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- Author or Editor: Norman F. Weeden x
Abstract
Genetic analysis of 11 allozyme polymorphisms was performed on the progeny of ‘Cayuga White’ × ‘Aurora’, two complex interspecific grape (Vitis) hybrids. Segregation for most of the polymorphisms closely approximated monogenic Mendelian ratios, and eight new isozyme loci were defined for grape. Joint segregation analysis among the isozyme loci revealed three multilocus linkage groups. These results demonstrate that sufficient allozyme polymorphism exists in grape to establish many multilocus linkage groups and that this genetic analysis can be accomplished using extant progeny or progeny readily produced from highly heterozygous clones.
One of the primary progenitors of the cultivated apple is Malus sieversii L., a species native to the forested regions of central Asia. Despite the horticultural importance of M. sieversii, little is known about genetic variation in this species. In this study, allozyme diversity at 18 loci was determined for 259 seedlings belonging to 31 sib families, each consisting of the set of offspring from a different open-pollinated maternal (seed) parent. Maternal parents belonged to 14 populations from four geographic regions. Genetic diversity statistics were computed from the resulting allele and phenotype frequencies. Cluster analysis of sib families showed that there was some grouping based on geographic region, but 16 of the sib families were most closely related to sib families from other regions. Analysis of molecular variance (AMOVA) indicated that 85% of the enzyme variability was due to differences among sib families within populations and 15% was due to differences among regions. No variability could be assigned to differences among populations within regions. In addition, no alleles were found that were fixed in a region and unique to that region. These results suggest that plants belonging to M. sieversii effectively form a panmictic population. Consequently, a thorough sampling of a few large populations will efficiently capture most of the genetic diversity present in wild M. sieversii.
One of the primary progenitors of the cultivated apple (Malus ×domestica) is M. sieversii, a species native to the forested regions of Central Asia. Despite the horticultural importance of M. sieversii, little is known about its genetic variation. In this study, isozyme diversity at 18 loci was determined for 259 open-pollinated offspring belonging to 31 different maternal half-sib families collected from 14 different populations in 4 regions of central Asia. Genetic diversity statistics were computed from the resulting allele and phenotype frequencies. Cluster analysis of half-sib families showed that there was some grouping based on geographic region, but 16 of the half-sib families were most closely related to half-sib families from other regions. AMOVA, the analysis of molecular variance, indicated that most of the enzyme variability (85%) was attributable to differences among half-sib families within populations, none could be assigned to populations within regions, and 15% was due to differences among regions. In addition, no alleles were found that were both fixed in a region and unique to that region. These results suggest that plants belonging to M. sieversii effectively form a single panmictic population. Thus, a thorough sampling of a few large populations will efficiently capture most of the genetic diversity present in wild M. sieversii.
The columnar mutation `Wijcik McIntosh' has attracted much attention because of its compact growth habit, which is compatible with high-density plantings. Using bulked segregant analysis, we identified several randomly amplified polymorphic DNA (RAPD) markers that displayed a close linkage with the columnar locus (Co). The RAPD marker that displayed the closest linkage was end sequenced to develop a sequence tagged site for rapidly screening segregating populations. A simple sequence repeat (SSR) of (GA)17 was identified within the DNA fragment. Four allelic forms, including an apparent null allele, could be distinguished among the cultivars tested. The null allele displayed close linkage with Co in two progenies, and we used this marker to identify the location of the gene on the apple linkage map.
The positions of over 50 SSR loci and other sequence tagged sites (STSs) have been located on the linkage maps of five apple cultivars (Rome Beauty, White Angel, Golden Delicious, Liberty, McIntosh) and two New York accessions. In most cases, the primers used produced single amplification products, permitting identification of homologous loci in the different cultivars and the precise alignment of the linkage maps generated for each. Based on this information, we present a general linkage map for apple with STS markers on each linkage group. The map consists of 17 linkage groups (equal to the haploid chromosome number for the species) with over 500 markers. The positions of several resistant gene analogues have been located on this linkage map. None of these sequences map near genes conferring resistance to scab or powdery mildew. SSR loci exhibited a tendency to cluster in certain regions of the linkage map. This clustering slightly reduces their effectiveness as genome markers for comparative mapping or germplasm diversity. However, the SSR markers definitely displayed a high level of polymorphism, making them particularly useful for genetic studies.
Bulked segregant analysis was used to identify RAPD markers that display tight linkage to the Vf gene in apple (Malus sp.) that confers resistance to five races of apple scab [Venturia inaequalis (Cke.) Wint.]. We identified several new RAPD markers linked to Vf. The most tightly linked marker in the test population, S52500, was cloned and sequenced. A linkage map of the Vf region was developed using these markers, RAPD markers previously described by other laboratories, and the isozyme locus Pgm-1. An assay was developed for Vf by multiplexing the two markers closely flanking the Vf locus. This assay has a theoretical `escape' value (discarding a resistant plant) of 3% and an error rate (selection of a susceptible plant) of 0.02%.
Fruit from seedlings selected from a population obtained by crossing superficial scald-resistant `White Angel' and superficial scald-susceptible `Rome Beauty' apple (Malus ×domestica Borkh.) were used to investigate the role of oxidative processes in the development of superficial scald. Selections were identified for study based on fruit coloration and scald susceptibility. Plant material had one of the following three physiognomies: 1) red-skinned fruit resistant to scald; 2) red-skinned fruit susceptible to scald; and 3) yellow-skinned fruit susceptible to scald. The concentrations of α-farnesene, conjugated triene (CT) species, hydrogen peroxide (H2O2), thiobarbituric acid-reactive substances, carbonyl groups, and the activities of superoxide dismutase, guaiacol-peroxidase, and catalase were measured at harvest and during 0.5 °C storage. Relationships were poor between scald susceptibility and α-farnesene and its oxidation products, CT258, CT281, and the CT258/CT281 ratio. Tissue concentrations of H2O2 were lower in scald-resistant than in scald-susceptible fruit at harvest and after storage, and these lower concentrations were associated with less damage to cellular membranes (lipid peroxidation) and proteins (carbonyl group content). Higher activities of the H2O2-degrading enzymes, guaiacol-peroxidases, and catalases, were related to lower H2O2 content and lower scald susceptibility. Activities of superoxide dismutase, a H2O2-generating enzyme, were not related to scald susceptibility or storage period. We hypothesize that fruit susceptibility to scald is strongly influenced by cellular efficiency in metabolizing active oxygen species such as H2O2.