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Fredrick A. Bliss

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Fredrick A. Bliss

The presence of arcelin protein in the seeds of common bean, Phaseolus vulgaris L., provides resistance to the Mexican bean weevil and to a lesser degree, the common bean weevil. Fast, accurate identification of single seeds containing arcelin facilitates the transfer of alleles for each of four different arcelin types through standard crossing procedures. Seed yields and other traits of near-isogenic lines that contain different alleles were comparable to the standard parent, Porrillo 70. Genotypic mixtures containing resistant and susceptible seeds produced seed yields comparable to Porrillo 70, which suggests that heterogeneous populations offer the potential for stable resistant cultivars.

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Fredrick A. Bliss

Fruits and vegetables are rich sources of the micro mineral elements and vitamins often lacking in diets based on cereals, grain legumes, and starchy roots and tubers, but void of animal products. When embarking on a breeding program to improve nutritional compounds, the way the fruit or vegetable is consumed in mixed diets must be considered. To alleviate nutritional problems, the nutrients must not only be present in the plant parts consumed, but also absorbed efficiently in the body. In some cases, it may be necessary to modify compounds to improve absorption as well as increase the concentration. Breeding to improve nutritionally related traits can be approached in a manner similar to that for other traits; i.e., identification of genetic variability, selection for enhanced levels using either individual phenotype or family mean values, and testing for field performance. In addition to improving amount and availability, avoidance of undesirable correlated responses due to genetic or physiological linkages between the trait of interest and other traits deleterious to either plant growth or the consumer is critically important during selection. The growing number of molecular marker-based linkage maps should prove especially useful for identifying genes of interest and employing marker-aided selection. When insufficient variability for amount or type of compound is present in the gene pool, strategies using transgenic plants may be useful.

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Fredrick A. Bliss

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Anne M. Gillen and Fredrick A. Bliss

Peach rootstock breeding may be accelerated by utilization of molecular markers linked to the root-knot nematode resistance locus (Mi) to screen segregating populations. A genetic linkage map was constructed using RFLP markers in an F2 population (PMP2) that is segregating for this locus. PMP2 is derived from a controlled cross of the relatively diverse peach rootstocks Harrow Blood (susceptible) and Okinawa (homozygous resistant). Bulked Segregant Analysis was applied using RAPD markers. A single small (227 base pairs) RAPD marker was found to be linked to the dominant resistant allele of Mi at a distance of 10 cM. This new marker joined the Mi locus to the RFLP linkage map and showed that two dominant RFLP markers are located between the RAPD marker and Mi. RFLPS are expensive, time-consuming and RAPD markers are unreliable, and therefore both are unsuitable for screening breeding populations. We attempted to convert the RAPD marker to a more breeder-friendly CAPS marker. The converted CAP marker was dominant. Attempts to convert the CAP marker to a co-dominant marker were not successful. The utility of the CAP marker was tested in an open pollinated F2 population derived from the F1 parent of PMP2 and in several rootstocks. The genetic linkage map was compared to other Prunus maps. The PMP2 linkage group containing the Mi locus can be related to the peach × almond linkage group which contains the phosphoglucomutase Pgm-1 locus.

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Marilyn L. Warburton and Fredrick A. Bliss

Previous studies of peach germplasm using pedigree information and isozyme polymorphism data have shown limited diversity in the U.S. gene pool. To further investigate the genetic diversity among peach cultivars grown in different regions of the United States, 94 RAPD markers were used to estimate the genetic distances among 136 cultivars. Of the 12 clusters formed in a dendrogram, the 90 U.S. cultivars and breeding lines and most of those from Europe and Latin America grouped to only three clusters, while the 23 peach entries from India, Pakistan, Russia, Okinawa, and China, as well as the almond cultivar used as an outgroup, were distributed among the other nine clusters. Therefore, the genetic diversity within temperate U.S. peach germplasm is quite limited, and to expand the variability, additional germplasm should be obtained, especially from Asia. Comparison of genetic similarity based on inbreeding coefficients with similarity coefficients based on the RAPD data produced a correlation of 0.395, which is comparable to values in similar investigations in other crops. Thus, similar conclusions can be drawn from these two sources of information. RAPD data are useful particularly when pedigree information is incomplete, there has been substantial selection within breeding populations, and a high proportion of alleles are identical in state but not by descent.

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Cameron P. Peace, Carlos H. Crisosto, Fredrick A. Bliss, and Thomas M. Gradziel

Candidate gene (CG) analysis can be an efficient approach for identifying genes controlling important traits in fruit production. Three chronological steps have been described for determining candidate genes for a trait—proposing, screening, and validating—and we have applied these to the problem of internal breakdown of peach and nectarine. Internal breakdown (IB), also known as chilling injury, is the collective term for various disorders that occur during prolonged cold storage and/or after subsequent ripening of stone fruit. Symptoms include mealiness, browning, and bleeding. Candidate genes for IB symptoms were proposed based on knowledge of the biochemical or physiological pathways leading to phenotypic expression of the traits. Gene sequences for proposed CGs were obtained primarily from the Genome Database for Rosaceae. Screening the CGs involved identifying polymorphism within a progeny population, relying mainly on simple PCR tests. Several polymorphic CGs were located on a peach linkage map and compared with phenotypic variation for IB susceptibility. A major QTL for mealiness coincided with the Freestone-Melting flesh locus, which itself is likely to be controlled by a CG encoding endopolygalacturonase, an enzyme involved in pectin degradation. Further gene sequences positioned on the consensus linkage map of Prunus by other researchers were co-located with QTLs for IB traits. Validation of the role of identified CGs will require detailed physiological or transgenic studies.