Submitted to: Crucifer Genetics Workshop
Publication Type: Abstract only
Publication Acceptance Date: 9/1/2006
Publication Date: 9/30/2006
Citation: Hale, A.L., Farnham, M.W. 2006. Expression of Heterosis in Green Sprouting Broccoli. Proceedings of the 15th Crucifer Genetics Workshop: Brassica 2006. 30 Sep-4 Oct 2006. Wageningen, The Netherlands. p 58. Interpretive Summary:
Technical Abstract: Through the 1960s most cultivars of broccoli (Brassica oleracea L., Italica Group) were open-pollinated populations developed by mass selection. In the late 1960s and early 1970s, the first F1 hybrid cultivars of broccoli were introduced. These hybrids were made by crossing two inbreds, and hybridization was typically facilitated by employing self-incompatibility in at least the female inbred of the cross. Today, virtually all commercial cultivars of broccoli are hybrids. The above change took place with little understanding of heterosis or hybrid vigor in this crop. The objectives of the present study were to: 1) assess the expression of heterosis in a set of hybrids derived by crossing relatively modern inbreds; and 2) determine if there is a relationship between the level of heterosis exhibited by a hybrid and the genetic similarity between the parents used to make it. A total of 36 hybrids formed by crossing nine different parents were evaluated for horticultural traits 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. Genetic similarity between parents was determined using about 600 polymorphic markers generated by SSR, AFLP, and SRAP techniques and ranged from 0.65 to 0.83 for the 36 combinations. When averaged across environments, only about half of the hybrids exhibited high parent heterosis for head weight (ranging from 1 to 30 g) and stem diameter (0.2 to 3.5 cm). On the contrary, almost all hybrids showed high parent heterosis for plant height (1 to 10 cm) and width (2 to 13 cm). Low parent heterosis for maturity was evaluated because hybrid vigor for this trait is usually expressed as earliness; however, only a minority of hybrids actually exhibited this type of heterosis. Significant decreases in heterosis for plant height and width were observed among hybrids as genetic similarity between parents increased; this same trend was not significant for the other traits. Using modern broccoli inbreds, expression of hybrid vigor for head characteristics appears less important than the expression for traits associated with plant vigor.