bean genotype. The objectives of this research were 1) to examine the potential for using previously developed molecular markers linked to CBB resistance in a marker-assisted selection (MAS) strategy; 2) to determine the effects on CBB resistance of
Patrick D. O'Boyle, James D. Kelly, and William W. Kirk
Matthew D. Robbins, Mohammed A.T. Masud, Dilip R. Panthee, Randolph G. Gardner, David M. Francis, and Mikel R. Stevens
selecting for resistance to multiple pathogens ( Yang and Francis, 2005 ). Marker-assisted selection (MAS) offers an opportunity to overcome some of the problems associated with phenotypic selection and facilitates combining multiple resistance genes
Nobuhito Mitani, Atsushi Kono, Masahiko Yamada, Akihiko Sato, Shozo Kobayashi, Yusuke Ban, Toshihito Ueno, Mikio Shiraishi, Shinya Kanzaki, Tomoyuki Tsujimoto, and Keizo Yonemori
.5% PCNA offspring on average ( Ikeda et al., 1985 ). So marker-assisted selection should be developed for selecting PCNA offspring efficiently. Most persimmon cultivars are hexaploid ( Tamura et al., 1998 ), and segregation analysis of molecular markers
M.A. Dalbó, G.N. Ye, N.F. Weeden, W.F. Wilcox, and B.I. Reisch
The efficiency of marker-assisted selection for powdery mildew (Uncinula necator (Schw.) Burr) resistance in grapes (Vitis L. sp.) was studied using molecular markers associated with a major QTL (quantitative trait loci) for this trait. Initially, genetic maps were constructed from a segregating population of the cross `Horizon' × Illinois 547-1 (a hybrid between V. rupestris Scheele and V. cinerea Engelm.). A major QTL from Ill. 547-1, the resistant parent, explained 41% of the variation. One RAPD (randomly amplified polymorphic DNA) marker and one AFLP (amplified fragment length polymorphism) marker, obtained by bulked segregant analysis, showed the highest association with powdery mildew resistance in the mapping population. Segregation of the QTL was followed in different crosses by CAPS (cleaved amplified polymorphic sequence) markers developed from these two markers. An allele-specific amplified polymorphism that segregates as present/absent was also developed from the CS25b locus. Powdery mildew resistance was evaluated visually on a 1 to 5 scale in four different seedling populations. Two populations originated from crosses using Ill. 547-1 as the resistant parent. Two other populations were from crosses with NY88.0514.03, a resistant seedling from the original `Horizon' × Ill. 547-1 mapping population. Segregation ratio distortions were observed in some crosses. In these cases, the allele associated with the QTL for powdery mildew resistance was less frequent than the alternate allele. In all crosses, the markers were closely associated with resistance. If selection were based on markers, the percentage of susceptible individuals (classes 4 and 5) would decrease from 24% to 52% to 2% to 18%. Selection efficiency was greatest in crosses where segregation distortion was most intense.
Majid R. Foolad
In tomato, Lycopersi conesculentum Mill., currently there are >285 known morphological, physiological and disease resistance markers, 36 isozymes, and >1000 RFLPs, which have been mapped onto the 12 tomato chromosomes. In addition, currently there are >162,000 ESTs, of which ∼3.2% have been mapped. Several tomato genetic maps have been developed, mainly based on interspecific crosses between the cultivated tomato and its related wild species. The markers and maps have been used to locate and tag genes or QTLs for disease resistance and other horticultural characteristics. Such information can be used for various purposes, including marker-assisted selection (MAS) and map-based cloning of desirable genes or QTLs. Many seed companies have adopted using MAS for manipulating genes for a few simple morphological characteristics and several vertical disease resistance traits in tomato. However, MAS is not yet a routine procedure in seed companies for manipulating QTLs although it has been tried for a few complex disease resistance and fruit quality characteristics. In comparison, the use of MAS is less common in public tomato breeding programs, although attempts have been made to transfer QTLs for resistances to a few complex diseases. The potential benefits of marker deployment to plant breeding are undisputed, in particular for pyramiding disease resistance genes. It is expected that in the near future MAS will be routine in many breeding programs, taking advantage of high-resolution markers such as SNPs. For quantitative traits, QTLs must be sought for components of genetic variation before they are applicable to marker-assisted breeding. However, MAS will not be a “silver bullet” solution to every breeding problem or for every crop species.
Wencai Yang and David M. Francis
The lack of resistance to bacterial diseases increases both the financial cost and environmental impact of tomato (Lycopersicon esculentum Mill.) production while reducing yield and quality. Because several bacterial diseases can be present in the same field, developing varieties with resistance to multiple diseases is a desirable goal. Bacterial spot (caused by four Xanthomonas Dowson species) and bacterial speck (caused by Pseudomonas syringae pv. tomato Young, Dye and Wilkie) are two economically important diseases of tomato with a worldwide distribution. The resistance gene Pto confers a hypersensitive response (HR) to race 0 strains of the bacterial speck pathogen. The locus Rx3 explains up to 41% of the variation for resistance to bacterial spot race T1 in field trials, and is associated with HR following infiltration. Both Pto and Rx3 are linked in repulsion phase on chromosome 5. We made a cross between two elite breeding lines, Ohio 981205 carrying Pto and Ohio 9834 carrying Rx3, to develop an F2 population and subsequent inbred generations. Marker-assisted selection (MAS) was applied to the F2 progeny and to F2:3 families in order to select for coupling-phase resistance. Thirteen homozygous progeny from 419 F2 plants and 20 homozygous families from 3716 F3 plants were obtained. Resistance was confirmed in all selected families based on HR in greenhouse screens using bacterial speck race 0 and bacterial spot race T1 isolates. Resistance to bacterial spot race T1 was confirmed in the field for 33 of the selected families. All selected families were also resistant to bacterial speck in the field. MAS was an efficient tool to select for desirable recombination events and pyramid resistance.
James J. Luby and Douglas V. Shaw
Matthew D. Robbins, Jack E. Staub, and Zhicheng Fan
To increase yield in cucumber (Cucumussativus L.), we designed a recurrent selection program utilizing phenotypic (PHE) and marker-assisted (MAS) selection for the development of multiple lateral branching (MLB; branches per plant), gynoecious, early genotypes possessing high fruit length to diameter ratio (L:D). These yield components are under genetic control of few quantitative trait loci (QTL; 2-6 per trait), which have been placed on a moderately saturated molecular linkage map. Four inbred lines, complementary for the target traits, were intermated and the resulting population underwent MAS and PHE, as well as random mating (RAN), for three cycles. Selections by PHE were visually made for all four traits at the whole plant level. Selections based on MAS contained the highest number of desired marker genotypes from 20 marker loci (SSR, RAPD, SCAR, SNP). Using the same selection scheme and intensity allowed a direct comparison of MAS to PHE. Selection was equally effective for MLB and L:D by MAS (3.5 and 3.0) and PHE (3.6 and 3.0), which were both superior to RAN (2.8 and 2.8). For earliness (days to anthesis) and gynoecy (percent female), MAS (41.8 and 26.6) was less effective than PHE (40.5 and 81.8) and RAN (41.0 and 80.9), which were equal. For yield (fruit per plant), RAN (1.90) and MAS (1.88) were equal, but less than PHE (2.15). After three cycles of PHE, further selection by MAS identified superior genotypes, which were intermated. Superior hybrids were selected by MAS and underwent one backcross generation. In some backcrosses, gains were made in every trait compared to the PHE Cycle 3 mean, while in other backcrosses, gains were made only in some traits. Improvement by MAS was very effective during line extraction for these yield components.
Joseph N. Wolukau, Xiaohui Zhou, and JinFeng Chen
. Selection for resistance to GSB is of pivotal importance to melon breeding programs. Phenotypic selection is tedious and sensitive to environmental conditions. Identification of linked markers is an essential step toward both marker-assisted selection (MAS
Sogo Nishio, Masahiko Yamada, Yutaka Sawamura, Norio Takada, and Toshihiro Saito
The effectiveness of detected quantitative trait loci (QTLs) and molecular markers associated with them in tree fruit breeding is measured by the percentages of the variance associated with detected QTL effects accounting for not phenotypic variance, but genetic variance of the trait. The genetic variance can be obtained by subtracting environmental variance from the phenotypic variance. Once accurate environmental variance components are obtained for a given selection field, environmental variances under any number of replications and measurement repetitions can be estimated. We estimated environmental variance components of fruit ripening date measured by days in a Japanese pear (Pyrus pyrifolia Nakai) breeding field in the National Institute of Fruit Tree Science, Tsukuba, Ibaraki, Japan. We estimated variance among fruits within a tree (σf 2) as 25.6, among trees within a genotype (σt 2) as 0.2, among years (σy 2) as 9.4, associated with genotype × year interaction (σgy 2) as 7.9, and associated with tree × year interaction (σty 2) as 1.2. Because σf 2 was the largest environmental variance component, increasing the number of fruit evaluated would most effectively reduce the environmental variance, and tree replication would not because of very small σt 2 and σty 2. The 95% confidence limit of a genotypic value was ± 10 days in the evaluation of five fruits on a single tree in a year and ± 7 days over 2 years. Broad-sense heritability in a family, each offspring in which was evaluated using five fruits on a single tree in a single year, was estimated at 0.83 for three full-sib families analyzed.