Florida is the primary source of strawberry fruit for the eastern United States and eastern Canada from December to late March. The state is second to California in total U.S. production with a harvested area of greater than 3600 ha during the 2010–11 season (U.S. Department of Agriculture, 2011). Production in Florida shares similarities with major production regions such as Australia, southern California, and Spain, where strawberries are grown using bare-root transplants in intensive, annualized systems for winter and spring markets.
In 1968 the University of Florida (UF) started a strawberry breeding program (Whitaker et al., 2011), although some open-pollinated seedling selection was performed before that time. Since that time, the breeding population has been continually improved for multiple plant and fruit traits through recurrent selection. Typically, ≈100 controlled crosses have been made each year among ≈30 or more parental genotypes in the main breeding population with additional crosses made for germplasm development efforts. Pedigree records have been maintained to monitor parentage and inbreeding, and full-sib crosses have almost always been avoided. To replicate commercial nursery conditions, each seedling genotype is asexually propagated through stolons (runners) in a temperate summer nursery to produce bare-root transplants for evaluation in the fruiting field. In this way multiple runner plants per seedling genotype may be evaluated; the original seedling plant is not evaluated.
West–central Florida is characterized by periodic rainfall, high humidity, fluctuating temperatures, and occasional freezes, which inhibit pollination and fruit development, resulting in unmarketable fruit. Therefore, reducing the proportion of unmarketable fruit and thereby increasing marketable yield is an important breeding objective. The seasonality of fruit production for a strawberry cultivar is also of vital importance, in which an ideal pattern consists of large early-season yields from late November through January when the value of the crop is greatest and moderated late-season yields during February and March when overproduction can result in reduced market prices. Large average fruit size is also a breeding objective as well as favorable levels of traits that affect flavor perception such as soluble solids content (SSC) and titratable acidity (TA) (Joquand et al., 2008). In addition, the plant must be vigorous enough to establish well in the field and support high yields but no so large and dense as to restrict air movement and obscure the fruit from harvesters.
A historical trial of cultivars and advanced selections from the UF strawberry breeding program revealed gains over time for fruit size and proportion of marketable fruit (Whitaker et al., 2011). Although SSC and TA varied widely among genotypes, clear trends over time could not be observed for these traits. Until recently, there have been no published reports of genetic parameters such as heritabilities and genetic correlations for the UF strawberry breeding population, which would be desirable for shaping breeding and selection strategies (Hasing et al., 2011). They provide an understanding of the effects of trait selection in the long term and the behavior of correlated traits that, if adverse, may hinder breeding progress if they are ignored during selection.
Previous studies have reported genetic parameters for plant and fruit traits of strawberries in both annual and perennial production systems (reviewed by Galleta and Maas, 1990; Hancock et al., 2008). Information on genetic parameters for annual production systems have mainly been generated using the University of California–Davis breeding population. Narrow-sense heritabilities for plant growth traits such as plant diameter have been low to moderate with little contribution of non-additive variance (Fort and Shaw, 2000; Shaw, 1993). Substantial amounts of additive variance for yield and fruit size have been demonstrated, although the relative proportions of additive and dominance variance have varied widely across testing environments and propagule types (Fort and Shaw, 2000; Pringle and Shaw, 1998; Shaw, 1989; Shaw et al., 1989; Shaw and Larson, 2005). Greater dominance variance was typically found for SSC and TA (Shaw et al., 1987). Gains from selection for SSC were predicted to be poor based on clonal trials of selected individuals, mainly as a result of large interactions with cultural environments and harvest dates (Shaw, 1988, 1990).
Although these previous studies provide important benchmarks, they may not be reflective of the germplasm, population history, and testing environments of the UF strawberry breeding program. In this study, we explore the genetic basis of several important fruit and vegetative traits in the UF strawberry breeding population by conducting clonal tests of seedling, parental, and control genotypes across two environments and performing genetic analyses that incorporate pedigree records spanning 15 generations. Specifically we aim to: 1) obtain estimates of narrow-sense heritability, broad-sense heritability, and genotype by environment interactions; 2) estimate phenotypic, genotypic (additive plus non-additive genetic effects), and genetic (additive) correlations among the traits of interest; and 3) predict genetic gains from multivariate selection.
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