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Early blight (EB), caused by the fungus Alternaria solani, is a destructive disease of tomato (Lycopersicon esculentum) worldwide. Sources of genetic resistance have been identified within related wild species, including green-fruited L. hirsutum and red-fruited L. pimpinellifolium. We have employed traditional protocols of plant breeding and contemporary molecular markers technology to discern the genetic basis of EB resistance and develop tomatoes with improved resistance. Backcross breeding has resulted in the development of germplasm with improved resistance; however, linkage drag has been a major obstacle when using L. hirsutum as a donor parent. To identify and map QTLs for EB resistance, we used several filial and backcross populations derived from interspecific crosses between L. esculentum and either L. hirsutum or L. pimpinellifolium. In each population, an average of seven resistance QTLs were detected. While similar QTLs were detected in different generations of the same cross, generally different QTLs were identified in populations derived from different crosses. The results suggested stability of QTLs across environments and generations but variation in QTLs in different interspecific populations. It is expected that marker-assisted pyramiding of QTLs from different sources results in development of germplasm with strong and durable resistance. Further inspection of the results led to the identification and selection of six QTLs with stable and independent effects for use in marker–assisted selection (MAS). However, to facilitate “clean” transfer and pyramiding of these QTLs, near-isogenic lines (NILs) containing individual QTLs in a L. esculentum background should be developed.
Black rot, caused by Xanthomonas campestris pv. campestris (Pam.) Dawson (Xcc), is a major bacterial disease of Brassica oleracea L. vegetables. In this study the related species Brassica carinata Braun (ethiopian mustard), which can be used to generate interspecific crosses with B. oleracea was evaluated for resistance to Xcc. Fifty-four accessions and susceptible control plants were wound inoculated with four isolates of Xcc race 4 at the juvenile stage. Of the 54 accessions tested, A 19182 and A 19183 exhibited no symptoms when inoculated with Xcc for all plants tested, and the accessions including PI 199947, PI 199949 and PI 194256 segregated for resistance to Xcc.
Tepary beans (Phaseolus acutifolius A. Gray) are considered drought and heat tolerant, desirable characteristics for arid regions. Knowing the genetic distances among tepary lines can indicate both compatibility for intraspecific crosses and potential for Interspecific P. acutifolius × P. vulgaris hybrids. Fifteen tepary lines, including cultivars and landraces, were compared to two pinto bean varieties using random amplified polymorphic DNA's (RAPDs). At the present time polymorphisms have been clearly identified between wild and cultivated teparies and the pinto bean. An ammo acid profile is also being determined using HPLC. More work needs to be completed before relationships among cultivated teparies can be established.
A genetic linkage map of Prunus has been constructed using an interspecific F2 population generated from self-pollinating a single F1 plant of a cross between a dwarf peach selection (54P455) and an almond cultivar (Padre). This map consists of approximately 80 markers including 10 isozymes. 12 plum genomic, 19 almond genomic and 40 peach mesocarp specific cDNA clones. The backbone map will be used for identifying the genomic locations and characterization of genes governing important economic traits in the genus Prunus. Of particular interests are those genes associated with fruit ripening and mesocarp development in peach and almond.
Two diploid yellowleaf Vaccinium elliottii Chapmn. clones were pollinated with pollen from the tetraploid southern highbush cultivar `Misty' (largely V. corymbosum L). These interspecific crosses, which normally yield few hybrids because of a triploid block, were made with and without the use of V. elliottii mentor pollen mixed with V. corymbosum pollen. Mentoring had no effect on the number of hybrids produced when V. elliottii `Silverhill' was the seed parent, but when V. elliottii `Oleno' was the seed parent, no hybrids were produced unless mentor pollen was utilized. The difference was postulated to be a greater ability to produce one-seeded berries in `Silverhill' than in `Oleno'.
Over a 2-year period, disease incidence and severity caused by tomato-yellow-leaf-curl-virus (TYLCV) in lines of tomato (Lycopersicon esculentum) derived from interspecific hybridization with wild species of L. pimpinellifoliuim, L. hirsutum and L. peruvianum ranged from non to intermediate. The average fruit weight of these lines ranged from 25 to 90 grams. Crosses between TYLCV-resistant lines derived from the same wild species produced progenies similar to their parents in the level of resistance. However, progenies of 2-wild species combination showed little higher in levels of resistance than their parents. Analysis of F1, F2 and backcross populations from crosses of the most promising TYLCV-resistant lines with the susceptible cultivar showed that resistance appeared to be incomplete dominant and controlled by few genes.
Sarracenia L. is a genus of insectivorous plants confined to wetlands of the United States and Canada. Green mutants, lacking red pigmentation in the leaves, flowers, and growing point, have been found in most Sarracenia species. Controlled crosses were made using green mutants from S. rubra Walter ssp. gulfensis Schnell, S. purpurea L., S. psittacina Mich., and S. leucophylla Raf. Self-pollination of mutant green individuals in four different species resulted in green offspring, whereas reciprocal crosses with respective wild-types resulted in red offspring. Three of six self-pollinated heterozygous S. rubra ssp. gulfensis yielded offspring exhibiting a 3 red : 1 green ratio. Progeny from a testcross and two self-pollinated heterozygous plants of S. purpurea fit the expected ratios, whereas offspring from two S. purpurea crosses had significant deviations in field and laboratory sowing experiments. Offspring from testcrosses with S. rubra Walter ssp. jonesii (Wherry) Wherry met expected ratios under field conditions. Interspecific crosses between green individuals resulted in green offspring. These results suggest that anthocyanin pigmentation is controlled by two alleles at a single locus, with red dominant to green.
Skewed segregations are frequent events in segregating populations derived from different interspecific crosses in tomato. To determine a basis for skewed segregations in the progeny of the cross between Lycopersicon esculentum and L. pennellii, monogenic segregations of 16 isozyme loci were analyzed in an F2 and two backcross populations of this cross. In the F2, nine loci mapping to chromosomes 1, 2, 4, 9, 10, and 12 exhibited skewed segregations and in all cases there was an excess of L. pennellii homozygotes. The genotypic frequencies at all but one locus were at Hardy–Weinberg equilibria. In the backcross populations, all except two loci exhibited normal Mendelian segregations. No postzygotic selection model could statistically or biologically explain the observed segregation patterns. A prezygotic selection model, assuming selective elimination of the male gametophytes during pollen function (i.e., from pollination to karyogamy) adequately explained the observed segregations in all three populations. The direction of the skewed segregations in the F2 was consistent with that expected based on the effects of unilateral incompatibility reactions between the two species. In addition, the chromosomal locations of five of the nine markers that exhibited skewed segregations coincided with the locations of several known compatibility-related genes in tomato. Multigenic unilateral incompatibility reactions between L. esculentum pollen and the stigma or style of L. pennellii (or its hybrid derivatives) are suggested to be the major cause of the skewed segregations in the F2 progeny of this cross.
Interspecific crosses with Fragaria moschata (6x) have been hampered by ploidy level differences, poor seed set, and extremely poor seed germination. Modification of pollination practices, embryo rescue, and use of several genotypes has allowed over 80 synthetic tetraploids to be created from 14 cross combinations. Germplasm for the experiment consisted of eight selections of F. moschata (6x), two of F. nubicola (2x), and two of F. viridis (2x). Both 2x × 6x and 6x × 2x crosses were performed. Initially, negligible seed set occurred on F. nubicola and F. viridis when multiple flowers per truss were pollinated. When only one cross was performed per truss, with other flowers removed, seed set was greatly enhanced. F. moschata was much more tolerant of multiple crosses per truss. The crossing combination of F. moschata × F. nubicola gave the worst seed production. Other species combinations were capable of producing good seed set with noticeable differences between individual selections. When achenes were halved, only 1% appeared normal, 2% were underdeveloped or shrunken, the remainder were empty. Many of the malformed and most of the normal embryos germinated using the cut achene method. Achenes were surface-sterilized, cut in half, and placed on MS media with activated charcoal (3g·L–1), sucrose (30g·L–1), and no hormones. Germination occurred only from achenes from fully ripened fruit. Viable hybrids were obtained from 2x × 6x as well as 6x × 2x crosses. Fragaria viridis–F. moschata hybrids closely resembled F. moschata while F. nubicola–F. moschata hybrids were more intermediate in leaf morphology.
Abstract
Large scale commercial cultivation of blueberries and the breeding of improved cultivars both began after 1900 (33). Interspecific hybridization has been important in the development of commercial cultivars, and many combinations of species have been crossed in the course of taxonomic studies and in cultivar breeding. Nonetheless, the experimental study of blueberry interspecific hybridization is still in its infancy.