF MEAN YIELD COMPONENTS AND FIBER QUALITY PARAMETERS OF ADVANCED BULK GENERATIONS IN F2, F3 AND F4 INTERSPECIFIC AND INTRASPECIFIC COTTON POPULATIONS

The performance of interspecific and intraspecific cotton hybrid populations were compared to investigate the correlation among F2, F3 and F4 bulk generations in terms of yield, lint percentage, and fiber quality during 2009 to 2011. In addition to that, this research investigated the effect of combining abilities on hybrid performance at further generations. For this purpose, nine interspecific and six intraspecific hybrids with eight parents were evaluated under west part of Turkey (Aegean region) environmental conditions. With the generation progresses, the highest drop in fiber length and fiber strength was detected in interspecific duo to the inbreeding depression and genetic breakdown in advanced populations. Based on 15 tested hybrids, the observed mean yield and fiber quality in the F2 does not adequately predict the performance of hybrids in the F4 generation. Thus, selection of individual plants should be delayed until the F4 generation especially for interspecific hybrids, but individual plants would be selected at early generation in intraspecific hybrid populations. In the study it was found that a higher general combining ability (GCA) does not necessarily confer a higher specific combining ability (SCA) and that the GCA and SCA were independent of one another, and that in F4 high yielded hybrid population was obtained from cross which at least one parent of hybrid has maximum positive GCA effects. These results show that instead of use only one criteria, F2 performance for intraspecific hybrids and F3 performance for interspecific hybrids, low heterosis and inbreeding depression, combining ability of parents could be used together to determine the most promising hybrid populations to be used as a source for further selection.

Heterosis and Genetic Variance in Soybean Recombinant Inbred Line Populations

One of the greatest challenges in plant breeding is the selection of parental combinations that will produce progeny that are superior to either parent. The objective of this research was to determine if yield heterosis in the F2 bulk of a soybean [Glycine max (L.) Merr.] single cross is a predictor of significant genetic variance for yield and superior line yields among random inbred lines derived from the F2 generation. In one experiment, thirty‐seven F6:7 inbred lines from each of five bi‐parental populations along with parents and the bulk F2 generation were yield tested at three locations. Of the five populations, three had significant genotypic variance for yield. The only two transgressive segregating lines came from the population with the greatest genotypic variation for yield and the only one with mid‐parent F2 yield heterosis (15% at P > 0.06). In another experiment, random inbred lines were derived from four single crosses chosen from a seven parent half diallel cross with similar mid‐parent yields. Two exhibited F2 yield heterosis and two did not. Derived F5:6 lines were yield tested in three environments. After adjustment for maturity, both populations derived from the non‐heterotic crosses had zero genetic variation. Genetic variance estimates for the two populations derived from heterotic crosses were larger than their standard errors and significant at P > Z = 0.07 and P > Z = 0.05. These experiments provide evidence that F2 bulk heterosis can be a predictor of significant genetic variation among inbred soybean lines derived from a single cross.

Mapping and candidate gene screening of tomato Cladosporium fulvum-resistant gene Cf-19, based on high-throughput sequencing technology.

BackgroundTomato leaf mold is a common disease in tomato cultivation. This disease is caused by Cladosporium fulvum, which has many physiological races and differentiates rapidly. Cf genes confer resistance to C. fulvum, and the C. fulvum-tomato pathosystem is a model for the study of gene-for-gene interactions. Plants carrying the Cf-19 gene show effective resistance to C. fulvum in the field, and can be used in breeding and resistance mechanism studies as new resistant materials. In this study, we used F2 bulk specific-locus amplified fragment sequencing (SLAF-seq) and parental resequencing methods to locate and characterize the Cf-19 gene.ResultsA total of 4108 Diff_markers and three association regions were found in association analysis. A 2.14-Mb region containing seven Cf-type genes was identified in further analysis based on data from SLAF-seq and parental resequencing. Two candidate genes, Solyc01g006550.2.1 and Solyc01g005870.1.1, were screened out by quantitative real-time PCR (qRT-PCR) analysis. Sequence analysis showed that Solyc01g006550.2.1 (an allelic locus of Cf-0) in CGN18423 was a novel homologue of the Cladosporium resistance gene Cf-9 (Hcr9s) in the Cf-4/9 locus. The marker P7, which cosegregated with the resistant trait, was developed based on sequence mutation of the Solyc01g006550.2.1 locus in CGN18423.ConclusionsThe Cf-19 gene was mapped to the short arm of chromosome 1. The candidate genes Solyc01g006550.2.1 and Solyc01g005870.1.1 showed related amino acid sequence structures and expression patterns. Solyc01g006550.2.1 had a close evolutionary relationship with the functional Hcr9 members Cf-4 and Cf-9, and was very different from non-functional members. The results from this study will facilitate the breeding of cultivars carrying the Cf-19 gene and provide a basis for further gene cloning, resistance gene evolution and plant resistance mechanism studies.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0737-0) contains supplementary material, which is available to authorized users.

Identification of microsatellite markers linked to thermotolerance in silkworm by bulk segregant analysis and in silico mapping

Silkworm, being a poikilothermic insect, its growth and development is affected by environmental factors especially, temperature. In tropical countries like India, it has considerable effect on silk production due to the prevailing of hot climatic conditions. Previous attempts to evolve silkworm breeds and hybrids tolerant to high temperature by traditional breeding methods have not yielded the desired results. Hence application of new strategies like marker assisted selection (MAS) could be the most effective strategy for developing a thermo-tolerant bivoltine silkworm for sustainable silk production in India. As a prelude, in this study it is aimed to identify simple sequence repeat (SSR) markers closely linked with thermotolerance in silkworm. To do so, 20 silkworm breeds were evaluated at high temperature (36?C) and based on pupation percentage, two multivoltines (Nistari and Cambodge) and two bivoltines (SK4C and BHR3) were identified as thermo-tolerant and one bivoltine (CSR2) was identified as the susceptible breed. These breeds were screened with 85 SSR markers drawn from different linkage groups and out of those, only 11 markers (12.9%) showed distinct polymorphism between thermo-tolerant and susceptible breeds. Further, bulked segregant analysis (BSA) was performed using 11 polymorphic SSR primers, by comparing the SSR profiles of the tolerant (Nistari) and susceptible (CSR2) parents, their F1 and F2 bulks. Nevertheless, only 5 markers generated clear differences in the amplified DNAs between the bulks corresponding to that of the parents suggesting that the DNA regions amplified by these SSR markers are closely linked to thermotolerance in B. mori. The results obtained through bulk segregant analysis was further confirmed by genotyping 5 linked SSR markers using 140 individual F2 progenies. Of these 5 markers, highest Spearman's rho correlation coefficient was shown by S0816 indicating a high degree of closeness between the genotypic and phenotype variations in F2 population. Furthermore, we have also attempted to locate the genes near to S0816 by in silico approach and upshot revealed 3 genes nearer to its sequence on the B. mori genome. The BGIBMGA005249 gene was found to be located nearest to S0816 at a distance of 14.8 Kb. But, further studies are required in this regard to derive a relationship between the thermotolerance and the functional role of identified genes nearer to the closest marker, so that the identified markers can be used to develop a thermo-tolerant silkworm breed through MAS.

Detection of a single nucleotide polymorphism (SNP) DNA marker linked to cocoon traits in the mulberry silkworm, Bombyx mori (Lepidoptera: Bombycidae)

Cocoon weight and shell weight are the key economic traits ultimately determining silk yield. In order to detect the main quantitative trait loci (QTL) associated with the cocoon traits of the mulberry silkworm, Bombyx mori, the parents of larvae that pro- duced cocoons that differed greatly in weight and shell weight were screened using 240 primer pairs of single nucleotide polymor- phic markers (SNPs) representing all the 28 linkage groups in silkworm. Out of the 240 primer pairs, 48 (20%) revealed distinct polymorphism between the parents, which was confirmed by the co-dominant expression of both polymorphic PCR products in the F1 generation. The bulked segregant analysis (BSA) was used to compare the SNP profiles of the parents, F1 and F2 bulks using the 48 informative SNP primers. This revealed that out of 48 primer pairs, only one pair, i.e., No. 04124 of the linkage group 4 showed clear differences in the amplified products between the bulks corresponding to that of the parents with different cocoon traits sug- gesting that the DNA regions amplified by this primer pair are closely linked to the QTL controlling the cocoon traits. The results were also confirmed by screening the backcross (BC) progeny. This is the first report of the identification of a QTL using SNPs with BSA. The results of the present study indicate that it might be possible to use SNPs for marker assisted selection (MAS) in silkworm breeding programs aimed at improving cocoon traits.

RAPD markers for resistance to Alternaria solani (early blight) in cultivated tomato (Lycopersicon esculentum L.)

SummaryAn attempt was made to understand the genetics of resistance against Alternaria solani in tomato (Lycopersicon esculentum L. cv. ‘Megha’), and to develop molecular markers as an effective strategy to assist in the transfer of resistance. The F2 segregation pattern of a resistant susceptible (‘Megha’ ‘Pusa Gaurav’) cross revealed the trait to be governed by two recessive genes. Efforts were made to develop RAPD markers to these two recessive resistance genes. Out of 14 random primers that were polymorphic between the parents, two primers (OPG19 and OPE11) produced unique amplicons 1,350 bp and 1,300 bp in length, respectively, that co-segregated in repulsion phase (i.e., were linked to the susceptible disease reaction in F2 bulks). Single marker analysis also revealed significant differences between the marker genotypes of 75 F2 individuals, indicating a linkage between the disease reaction and the individual markers OPG191350 and OPE111300. A best-fit ratio of 9:3:3:1 between the two markers, OPG191350 and OPE111300, revealed that they were not linked to each other, suggesting that the markers were linked to two different loci governing the resistance trait. This is the first report of RAPD markers for resistance to Alternaria solani in tomato.

Segregation distortion of Brassica carinata derived black rot resistance in Brassica oleracea

Three segregating F2 populations were developed by self-pollinating 3 black rot resistant F1 plants, derived from across between black rot resistant parent line 11B-1-12 and the susceptible cauliflower cultivar ‘Snow Ball’. Plants were wound inoculated using 4 isolates ofXanthomonas campestris pv. campestris (Xcc) race 4, and disease severity ratings of F2 plants from the three populations were scored. A total of 860 arbitrary oligonucleotide primers were used to amplify DNA from black rot resistant and susceptible F2 plants and bulks. Eight RAPD markers amplified fragments associated with completely disease free plants following black rot inoculation,which segregated in frequencies far lower than expected. Segregation of markers with black rot resistance indicates that a single, dominant major gene controls black rot resistance in these plants. Stability of this black rot resistance gene in populations derived from 11B-1-12 may complicate introgression into B. oleracea genotypes for hybrid production.

Performance of cotton F1 hybrids and its relation to the mean yield of advanced bulk generations

The performance of intraspecific (Gossypium hirsutum) and interspecific (G. hirsutum × G. barbadense) F1 cotton hybrids and its correlation with the mean yield of advanced bulk generations was assessed under Greek environmental conditions. For this, 14 intraspecific and five interspecific cotton F1 hybrids were evaluated at two locations (Sindos and Vardates) during the 1985–88 period. In addition, two interspecific hybrids (5, 6), the three best (8, 7, 11) and two among the worst (4, 10) intraspecific F1s and their F2 bulks were also evaluated at Sindos in 1986. This evaluation was repeated in the following years. In each successive year the new generation (up to the F5 bulk) was included.Intraspecific F1 hybrids outyielded the best variety at both locations. Lint yield of the F1 hybrids ranged from 99 to 158% with a mean value of 122%. Useful F1 heterosis (hybrids with higher values than the best check cultivar) was also expressed for boll weight and Verticillium wilt infestation. The latter was more pronounced in hybrids derived from tolerant parents.All F1 interspecific hybrids were dominated by the barbadense phenotype and had its superior fiber lint quality, lower lint percentage, and resistance to Verticillium wilt. In addition, they expressed positive heterosis for lint yield at Sindos and negative at Vardates, compared with the check variety. The frequency of the hirsutum phenotype progressively increased in the F2 through F5 generations, with plant height, boll weight, earliness, fiber quality and wilt tolerance moving toward the hirsutum values.In the F2 generation yield dropped dramatically (>50%) in the interspecific hybrids, whereas in the intraspecific hybrids inbreeding depression was much less and varied from hybrid to hybrid. The good F1s intraspecific hybrids maintained part of their useful heterosis in the F2. From the F3 onwards the yield improved in the interspecific crosses and stabilized around 75% of the yield recorded in the best check variety. In contrast, the highest yielding F1 intraspecific hybrids yielded progressively less in subsequent generations, and in the F5 they produced the same yield as the best cultivar, whereas the yield of the lowest yielding intraspecific hybrids was stabilized around 95% of the yield recorded in the best cultivar. It was concluded that yield of the F1 did not predict the yield of the bulks in the following generations. Yet, the combined performance of the hybrids in the F1 and F2 generation could be a good indicator to identify the most promising populations to be utilized either as F2 hybrids or as a source population for further selection.

Early generation testing of common bean (Phaseolus vulgaris L.) populations in sole crop and in maize/bean intercrop

Few plant-breeding studies have examined methodology for improving common bean (Phaseolus vulgaris L.) yields by selecting in an intercrop situation. We hypothesized that early-generation testing would be as useful in a maize (Zea mays L.)/bean intercrop as in sole crop for identifying superior bean populations for yield. F2 to F5 bulks of six selected crosses and their F5-derived advanced lines were evaluated in sole crop or intercrop. The F2 and F5 bulks were evaluated together in a preliminary trial in one location, while the advanced lines were evaluated with the F3s in one location, and with the F4s in two locations. Within sole crop, selection of the best three populations, based on F2 performance, provided 67% of the top advanced lines. The rank correlation between average bulk yield across generations and the average line yield was positive and significant. Within intercrop, selection of the best three populations provided 56% of the top advanced lines. The rank correlation between advanced line yield and bulk yield across generations was positive but not significant. Also, the top three F2 populations selected in sole crop produced 89% of the top advanced lines in intercrop. Advanced line performance showed a positive significant correlation with mean F4 bulk performance for grain yield, 100-seed weight and seeds per pod within sole crop, while positive significant correlation was seen with pods per plant and seeds per pod in intercrop. Results indicate that F2 bulk yields can be used to discard the least promising crosses in both cropping systems. Key words: Early generation testing, Phaseolus vulgaris, intercropping

Genetic Diversity and Heterosis of Spring Wheat Crosses

Genetic diversity between parents may contribute positively to both heterosis and transgressive segregation. The objectives of this study were to determine (i) whether genetic diversity in spring wheat (Triticum aestivum L.) affected F2 bulk heterosis and (ii) whether genetic diversity affected the amount of transgressive segregation among progeny from a cross. Genetic diversity was measured by pedigrees, morphology, and gliadin seed proteins. Heterosis was evaluated for 137 F2 bulks tested at two Minnesota locations in 1993 and three locations in 1994. These bulks were developed from crosses among 91 cultivars. No linear relationship between genetic distance and F2 bulk heterosis was detected. However, when crosses were divided into related and unrelated groups, crosses with parents unrelated by pedigree or morphology expressed greater heterosis than crosses with related parents. Bulks of crosses between cultivars that were classified as unrelated by two or more of the distance measures showed more heterosis than related crosses. Thus, part of heterosis seems to be due to parental diversity, though not in a linear fashion. Expression of greater‐than‐average heterosis by the highest yielding bulks, without an increase in genetic distance, suggested additional factors were influencing the expression of heterosis. Transgressive segregation was evaluated for six of the 137 crosses; 30 to 40 inbred lines from each of the crosses was evaluated in three environments. Data for these populations were consistent with an additive genetic model. The expression of heterosis was due in part to genetic diversity but was unpredictable and also depended on factors not elucidated by this study.

The expression of aluminum stress induced polypeptides in a population segregating for aluminum tolerance in wheat (Triticum aestivum L.)

This study examined the changes in gene expression induced by aluminum (Al) stress in wheat root tips. Seedlings of Triticum aestivum L. cvs. Katepwa (Al sensitive), Maringa (Al tolerant), and Alikat (Al tolerant near isoline; 'Katepwa'*3/'Maringa') and a F2 population derived from 'Katepwa' x 'Alikat', were grown for 3 days in either 0 or 1 μg∙mL−1 Al. Polypeptides were labeled with 35S-methionine prior to separation by gel electrophoresis. There were a few polypeptides from whole cell lysates that showed enhanced expression in all of the genotypes in 1 μg∙mL−1 Al, however, the whole cell lysate and microsomal polypeptide profiles also revealed numerous unique changes in gene expression in Al-sensitive 'Katepwa' at 1 μg∙mL−1 Al; the latter cosegregated with only the Al-sensitive F2 bulks. The microsomal polypeptide profiles of the Al-tolerant lines 'Maringa' and 'Alikat' changed marginally in the presence of Al and these changes were also reflected in the Al-tolerant F2 bulks. The data showed that there were many changes in gene expression which cosegregated with Al sensitivity and suggest that Al tolerance in wheat may rely on constitutively expressed polypeptides.

Potential of wild common bean for seed yield improvement of cultivars in the tropics

Thirty nine wild or weedy common bean (Phaseolus vulgaris L.) accessions, representing the two extremes of geographical range of distribution and domestication of cultigens in the Americas, were crossed to a high-yielding, small-seeded cultivar, ICA Pijao. The resulting F1, F2, and F3 population bulks, along with ICA Pijao, were evaluated for seed yield, days to maturity, and 100-seed weight under favorable growing conditions at two locations in Colombia. The F3 and F4 bulks were tested separately under soil-fertility and moisture stresses. Thirty random F5-derived F8 lines from the highest yielding population involving both Andean and Middle American wild beans, along with the parents and a control cultivar, were also evaluated for 2 yr, under favorable conditions. No F1 hybrid, population bulk, or F5-derived F8 line significantly outyielded ICA Pijao in any test environment. The mean yield of F1 hybrids, population bulks, and F5-derived F8 lines involving wild and weedy beans of Middle America was higher than that involving Andean South American wild beans. Heritability for seed yield, seed weight, and days to maturity, as well as gains from selection were comparable to those obtained in crosses among cultivars. The small seed of the progenies and the positive correlation between seed size and seed yield suggest that alternative mating schemes that increase the recovery of progenies with a cultivated phenotype should be investigated. Until this research is done, it would be premature to draw conclusions about the yield improvement potential of wild beans. Key words:Phaseolus vulgaris, common bean (wild), yield, heterosis, heritability, selection gain