Interspecific hybridization, pollen-stigma incompatibility, and DNA sequence analysis were used to study the relationships among hazelnut (Corylus) species. Interspecific crosses resulted in a wide range of cluster set from 0% to 65%. Reciprocal differences were common. In general, crosses involving C. avellana and C. heterophylla were more successful when used as pollen parents, but crosses involving C. americana were more successful when it was the female parent. C. cornuta, C. californica and C. sieboldiana intercrossed freely in both directions, as did C.colurna and C.chinensis. The Asian species, C. sieboldiana, C.heteropyhlla, and C. chinensis, were not cross-compatible with each other. Fluorescence microscopy showed that pollen-stigma incompatibility exists within and among wild hazelnut species, in addition to the cultivated European hazelnut C. avellana. Pollen-stigma incompatibility and embryo abortion (blank nuts) appear to be major blocks to interspecific gene flow. In addition, the chloroplast matK gene and the Internal Transcribed Spacer (ITS) region of the nuclear ribosomal DNA (nrDNA) were amplified and sequenced. The matK sequence was highly conserved and thus was not informative. However, the ITS sequence was highly informative and parsimony analysis agreed with morphological similarities. Corylus species were placed into four groups: 1) C. avellana, C. maxima, C. americana and C. heterophylla 2) C. colurna, C.chinensis, and C. jacquemontii 3) C. cornuta, C. californica and C. sieboldiana 4)C. ferox.
V. Erdogan and S.A. Mehlenbacher
Yiping Zhang, Vince Lackney and Molly Kyle
Genetic resistance to cucumber mosaic virus (CMV) in pepper (Capsicum spp.) is recessive, polygenic and, therefore, has been difficult to transfer in breeding programs. Although a few varieties have been released with some resistance, in our tests, these develop severe symptoms that are eventually indistinguishable from the susceptible reactions. Furthermore, accurate and consistent screens for the disease can be relatively difficult; therefore, we report on the detection molecular markers linked to two CMV resistance genes using distributional extreme analysis to identify the relevant quantitative trait loci (QTL). The 12 most resistant and 15 most susceptible individuals were selected from a segregating F2 population of 316 individuals that were derived from the interspecific cross (C. annuum `Jupiter' × C. frutescens BG2814-6). A total of 132 tomato genomic, cDNA, and pepper genomic clones were hybridized to filters with DNA extracted from the distributional extremes. These clones included framework markers representing all pepper linkage groups and also selected clones from regions of the genome identified in a preliminary analysis as possibly involved with CMV resistance. Several clones from the two regions of the genome previously identified appear to be nonrandomly cosegregating with the CMV resistance phenotype in this larger population. Further analysis will be done by adding more markers in the regions and refining the positions of the resistance QTL.
W.R. Maluf, L.V. Barbosa and L. V. Costa Santa-CecÌlia
Oviposition and feeding of Scrobipalpuloides absoluta was studied on plants of seven different genotypes with varying 2-tridecanone (2-TD) foliage concentrations: Lycopersicon esculentum var. glabratum PI 1344417 (GLA), L. esculentum TSWV-547 (ESC), F1 (ESC × GLA), and four F2 genotypes—two with high 2-TD (HI1, HI2) and two with low foliage 2-TD concentrations (LO1, LO2). GLA, HI1, and HI2 showed 2-TD concentrations above 185 × 10–12 mol·cm2, while ESC, LO1 and LO2 had 2-TD below 40 × 10–12 mol·cm2; F1 had intermediary levels of 2-TD (83.5 × 10–12 mol 2-TD/cm2). Ovipositioning was substantially higher in the low 2-TD than on either the high 2-TD genotypes or in the F1, especially in the upper portion of the plants. Scores for leaf lesion type (LLT), overall plant damage (OPD) and percent leaflets attacked (PLA) were substantially higher for the low 2-TD than for either the high 2-TD genotypes or the F1. The results indicate that 2-TD mediates resistance to Scrobipalpuloides absoluta in the interspecific cross, and strongly suggest that 2-TD acts as both an ovipositioning and feeding deterrent for this insect.
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.
Philipp W. Simon
Central Asia is the center of origin for many Allium species and a rich genetic source of wild relatives of onion and garlic. For this reason germplasm collections of cultivated Alliums have targeted the acquisition of seed and bulb samples from this region, and several plant expeditions from Asia, Europe, and North America have collected Allium germplasm in Central Asia. Central Asian Allium germplasm has been valuable both as raw materials for scientific research leading to published data, and as starting materials for genetic improvement of the crop. Utilizing this germplasm it has been possible to improve garlic so it can be bred like other seed-propagated crops. Several interspecific crosses have been made between onion and other Central Asian wild relatives and these crosses have yielded useful traits for onion improvement. Allium germplasm from this region has also been important in elucidating the systematics and origins of diversity in onion and garlic. By any of these measures, Central Asian Allium collections have been valuable. Challenges and successes in collecting, maintaining, evaluating, and using these collections remain.
Philip M. Sheridan and Richard R. Mills
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.
Majid R. Foolad, Arun Sharma, Hamid Ashrafi and Guoyang Lin
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.
Rebecca Nelson Brown and James R. Myers
A molecular and morphological marker map would improve our knowledge of Cucurbita genetics, and would facilitate efforts to breed improved summer and winter squash cultivars. Random amplified polymorphic DNA (RAPD) markers were used to construct a partial map of the Cucurbita genome. The mapping population was the BC1 progeny of the Cucurbita pepo L. yellow straightneck inbred A0449 and the tropical Cucurbita moschata Duchesne ex Lam. landrace `Nigerian Local'. A0449 was the recurrent parent. This cross was chosen because of the relatively greater economic importance of summer squash, traits of value to be introgressed from the C. moschata parent, and maximized genetic variation from the interspecific cross. The map contains 148 RAPD markers in 28 linkage groups. Loci controlling five morphological traits were placed on the map. The map covers 1,954 cM, which is estimated to be 75% of the Cucurbita genome. The qualitative traits placed on the map include the B gene for fruit which turn yellow before anthesis, the M gene for silver mottling of leaves, and a locus controlling the intensity of rind color on mature fruit. Quantitative trait loci (QTL) associated with fruit shape and the depth of the indentations between primary leaf veins were identified.
Maureen C. O'Leary and Thomas H. Boyle
Polyacrylamide gel electrophoresis was used to study inheritance and linkage of isozymes in Easter cactus (Hatiora species and interspecific hybrids). Five isozyme systems were analyzed: aspartate aminotransferase (AAT), glucose-6-phosphate isomerase (GPI), malate dehydrogenase (MDH), phosphoglucomutase (PGM), and triosephosphate isomerase (TPI). F1, F2, BC1, and S1 progeny were used for inheritance studies. Six polymorphic loci (Aat-1, Gpi-1, Mdh-1, Pgm-1, Pgm-2, and Tpi-2) were identified. Aat-1 and Pgm-1 were linked (recombination frequency = 26% ± 7%), but the other isozyme loci assorted independently. Aberrant segregation ratios were observed in at least one segregating family for all six isozyme loci. We hypothesize that segregation distortion was due to linkage between isozyme loci and other genes subject to pre- or postzygotic selection. The existence of five additional isozyme loci (Aat-2, Gpi-2, Mdh-2, Mdh-3, and Tpi-1) was inferred from segregation patterns and by comparison of isozyme profiles from phylloclades and pollen. These isozyme loci may prove useful for confirming hybridity in intra- and interspecific crosses, determining parentage of cultivars, and assessing genetic diversity in germplasm collections.
Ying Wang, Gregory L. Reighard, Desmond R. Layne, Albert G. Abbott and Hongwen Huang
Pawpaw (Asimina triloba) produces the largest fruit native to the United States. Six linkage groups were identified for A. triloba using the interspecific cross [PPF1-5 (A. triloba) × RET (A. reticulata Shuttlw. ex Chapman)], covering 206 centimorgans (cM). A total of 134 dominant amplification fragment length polymorphism (AFLP) markers (37 polymorphic and 97 monomorphic) were employed for estimating the genetic diversity of eight wild populations and 31 cultivars and advanced selections. For the wild populations, the percentage of polymorphic loci over all populations was 28.1% for dominant markers and Nei's genetic diversity (He) were 0.077 estimated by 134 dominant markers. Genetic diversity and the percentage of polymorphic loci estimated using only polymorphic dominant AFLPs were 0.245 and 79%, respectively, which are comparable with other plant species having the same characteristics. Estimated genetic diversity within populations accounted for 81.3% of the total genetic diversity. For cultivars and advanced selections, genetic diversity estimated by 134 dominant markers was similar to that of wild pawpaw populations (He = 0.071). Thirty-one cultivars and advanced selections were delineated by as few as nine polymorphic AFLP dominant loci. Genetic relationships among wild populations, cultivars and advanced selections were further examined by unweighted pair group method with arithmetic mean (UPGMA) of Nei's unbiased genetic distance. The genetic diversity estimated for wild populations using the clustered polymorphic markers was lower than the result estimated using the nonclustered polymorphic markers. Therefore, this study indicates that the number of sampled genomic regions, instead of the number of markers, plays an important role for the genetic diversity estimates.