Breeding synthetic varieties in annual caraway: observations on the outcrossing rate in a polycross using a high-throughput genotyping system

Caraway (Carum carvi) is an economically important spice and medicinal plant of the Apiaceae family (syn. Umbelliferrae). Farmers often favor annual cultivation of caraway. However, the annual varieties, which are currently available, do not provide satisfying seed yields. Employing heterosis can be a promising approach to increase yield. Breeding of synthetic varieties utilizes heterosis and may be the method of choice for future caraway breeding. Knowledge of the outcrossing rate is important to evaluate the effectiveness of this breeding method. However, the outcrossing rate of caraway is unknown so far. We estimated the outcrossing rate of seven inbred lines under field conditions in a neighbor-balanced polycross design. For this purpose, we implemented a high-throughput genotyping system (PACE), accompanied by a high-throughput method for DNA extraction adapted to caraway. In total, more than 1300 individual plants were genotyped. We found a high variability of lines regarding outcrossing rate and other traits associated with flowering. The outcrossing rate was on average 66.5% and ranged from 51.6 to 82%. We discussed implications of our findings on the targeted breeding method.