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- Author or Editor: F. A. Bliss x
Control of pollen production without the difficulties of mechanical emasculation allows crosses to be made efficiently in many plant species that would otherwise be disregarded commercially because of problems of hybridization. Development of male-sterile inbreds is facilitated by the use of cytoplasmic-genic male sterility in a manner similar to the system proposed by Jones and Clarke for use in onions (5). This method is now commonly used in F1 hybrid development of such crops as maize (6), sorghum (9), sugar beets (8), and sudan grass (1), and has been suggested for use in other crops (4).
The germplasm for a cultivated crop is generally regarded as the gene pool of cultivars, species and genera that can be utilized as sources of additional genetic variation for crop improvement. When developing strategies for the optimum use of vegetable germplasm, several problems not found in field crops, ornamentals and fruits should be considered. In vegetables, a part of the immature plant is often utilized. Because it is still physiologically immature, arbitrary judgments concerning time of harvest may greatly affect yield and quality, compared to crops in which the mature seeds are harvested. Second, unlike most ornamentals, tree fruits and small fruits, many vegetables are propagated by seed rather than asexually. An efficient means of stabilizing the commercial genotype(s) must be developed before widespread use is feasible.
The precision of a 9-plant hill-plot design in which plants were sown 15 cm apart in a 3 × 3 arrangement compared favorably to that of 3-m-row plots containing approximately 75 plants for the estimation of pod yield of snap beans Phaseolus vulgaris L. Quality traits and sieve size distribution based on pod diameter in both plot designs were similar. Using the square design, the entire 9-plant plot can be taken as the unit of selection, or single plant selection can be practiced when the test plant is grown in the center hill surrounded by 8 uniform guard plants. Single plant selection using this design has been used effectively to modify traits of beans having moderate to high heritability. Selection based on family means should be used for traits with low heritability. Efficiency of the hill-plot design is realized in terms of smaller plot size, fewer required seeds per plot and reduced harvest time.
Four true bush snap bean cultivars were hybridized with OSC410, a determinate, sprawling ‘Blue Lake’ bush breeding line. Observed segregation data fit a single gene hypothesis with the sprawling habit completely dominant to true bush type. Crosses of OSC410 to 2 indeterminate cultivars provided evidence for 2 additional patterns of inheritance. When ‘Polaris’ was used as the indeterminate parent, segregation ratios indicated that indeterminance was dominant and controlled by a single gene. Epistasis of 2 genes was suggested from the cross Mexico 80R' (normal) × OSC410, with indeterminance dominant.
Seedlings of Prunus mahaleb are often used as rootstocks for sweet cherry (P. avium) scion cultivars in commercial orchards. While they are desirable based on ease of propagation and economical production of nursery stock, seedlings may be variable resulting in nonuniform compound trees, and they are susceptible to several important diseases. Seedling sources have shown substantial variability for population uniformity of plant growth, and reaction to crown gall, powdery mildew and Phytophthora root rot. Segregating families also vary for pollen fertility, inbreeding response and control of scion growth. Multiple screening for favorable trait combinations is underway to develop improved sources of cherry rootstocks.
Crown gall incited by Agrobacterium tumifaciens is an important problem for nursery and field production of stone fruit and nut crops. Genotypes reportedly differ for crown gall reaction, but there is little information about resistance of Prunus accessions used as rootstocks. From among four wild-type strains of A. tumifaciens-virulent on apricot and almond, K12 was selected for inoculation of 6-month-old seedlings of cherry, plum, peach, almond, apricot, and miscellaneous species. The large majority of seedlings were very susceptible to crown gall, but some had few or no galls. Cherry, especially some lines of P. mahaleb, showed the most resistant or moderately resistant seedlings, while some accessions of plum, especially P. cerasifera, P. angustifolia, and P. insititia had the most resistant seedlings. Plants with different reactions were propagated to determine adult plant resistance and to study the heritability of crown gall reaction.
Genetic linkage maps for many organisms are being produced using molecular markers. The utility of these maps depends on the ability to place genes of known, important effects on the map. It is often useful lo saturate the chromosome around these loci with many linked molecular markers. This study used Bulked Segregant Analysis and Random Amplified Polymorphic DNA to identify linked markers to loci in peach, Prunus persica L. Batch and almond Prunus dulcis Mill populations. Linkages to isozyme loci were first sought to test the suitability of this technique to long-lived perrenials. Several RAPD markers were found to be linked to three isozyme loci in a segregating F3 population from a peach × almond cross. PAPD markers have also been identified which are linked to the yellow-flesh locus of peach in a heterozygous peach population. Thus, this method has proven useful for identifying molecular marker linkages to important loci in peach and almond. These RAPDs may now be placed on a linkage map generated in our lab using a peach/almond hybrid population which will allow these loci to be studied and manipulated more easily in a breeding program.
Studies of genetic variation at the DNA level in the tree fruit and nut crop species of Prunus have been very limited. Recently molecular markers based on random amplified polymorphic DNA (RAPD) markers have been shown to be highly useful and efficient gene markers in other plant and animal species. We have used a total of 50 primers (10-mers) with arbitrary nucleotide sequence to identify cultivars of cherry, plum, apricot, peach and almond. A total of 120 accessions of different cultivars were assayed. The variation revealed by RAPD markers was highly species specific in the five Prunus species examined. High levels of polymorphism were observed for almond cultivars whereas sweet cherry revealed the lowest levels of polymorphism for the RAPD primers examined. The implications of these results in the germplasm diversity in the cultivated species of Prunus will be discussed.
Kiwifruit (Actinidia deliciosa) is a functionally dioecious plant where fruit size is dependent on number of seeds set. Pollen fertility was estimated in 1990 and 1991 by percentage stainability and percentage germinability in vitro. Profiles of the isozymes AAT, GPI and PGM were used to assess if any large differences in pollen fertility could be attributed to genotypic variation. Based on these three isozymes, eight different genotypes were discovered. Although significant differences were found among vines within orchards and among orchards, all vines can be considered good pollenizers (stainability > 87%). A positive correlation was found in 1991 between percentage stainability and percentage germination.
A male sterile bean plant was observed in the F2 population of WI 74-2047 × ‘Swedish Brown’. Male sterile phenotypes were characterized by all pollen grains being shriveled and non-functional, but with female fertility unaffected. Expression of sterility was genic, involving 2 unlinked loci, Ms 1 and In-ms 1, which interact to give a 13:2:1 F2 ratio of non-sterile:sterile:lethal individuals. ‘Swedish Brown’ contained fully fertile and semisterile plants, the latter presumed to be caused by the presence of a reciprocal translocation. One of the loci involved in male sterility is loosely linked to one of the break points of the translocation, and lines segregating for sterility but not semisterility were obtained.