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  • Author or Editor: Fenny Dane x
  • Journal of the American Society for Horticultural Science x
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Genetic variation among nine populations of Ozark chinkapin [Castanea pumila (L.) Mill. var. ozarkensis (Ashe) Tucker], threatened by their susceptibility to chestnut blight (Cryphonectria parasitica (Murrill) Barr), was investigated. Population genetic parameters estimated from isozyme variation suggest the populations have a higher genetic diversity (He = 0.227) than populations of the other Castanea Mill. species on the North American continent, the American chestnut (C. dentata (Marsh.) Borkh.) High levels of heterozygosity were detected within the populations, but nonsignificant differences in genetic diversity were observed among the different populations. Principal component analysis based on isozyme allele frequencies or randomly amplified polymorphic DNA phenotype frequencies showed clustering of the same populations. Populations with high levels of genetic diversity and unusual alleles should be the focal point of conservation biologists for capturing much of the genetic variation of the species.

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Allozyme polymorphism in chestnut (Castanea) species was investigated using isoelectric focusing in thin-layer polyacrylamide slab gels. Genetic analysis of the progenies of intraspecific crosses and interspecific F2s and backcrosses (BC1s) allowed the verification of 11 polymorphic isozyme loci from 11 enzyme systems. The following loci were defined: Acp, Adh, Est-1, Est-2, Est-5, Me, Prx-1, Prx-2, Prx-3, Skd-3, and Skd-4. All polymorphic loci behaved as single-locus Mendelian genes. Skd showed unique species specificity. Skd-1 and Skd-2 were unique to the American chestnut (C. dentata Borkh.) and the European chestnut (C. sativa Mill.), whereas Skd-3 and Skd-4 were unique to the Chinese chestnut (C. mollissima Bl.) and the Japanese chestnut (C. crenata Sieb.). Linkage analysis revealed linkage for three pairs of loci: Skd-3/Skd-4, Est-1/Est-2, and Est-5/Prx-1. The single-tree progeny method was used successfully for isozyme genetic analysis. Forty-seven chestnut cultivars in six chestnut species were characterized using 12 isozyme loci and can be unambiguously identified by 12 multi-locus genotypes. The interspecific and geographic relationships among species were also discussed.

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Selected tomato (Lycopersicon esculentum Mill) genotypes were evaluated for their fruit-setting ability under high-temperature field conditions. A temperature-controlled greenhouse study was conducted to determine the percent fruit set from the total number of flowers and fruit produced per plant. Ratings for set obtained under high-temperature field conditions were significantly (P = 0.001) correlated with percent fruit set determined under similar greenhouse conditions. Most of the Asian Vegetable Research and Development Center (AVRDC) selections, Beaverlodge lines, `Nagcarlan', and `Red Cherry' could be considered heat-tolerant. Small-fruited, abundantly flowering genotypes were less affected by heat stress than larger-fruited cultivars. Prolonged periods of high temperature caused drastic reductions in pollen fertility in most genotypes, except `Red Cherry' and L. esculentum var. cerasiforme (PI 190256). Stigma browning and stigma exsertion commonly occurred on all lines, except AVRDC CL-5915-553 and PI 190256. Diallel analyses indicated that pollen fertility and fruit set under high field temperatures were primarily under additive gene control.

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Genetic relationships among 970 cucumber (Cucumis sativus L.) plant introductions (PIs) in the U.S. National Plant Germplasm System (NPGS) were assessed by observing variation at 15 isozyme loci. Allozyme frequency data for these PIs were compared to allozyme variation in heirloom and modern (H&M) cultivars released from 1846-1985 (H&M cultivars; 178 accessions), and experimental commercial (EC) germplasm (EC germplasm; 82 accessions) in use after 1985. Multivariate analysis defined four distinct groups of accessions (Groups A-D), where Group A consisted of PIs received by the NPGS before 1992, Group B contained PIs from India and China obtained by NPGS after 1992, Group C consisted of EC germplasm, and Group D contained H&M cultivars. Morphological, abiotic stress (water and heat stress tolerance) and disease resistance evaluation data from the Germplasm Resources Information Network (GRIN) for the PIs examined were used in conjunction with estimates of population variation and genetic distance estimates to construct test arrays and a core collection for cucumber. Disease resistance data included the evaluation of angular leafspot [Pseudomonas lachrymans (E.F. Smith) Holland], anthracnose [Colletotrichum lagenarium (Ross.) Ellis & Halst], downy mildew [Pseudoperonospora cubensis (Berk. & Curt) Rostow], rhizoctonia fruit rot (Rhizoctonia solani Kuhn), and target leafspot [Corynespora cassiicola (Berk. & Curt) Wei] pathogenicity. The test arrays for resistance-tolerance to angular leafspot, anthracnose, downy mildew, rhizoctonia fruit rot, target leafspot, and water and heat stress consisted of 17, 16, 17, 16, 17, 16, and 16 accessions, respectively. The core collection consisted of accessions in these test arrays (115) and additional 32 accessions that helped circumscribe the genetic diversity of the NPGS collection. The core collection of 147 accessions (115 + 32) represents ≈11% of the total collection's size (1352). Given estimates of genetic diversity and theoretical retention of diversity after sampling, this core collection could increase curatorial effectiveness and the efficiency of end-users as they attempt to identify potentially useful germplasm.

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Isozyme, randomly amplified polymorphic DNA (RAPD), and simple sequence repeats (SSR) markers were used to generate a linkage map in an F2 and F3 watermelon [Citrullus lanatus (Thumb.) Matsum. & Nakai] population derived from a cross between the fusarium wilt (Fusarium oxysporum f. sp. niveum) susceptible `New Hampshire Midget' and resistant PI 296341-FR. A 112.9 cM RAPD-based map consisting of 26 markers spanning two linkage groups was generated with F2 data. With F3 data, a 139 cM RAPD-based map consisting of 13 markers covering five linkage groups was constructed. Isozyme and SSR markers were unlinked. About 40% to 48% of the RAPD markers were significantly skewed from expected Mendelian segregation ratios in both generations. Bulked segregant analysis and single-factor analysis of variance were employed to identify RAPD markers linked to fusarium wilt caused by races 1 and 2 of F. oxysporum f. sp. niveum. Current linkage estimates between the resistance trait and the marker loci were too large for effective use in a marker-assisted selection program.

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