After chestnut blight, caused by the accidental introduction of the pathogenic fungusCryphonectria parasitica, ravaged American chestnut (Castanea dentata) populations, otherchestnut 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 ofgrafted cultivars is crucial for long-term commercial success. Due to the mixture of non-graftedseedling trees, hybrid trees, and grafted cultivars in Michigan orchards and the variability of nutproduction within orchards the two objectives of my study were to 1) genetically identifycommercially important chestnut cultivars currently growing in Michigan using simple sequencerepeat (SSR) markers, and 2) improve our understanding of chestnut pollination includingreproductive phenology, and nut-set using genetic analysis of parents and offspring.To genetically identify chestnut cultivars growing in Michigan, a total of 110samples representing 9 European hybrid cultivars and 2 Chinese cultivars were genotyped. Theefficacy of 5 previously determined SSR markers to describe the genetic diversity among 8chestnut cultivars was evaluated using IDENTITY, POPGENE and CERVUS software. Thenumber of alleles per locus ranged from 10 to 19 alleles with intermediate to high levels ofheterozygosity (0.457-0.923). Polymorphic information content (0.693-0.797) and power ofdiscrimination (0.707-0.819) were determined. High levels of genetic diversity were observed inthe chestnut population included in this study, where 56 genotypes were defined. The overallSSR profile of each cultivar consisted of alleles useful for the identification of each cultivarincluded in this study. Unique alleles were obtained with each SSR locus and useful for theidentification 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 werealways sufficient to identify each cultivar, however, the selection of useful primers requires priorknowledge of the cultivars being differentiated. These SSR primer sets were able to identify theparents of F1 progeny when two cultivars, (`Benton Harbor' and `Okei') pollinized a thirdcultivar (`Colossal'). The SSR-based identification of individual nuts could only be performed ifthe SSR alleles of the parental trees involved in the cross were known and partially unique toeach 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 duringthe 2008-2010 growing seasons. In controlled, natural pollination experiments, the interactionbetween pollen and flowers was monitored. For one cultivar ('Colossal'), female flowers werereceptive to pollen as early as 19-June, and as late as 3-August. In controlled pollinationexperiments 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 wasavailable at anthesis on 11-July, 6-July, and 3-July for the 2008, 2009, and 2010 growingseasons, respectively. Pollen application at anthesis was important to obtain a high number ofnuts, however, anthesis shifted from year to year. Studies of genetic characterization andpollination biology should provide opportunities that will help Michigan growers establish andmaintain high quality commercial chestnut plantings by improving cultivar identification as wellas issues related to pollen timing and nut-set.
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