Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2007
Publication Date: 6/7/2007
Citation: Hale, A.L., Farnham, M.W., Nzaramba, M.N., Kimbeng, C.A. 2007. Heterosis for Horticultural Traits in Broccoli. Theoretical and Applied Genetics. 115:351-360. Interpretive Summary: Over the last three decades, broccoli hybrids (made by crossing two inbred lines) replaced open-pollinated populations to become the predominant type of cultivar. The decision to market hybrid broccoli was made by seed companies with little or no understanding of hybrid vigor (the increased performance of an offspring over either of its parents) in this crop. Hybrid development is an expensive and time-consuming endeavor, and one of the most costly aspects of developing good hybrids is identifying parental combinations which produce superior offspring. The purpose of this study was to determine the amount of hybrid vigor expressed for important traits in broccoli, and to determine if molecular markers can aid in the selection of parents for increased hybrid vigor in the offspring. Nine parents were crossed in all possible combinations to produce 36 offspring, and these offspring were evaluated for head weight, stem diameter, plant height, plant width, and maturity. Almost all of the offspring were taller and wider than their parents, and roughly half had heavier heads and stem diameters. Molecular markers were used to determine genetic similarity, and it was determined that, for most traits, crossing more genetically similar parents resulted in less hybrid vigor in the offspring.
Technical Abstract: Over the last three decades, broccoli (Brassica oleracea L., Italica Group) hybrids made by crossing two inbred lines replaced open-pollinated populations to become the predominant type of cultivar. The change to hybrids evolved with only one documented result showing hybrid vigor for days from transplant to harvest in broccoli. Therefore, this study examined levels of heterosis in a set of hybrids derived from relatively elite, modern inbreds (n=9). An additional objective was to determine if PCR-based marker derived genetic similarities among the parents are correlated with heterosis in this crop. A total of 36 hybrids formed by crossing nine parents were evaluated for horticultural characters including head weight, head stem diameter, plant height, plant width (in a row), and maturity (e.g., days from transplant to harvest) in four environments. When averaged across all four environments, roughly half of the hybrids exhibited highparent heterosis for head weight (1 to 30 g) and stem diameter (0.2 to 3.5 cm). Almost all hybrids showed highparent heterosis for plant height (1 to 10 cm) and width (2 to 13 cm). Unlike other traits, there was negative heterosis for maturity, indicating that heterosis for this character in hybrids is expressed as earliness. Genetic similarity was significantly and negatively correlated with highparent heterosis for all traits except for stem diameter and days from transplant to harvest. With modern broccoli inbreds, heterosis for head characteristics appears less important than for traits that measure plant vigor.