The chestnut (Castanea spp.) has been an important food source in East Asia, Asia Minor, Europe, and North America for centuries. Three main chestnut species are currently produced commercially and are naturally distributed around the world: Japanese chestnut (C. crenata Sieb. et Zucc.) in Japan and the Korean Peninsula, Chinese chestnut (C. mollissima Bl.) in China, and European chestnut (C. sativa Mill.) in Europe and Asia Minor.
The history of chestnut use in Japan is very long. For example, a large number of chestnut pericarps and large chestnut timbers were found in a tower at the Sannai-Maruyama ruins (3500 to 2000 BCE) in Aomori Prefecture, northern Japan (Sawamura, 2006). For primitive Japanese people, chestnuts were important sources of food and construction materials. Even now, they are commercially grown throughout Japan for food, and chestnut orchards covered more than 21,400 ha in 2011, ranking fourth among woody fruit and nut crops in Japan after citrus, apple, and persimmon (FAOSTAT, 2013).
An organized chestnut breeding program started in 1947 at the National Institute of Fruit Tree Science in Japan and continues at the National Agriculture and Food Research Organization (NARO) Institute of Fruit Tree Science (NIFTS; Kotobuki et al., 1999; Pereira-Lorenzo et al., 2012). Currently, the most important target of the program is to release Japanese chestnut cultivars with an easily peeled pellicle. One such elite cultivar, Porotan, was released by NIFTS in 2006 (Pereira-Lorenzo et al., 2012; Saito et al., 2009). In addition, it is desirable to broaden the range of NHD to provide cultivars that are adapted to a much wider range of growing seasons. Simultaneously, large nut weight, the absence of PS, and freedom from II by the peach moth, Conogethes punctiferalis (Lepidoptera: Crambidae), are also important targets. The easily peeled pellicle trait in Japanese chestnut is controlled by a major gene at a single locus (Takada et al., 2012). Molecular markers associated with this trait are now being used in applied chestnut breeding programs (Nishio et al., 2013). On the other hand, the inheritance of NHD and NW has been insufficiently studied. Kotobuki et al. (1984) reported the narrow-sense heritability of NHD and NW using F1 progeny from the chestnut breeding program at NIFTS, but their experiment was not replicated over multiple years, and they did not estimate the environmental variance components. In addition, no reports are available for the inheritance of PS and II and the associated selection efficiency. However, breeders have recorded the performance data for those traits of cultivars/selections for many years in the NIFTS breeding program.
Environmental variance in an orchard comprises the variance among years and variance among trees within a genotype as well as the genotype × year interaction variance, the tree × year interaction variance, and the variance among nuts (fruits) within a tree, which can all be estimated by repeating measurements of replications of several genotypes in 2 or more years (Hansche and Beres, 1966; Hansche and Brooks, 1965; Sato et al., 2000; Yamada et al., 1993, 2002). Genetic and environmental variance has been estimated for many fruit crops such as sweet cherry (Hansche and Beres, 1966), Japanese persimmon (Yamada et al., 1993), strawberry (Sacks and Shaw, 1994; Shaw, 1991), peach (De Souza et al., 1998), grape (Sato et al., 2000), guava (Thaipong and Boonprakob, 2005), and Japanese pear (Nishio et al., 2011). Once accurate estimates of the components of environmental variance have been obtained under a given set of cultural conditions, the environmental variance under any given number of replications and measurement repetitions can be estimated for those conditions (Nyquist and Baker, 1991; Yamada et al., 1993, 2002).
It is necessary to estimate the broad-sense heritability to determine whether the breeder has made or will be able to make an effective selection. If the broad-sense heritability is zero, any selection is meaningless, and the breeder cannot select superior genotypes. Therefore, breeders should concentrate on traits with high heritability. Broad-sense heritability is defined as the ratio of genetic variance to phenotypic variance. However, the broad-sense heritability of a breeding offspring population is difficult to estimate using the seedlings from controlled crosses, because the use of seedlings does not permit replication of each seedling’s genotype and because the seedlings quickly outgrow the space allowed in the breeding nursery. Instead, we can estimate the broad-sense heritability using environmental variance obtained from cultivars/selections over 2 or more years.
Environmental variance and broad-sense heritability are important both for selection and for future genomics-based breeding. All genomics-based breeding techniques, including quantitative trait locus (QTL) analyses, genome-wide association studies (GWAS), and genomic selection (GS), need accurate phenotype evaluations to build an accurate phenotype database. Phenotypic variance contains both genetic and environmental variance components. The fluctuation of phenotypic values in response to changes in environmental conditions hinders the identification of useful molecular markers and can provide misleading results for marker effects detected by these analyses.
The objective of the present study was to accurately estimate the components of environmental variance and the broad-sense heritability of NHD, NW, PS, and II with the goal of supporting phenotypic selection in chestnut breeding programs, studies to detect linkages between molecular markers and traits of interest, and future genomics-based breeding.
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