Genetic diversity and heterosis of spring wheat crosses

Genetic diversity between parents may contribute positively to both heterosis and transgressive segregation. The objectives of this study were to determine (i) whether genetic diversity in spring wheat (Triticum aestivum L.) affected F-2 bulk heterosis and (ii) whether genetic diversity affected the amount of transgressive segregation among progeny from a cross. Genetic diversity was measured by pedigrees, morphology, and gliadin seed proteins. Heterosis was evaluated for 137 F-2 bulks tested at two Minnesota locations in 1993 and three locations in 1993. These bulb were developed from crosses among 91 cultivars. No linear relationship between genetic distance and F2 bulk heterosis was detected. However, when crosses were divided into related and unrelated groups, crosses,vith parents unrelated by pedigree or morphology expressed greater heterosis than crosses with related parents. Bulks of crosses between cultivars that were classified as unrelated by two or more of the distance measures showed more heterosis than related crosses. Thus, part of heterosis seems to be due to parental diversity, though not in a linear fashion. Expression of greater than-average heterosis by the highest yielding bulks, without an increase in genetic distance, suggested additional factors were influencing the expression of heterosis. Transgressive segregation was evaluated for six of the 137 crosses; 30 to 40 inbred lines from each of the crosses was evaluated in three environments. Data for these populations were consistent with an additive genetic model. The expression of heterosis was due in part to genetic diversity but was unpredictable and also depended on factors not elucidated by this study.