Disease resistance is an objective of most breeding programs for small fruits, tree fruits, and nuts. Often a moderate level of resistance is adequate, and must be combined with many other desirable horticultural characteristics. Classical methods (a segregating population of the host plant is inoculated with a virulent isolate of the pathogen under environmental conditions appropriate for disease development) have been used with great success and have incorporated both horizontal and vertical resistance. Molecular approaches offer new opportunities and are likely to be appropriate and cost-effective in a few situations. Transformation is not yet routine in fruit and nut crops, and there is a shortage of useful genes. Genetic maps are being constructed using RFLP and RAPD markers in several species, allowing determination of number and location of important genes as well as indirect selection based on linked markers. This presentation will include examples of both classical and molecular approaches as they are used in the genetic improvement of fruit and nut crops with an emphasis on fungal and bacterial diseases.
Shawn A. Mehlenbacher
Incompatibility in hazelnut (Corylus avellana L.) is of the sporophytic type and is under the control of a single S-locus with multiple alleles. Tests in recent years have identified four new alleles, bringing the total to 26. Improved pollen testers have been identified for several alleles. The S-alleles of more than 90 cultivars have been identified by fluorescence microscopy and will be presented. These cultivars (and their alleles) include Tonda di Giffoni (2 23), San Giovanni (2 8), Gasaway (3 26), Gunslebert (5 23), Kadetten (20 25), Lang Tidlig Zeller (4 20), Nocciolino Sangrato (7 17), Rode Zeller (6 11), Segorbe (9 23), and Simon (6 22).
Shawn A. Mehlenbacher
Chilling requirements of 44 genotypes of Corylus avellana L. were estimated by cutting shoots in the field at weekly intervals and forcing them in a warm greenhouse for four weeks. The chilling requirements of catkins, female flowers, and leaf buds were assumed to have been met when development occurred on more than half of the respective plant parts. Chilling requirements were lowest for catkins and highest for leaf buds, and ranged from <100 to 860 hours for catkins, 290-1550 hours for female flowers, and 365-1395 hours for leaf buds. The lowest chilling requirements were observed for the leading cultivars of Turkey and southern Italy. The yellow-leafed ornamental C. avellana var. aure a had very high chilling requirements for all plant parts.
Shawn A. Mehlenbacher
The european hazelnut (Corylus avellana L.) is native to most of Europe and nearby areas in Asia Minor and the Caucasus Mountains. Cross-pollination is enforced by sporophytic incompatibility under the control of a single locus with multiple alleles (haplotypes). Fluorescence microscopy is routinely used to determine if a pollination is compatible or incompatible, and use of an array of known testers allows identification of the alleles of cultivars and selections. Both alleles are expressed in the stigmas, but often only one is expressed in the pollen because of dominance. Cultivars are highly heterozygous diploids (2n = 2x = 22) and clonally propagated. Most of the world's leading cultivars were selected from local wild populations near where they are now planted on a commercial scale. Genetic improvement efforts are recent and, although tremendous genetic variability is available, such efforts have had little impact outside of Oregon and France. Studies of genetic diversity using simple sequence repeat markers have placed most cultivars in one of the four main groups: Spanish-Italian, Central European, English, or Black Sea. This study presents 17 years of data on incompatibility in hazelnut, including the discovery of six new S-alleles and determination of the dominance relationships among 105 new pairs of alleles. The total number of alleles now stands at 33. The S-alleles of 284 cultivars, 13 interspecific hybrids, and 522 selections of diverse origin are presented. The S-alleles identified in hazelnut cultivars is information that should be useful to breeders in the planning of crosses, to curators of germplasm collections, and to growers and nurseries as they choose cultivars and pollenizers when designing orchards. Differences in S-allele frequency seen in the cultivars and selections are related to geographic origin. The most common alleles of cultivars in the major geographical groups are S 2 in the Spanish-Italian group, S 5 in the Central European group, S 3 in the English group, and S 4 in the Black Sea group. Most selections belonged to the Black Sea group, and S 4 was by far the most common allele. Differences in allele frequency were also observed among seed lots within a country.
Shawn A. Mehlenbacher
Semihardwood cuttings of 82 genotypes of peach [Prunus persica (L.) Batsch] and complex hybrids of peach with almond (P. amygdalus Batsch), P. davidiana, P. kansuensis, and P. persica vulgaris siberica were rooted under mist. Average rooting percentages ranged from 7% to 100%. Peach selections generally rooted well with the exception of hardy rootstock selections. P. davidiana rooted poorly, but most of its hybrid progeny rooted well. P. kansuensis and its hybrid progeny rooted well. Peach x almond F1 hybrids generally rooted poorly; the rooting percentage was higher in backcrosses to peach.
Qiang Yao and Shawn A. Mehlenbacher
Seventy-seven trees representing 41 hazelnut (Corylus avellana L.) genotypes were to evaluate variance components and broad-sense heritability for 10 nut and kernel traits from 1994 to 1996. All effects in the models were assumed to be random. All traits had extremely high heritability. This indicated that nearly all of the phenotypic variation had a genetic basis. Knowledge of variance components may help us efficiently allocate resources. Broad-sense heritability estimates were larger than those in narrow sense, suggesting the presence of nonadditive genetic variation in the population.
Veli Erdogan and Shawn A. Mehlenbacher
Eight Corylus L. (hazelnut) species were intercrossed in all possible combinations to reveal genetic relationships. Pollinations were made on either individually bagged branches or trees covered entirely with polyethylene using mixtures of pollen of five genotypes to minimize low cluster set due to single incompatible combinations. Percent cluster set, seed germination, and hybrid seedling survival were determined. Hybridity of seedlings was verified by inspection of morphological traits. Based on percent cluster set, seed germination, and hybrid seedling survival along with observed morphological similarities, Corylus species were placed in three groups: 1) the tree hazels C. colurna L. (turkish tree hazel) and C. chinensis Franchet (chinese tree hazel), 2) the bristle-husked shrub species C. cornuta Marshall (beaked hazel), C. californica (A.DC.) Rose (california hazel), and C. sieboldiana Blume (manchurian hazel), and 3) the leafy-husked shrub species C. avellana L. (european hazel), C. americana Marshall (american hazel), C. heterophylla Fischer (siberian hazel), and C. heterophylla Fischer var. sutchuensis Franchet (sichuan hazel). The two tree hazel species crossed with each other readily, as did the three bristle-husked shrub species. The frequency of blanks was low (<20%) for crosses of the tree hazels, and <50% for interspecific crosses within the group of bristle-husked species. The leafy-husked shrub species could be crossed with each other in all directions, although cluster set on C. heterophylla was low. For crosses of species belonging to different groups, set was generally low and the frequency of blanks high. Nevertheless, a few hybrid seedlings were obtained from several combinations. When used as the female parent, C. californica set nuts when crossed with all other species, indicating possible value as a bridge species. Crosses involving C. avellana were more successful when it was the pollen parent. In crosses with C. avellana pollen, cluster set on C. chinensis was better than on C. colurna and the frequency of blanks was much lower, indicating that it might be easier to transfer nonsuckering growth habit from C. chinensis than from C. colurna. Reciprocal differences in the success of crosses was observed. The following crosses were successful C. californica × C. avellana, C. chinensis × C. avellana, C americana × C. heterophylla, C. cornuta × C heterophylla, C. californica × C. colurna, and C. americana × C. sieboldiana, but the reciprocals were not.
Gehendra Bhattarai and Shawn A. Mehlenbacher
From the genome sequence of hazelnut (Corylus avellana), 192 new polymorphic simple sequence repeat (SSR) markers were developed, characterized, and used to investigate genetic diversity in 50 accessions. Next-generation sequencing allows inexpensive sequencing of plant genomes and transcriptomes, and efficient development of polymorphic SSR markers, also known as microsatellite markers, at low cost. A search of the genome sequence of ‘Jefferson’ hazelnut identified 9094 fragments with long repeat motifs of 4, 5, or 6 base pairs (bp), from which polymorphic SSR markers were developed. The repeat regions in the ‘Jefferson’ genome were used as references to which genomic sequence reads of seven additional cultivars were aligned in silico. Visual inspection for variation in repeat number among the aligned reads identified 246 as polymorphic, for which primer pairs were designed. Polymerase chain reaction (PCR) amplification followed by agarose gel separation indicated polymorphism at 195 loci, for which fluorescent forward primers were used to amplify the DNA of 50 hazelnut accessions. Amplicons were post-PCR multiplexed for capillary electrophoresis, and allele sizes were determined for 50 accessions. After eliminating three, 192 were confirmed as polymorphic, and 169 showed only one or two alleles in each of the 50 cultivars, as expected in a diploid. At these 169 SSRs, a total of 843 alleles were found, for an average of 4.99 and a range of 2 to 17 alleles per locus. The mean observed heterozygosity, expected heterozygosity, polymorphism information content, and the frequency of null alleles were 0.51, 0.53, 0.47, and 0.03, respectively. An additional 25 primer pairs produced more than two bands in some accessions with an average of 6.8 alleles. The UPGMA dendrogram revealed a wide genetic diversity and clustered the 50 accessions according to their geographic origin. Of the new SSRs, 132 loci were placed on the linkage map. These new markers will be useful for diversity and parentage studies, cultivar fingerprinting, marker-assisted selection, and aligning the linkage map with scaffolds of the genome sequence.
Kahraman Gürcan and Shawn A. Mehlenbacher
Microsatellite-containing sequences for the Betulaceae (Betula, Corylus, and Alnus) were retrieved from GenBank and used to develop twelve new microsatellite marker primer pairs that amplified and were polymorphic in european hazelnut (Corylus avellana). The primer pairs were characterized using 50 european hazelnut accessions. Nine of these microsatellites that segregated in a mapping population were assigned to linkage groups. The 12 new primer pairs will be useful in genetic studies in Corylus and Betula. To investigate transferability of microsatellite primer pairs in the family Betulaceae, we assessed the ability of 129 simple sequence repeat (SSR) primer pairs (75 from Corylus, 52 from Betula, and two from Alnus) to amplify DNA of 69 accessions representing diverse taxa. Microsatellite primer pairs from Betula amplified 92% of Betula, 51% of Alnus, 41% of Corylus, 37% of Carpinus, 35% of Ostryopsis, and 34% of Ostrya accessions. In the 69 accessions, microsatellite primer pairs from Corylus amplified 81% of Corylus, 55% of Carpinus, 53% of Ostrya, 51% of Ostryopsis, 41% of Alnus, and 39% of Betula accessions. An additional 147 SSR primer pairs developed from Corylus, used to amplify a subset of 32 accessions, gave similar values: 92% in Corylus, 33% in Carpinus, 33% in Ostrya, 44% in Ostryopsis, 35% in Alnus, and 54% in Betula. The high transferability (>39%) of microsatellite primer pairs between Betula and Corylus will allow comparative studies of the two genera with the greatest economic importance.