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.
Yiping Zhang, Vince Lackney, and Molly Kyle
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.
Robert H. Bors and J. Alan Sullivan
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.
Neil O. Anderson and Peter D. Ascher
Male and female fertility, seed germination, and progeny fertility were used to determine cultivar fertility in species of Lythrum. One short-, 11 mid-, and six long-styled cultivars were included in this study. Duplicates of several cultivars from different nurseries and three unknown cultivars from Minnesota gardens were also collected. Plants from 17 Minnesota and one Wisconsin population of L. salicaria served as fertile male and/or female testers. Pollen stainability (usually 100%) showed low levels of male gamete abortion. Pollen size within and among anther type varied widely; possible 2n gametes were present in primarily the short- and mid-anther morphs. Seed production per capsule from legitimate cross-pollinations, using cultivars as male parents with Minnesota or Wisconsin female testers, averaged 48 ± 36 across style morphs. Cultivars differed as males, as did anther morphs. With female fertility tests, seed set per capsule ranged from zero to 152 and averaged 54 ± 40 in legitimate pollinations (i.e., pollinations between stamen and styles of the same length). Seed set for other crosses showed similar trends. Only `Morden Gleam' produced no seed with all legitimate pollinations, although illegitimate selfs or interspecific crosses produced seed. Seed from legitimate crosses of L. salicaria × cultivars had 30% to 100% germination. Common male and female parents within each legitimate crossing group were not significantly different. This study showed that the cultivars are highly fertile when used as male or female parents with wild purple loosestrife, native species (L. alatum Pursh.), or other cultivars. Thus, cultivars grown in gardens could serve as pollen or seed sources for the continued spread of purple loosestrife. The implications of cultivar fertility, especially interspecific F1 hybrids, is discussed in relation to the spread of noxious weeds in wetlands.
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.
William R. Graves and Anthony S. Aiello
Information on the heat resistance of silver maple (Acer saccharinum L.) could help develop stress-resistant Freeman maples (Acer ×freemanii E. Murray). Our first objective was to determine how 26, 30, 32, 34, and 36 °C in the root zone affect growth and water relations of plants from rooted cuttings of a silver maple clone indigenous to Mississippi (33.3 °N latitude). Fresh mass increased over time for plants at all temperatures and was highest for plants with root zones at 30 °C. Quadratic regression functions predicted maximal plant dry mass, leaf surface area, and stomatal conductance at 29, 29, and 28 °C, respectively. Stem xylem water potential (ψ) during the photoperiod decreased linearly with increasing root-zone temperature from -0.83 MPa at 26 °C to -1.05 MPa at 36 °C. Our second objective was to compare six clones of silver maple from the Mississippi location with six clones from 44.4 °N latitude in Minnesota for effects of 35 °C in the root zone on plant growth, stomatal conductance, and stem ψ. Provenance and temperature main effects were significant for most dependent variables, but there were no provenance × temperature interactions. Over both provenances, plant fresh and dry mass, leaf surface area, stomatal conductance, and stem ψ during the photoperiod were higher at 29 than 35 °C. Over both temperatures, plants from Minnesota clones had higher fresh and dry mass and more leaf surface area than plants from Mississippi clones. The lack of temperature × provenance interactions suggests that ecotypic or clinal variation in heat resistance is minimal and will not be useful for identifying superior genotypes for use in interspecific crosses with red maple (Acer rubrum L.).
William A. Hoch, Geunhwa Jung, and Brent H. McCown
A significant pest affecting commonly planted Betula spp. is the birch leafminer (Fenusa pusilla Lepeletier), an insect that can be present in large populations in the landscape and can greatly reduce the vigor and ornamental value of these trees. Twenty-two interspecific crosses were performed between leafminer resistant and susceptible Betula species in an attempt to create the novel combination of ornamental white bark and significant leafminer resistance. Of the nine successful crosses, two produced resistant offspring. Progeny of the diploid × hexaploid cross B. turkestanica Litvin (2x) × B. alleghaniensis Britt. (6x) displayed a broad range of resistance levels, likely the result of segregating alleles contributed by the hexaploid parent. All crosses involving highly resistant individuals of B. costata Trautv. (2x) yielded leafminer susceptible progeny. These results suggest that the larval antibiosis demonstrated by B. alleghaniensis and B. costata is inherited as a recessive trait, and exhibits a gene dosage effect as evidenced by the B. turkestanica × B. alleghaniensis offspring. While most progeny of the B. populifolia Marsh (2x) × B. maximowicziana Regal (2x) cross were susceptible, a single resistant offspring, which was found to be triploid (3x), displayed a mechanism of resistance similar to that of a hypersensitive response. No strong intersectional barriers to hybridization were observed and all interploidy crosses were successful. The chromosome numbers of B. costata (2n = 2x = 28) and B. turkestanica (2n = 2x = 28) are reported here for the first time. The results of this study indicate that the potential exists for the development of insect resistant, ornamental white-barked birch clones through the implementation of a planned, systematic breeding program.
Hongwen Huang, Desmond R. Layne, and Thomas L. Kubisiak
Twelve, 10-base primers amplified a total of 20 intense and easily scorable polymorphic bands in an interspecific cross of PPF1-5 pawpaw [Asimina triloba (L.) Dunal.] × RET (Asimina reticulata Shuttlew.). In this cross, all bands scored were present in, and inherited from, the A. triloba parent PPF1-5. Nineteen of the 20 bands were found to segregate as expected (1:1 or 3:1) based on chi-square goodness-of-fit tests, and were subsequently used to evaluate genetic diversity in populations of A. triloba collected from six states (Georgia, Illinois, Indiana, Maryland, New York, and West Virginia) within its natural range. Analysis of genetic diversity of the populations revealed that the mean number of alleles per locus was A = 1.64, percent polymorphic loci was P = 64, and expected heterozygosity was He = 0.25. No significant differences were found among populations for any of the polymorphic indices. Partitioning of the population genetic diversity showed that the average genetic diversity within populations was Hs = 0.26, accounting for 72% of the total genetic diversity. Genetic diversity among populations was Dst = 0.10, accounting for 28% of the total genetic diversity. Nei's genetic identity and distance showed a high mean identity of 0.86 between populations. Genetic relationships among the populations examined by unweighted pair-group mean clustering analysis separated the six populations into two primary clusters: one composed of Georgia, Maryland, and New York, and the other composed of Illinois, Indiana, and West Virginia. The Georgia and Indiana populations were further separated from the other populations within each group. This study provides additional evidence that marginal populations within the natural range of A. triloba should be included in future collection efforts to capture most of the rare and local alleles responsible for this differentiation.
R.L. Fery and J.A. Thies
The peanut root-knot nematode (Meloidogyne arenaria race 1) is potentially a major pest of pepper cultivars belonging to the species Capsicum chinense. Greenhouse tests were conducted to: 1) compare the level of resistance to the peanut root-knot nematode exhibited by the recently released C. chinense germplasm line PA-353 to that exhibited by the C. annuum cv. Carolina Cayenne; 2) to determine the inheritance of the resistance in the C. chinense germplasm line PA-353; and 3) to determine the genetic relationship between the resistance exhibited by the C. chinense germplasm line PA-353 and that exhibited by the C. annuum cv. Carolina Cayenne. The level of resistance exhibited by the C. chinense germplasm line PA-353 was equal to the high level of resistance of the C. annuum cv. Carolina Cayenne. Evaluation of parental, F1, F2, and backcross populations of the cross between the resistant C. chinense germplasm line PA-353 and the susceptible C. chinense accession PA-350 indicated that the resistance in C. chinense is conditioned by a single dominant gene. The F2 population of the interspecific cross between the resistant C. chinense germplasm line PA-353 and the resistant C. annuum cv. Carolina Cayenne did not segregate for resistance, indicating that the dominant resistance gene in C. chinense is likely allelic to or closely linked to a gene conditioning resistance in C. annuum. The availability of a simply inherited source of outstanding resistance makes breeding for peanut root-knot nematode resistance a viable objective in C. chinense breeding programs.
Michael E. Compton, Brenda L. Fuchs, and Jack E. Staub
Cucumis hystrix Chakr. is a rare cucurbit species native to Asia. The species is valued by breeders because of its multiple branching habit and has been used in interspecific crosses with Cucumis sativus. However, individual C. hystrix plants have not been identified in the wild since 1990. Therefore, it was our objective to develop a micropropagation protocol that would allow us to clonally propagate plants in cultivation. Shoots tips (2 cm) were excised from a single C. hystrix plant grown in the greenhouse. All tendrils and leaves were removed before surface-sterilization in 1.25% NaOCl for 5 or 10 min and rinsed six times with sterile distilled water. Shoot tips were trimmed to 1 cm (meristem with two to three young leaf primordia) and placed into 25 × 125-mm test tubes containing 25 ml of initiation medium [MS plus (per liter) 100 mg inositol, 30 g sucrose and 5 g Agargel; pH 5.7-5.8]. PGR combinations tested were initiation medium with 1 μM BA, and initiation medium with 1.7 μM IBA, 0.5 μM kinetin and 0.3 μM GA3 (IKG). Explant survival was greater when shoot tips were surface-sterilized for 5 min (75%) compared to 10 min (33%). More axillary shoots formed when shoot tips were cultured in IKG medium (10.8) than in medium with BA (5.5). Shoots were considerably longer (10 mm) when cultured in medium with IKG compared to BA (1.5 mm). About 64% of shoots place in medium containing 8 μM NAA formed roots and were acclimatized to greenhouse conditions.