| After chestnut blight, caused by the accidental introduction of the pathogenic fungus Cryphonectria parasitica, ravaged American chestnut (Castanea dentata) populations, other chestnut species attracted more interest because blight resistance was of primary concern. Therefore, the Michigan chestnut population consists of American, Chinese (C. mollissima), Japanese (C. crenata) and European (C. sativa) trees and their hybrids. For orchards, selection of grafted cultivars is crucial for long-term commercial success. Due to the mixture of non-grafted seedling trees, hybrid trees, and grafted cultivars in Michigan orchards and the variability of nut production within orchards the two objectives of my study were to 1) genetically identify commercially important chestnut cultivars currently growing in Michigan using simple sequence repeat (SSR) markers, and 2) improve our understanding of chestnut pollination including reproductive phenology, and nut-set using genetic analysis of parents and offspring.;To genetically identify chestnut cultivars growing in Michigan, a total of 110 samples representing 9 European hybrid cultivars and 2 Chinese cultivars were genotyped. The efficacy of 5 previously determined SSR markers to describe the genetic diversity among 8 chestnut cultivars was evaluated using IDENTITY, POPGENE and CERVUS software. The number of alleles per locus ranged from 10 to 19 alleles with intermediate to high levels of heterozygosity (0.457-0.923). Polymorphic information content (0.693-0.797) and power of discrimination (0.707-0.819) were determined. High levels of genetic diversity were observed in the chestnut population included in this study, where 56 genotypes were defined. The overall SSR profile of each cultivar consisted of alleles useful for the identification of each cultivar included in this study. Unique alleles were obtained with each SSR locus and useful for the identification of 5 out of the 11 chestnut cultivars ('Colossal', 'Benton Harbor', 'Everfresh', 'Nevada', and 'Okei'). Out of the 5 SSR primer sets used, a combination of two primer sets were always sufficient to identify each cultivar, however, the selection of useful primers requires prior knowledge of the cultivars being differentiated. These SSR primer sets were able to identify the parents of F1 progeny when two cultivars, ('Benton Harbor' and 'Okei') pollinized a third cultivar ('Colossal'). The SSR-based identification of individual nuts could only be performed if the SSR alleles of the parental trees involved in the cross were known and partially unique to each chestnut cultivar. The SSR profile resulting from the primer sets (EMCs15 and CsCAT1) was sufficient to identify the paternal parent of each nut recovered from the pollination event.;To better understand chestnut pollination, experimental crosses were conducted during the 2008-2010 growing seasons. In controlled, natural pollination experiments, the interaction between pollen and flowers was monitored. For one cultivar ('Colossal'), female flowers were receptive to pollen as early as 19-June, and as late as 3-August. In controlled pollination experiments performed in mid-Michigan, pollen was made available to flowers at pre-anthesis, anthesis, and post-anthesis. The highest level of nut production occurred when pollen was available at anthesis on 11-July, 6-July, and 3-July for the 2008, 2009, and 2010 growing seasons, respectively. Pollen application at anthesis was important to obtain a high number of nuts, however, anthesis shifted from year to year. Studies of genetic characterization and pollination biology should provide opportunities that will help Michigan growers establish and maintain high quality commercial chestnut plantings by improving cultivar identification as well as issues related to pollen timing and nut-set. |
