GENETIC DIVERSITY AND HETEROSIS OF SPRING WHEAT CROSSES

Genetic diversity between parents may contribute positively to both he terosis and transgressive segregation. The objectives of this study we re to determine (i) whether genetic diversity in spring wheat (Triticu m aestivum L.) affected F-2 bulk heterosis and (ii) whether genetic di versity 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 detec ted. However, when crosses were divided into related and unrelated gro ups, crosses,vith parents unrelated by pedigree or morphology expresse d greater heterosis than crosses with related parents. Bulks of crosse s between cultivars that were classified as unrelated by two or more o f the distance measures showed more heterosis than related crosses. Th us, part of heterosis seems to be due to parental diversity, though no t 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 hetero sis. Transgressive segregation was evaluated for six of the 137 crosse s; 30 to 40 inbred lines from each of the crosses was evaluated in thr ee environments. Data for these populations were consistent with an ad ditive genetic model. The expression of heterosis was due in part to g enetic diversity but was unpredictable and also depended on factors no t elucidated by this study.