Extraordinary Heterosis Found In A Forage Grass Hybrid, Festuca apennina × F. pratensis

Abstract Background: Heterosis (or hybrid vigor) is the over-performance of a hybrid over its parents in a specific trait or a set of traits. As such, hybrid breeding serves as a tool to efficiently trigger gains in breeding programs. Moreover, hybrids of genetically distant landraces, varieties or even species may become evolutionary successful. In Swiss alpine swards, we observed frequent prevalence of triploid hybrids of Festuca pratensis × F. apennina with outstanding competitiveness relative to their parental species in the sites of sympatric occurrence. Results: Observations of these highly vigorous hybrids prompted the study on their heterosis across various environmental conditions. Phenotypic observations during three years at four locations at different altitudes (from 200 m a.s.l. to 1850 m a.s.l.) have shown significant heterosis for dry biomass production at all sites during the first and second year, and at the mid- and high altitude sites also in the third year. At mid-altitude (1000 m a.s.l.), heterosis increased steadily and reached a maximum of +508 % for annual yield (+626% for a single cut) in the third year. This is by far the highest value of heterosis ever reported for annual dry mass yield of a forage grass. Conclusions: Further utilization of triploid hybrids in forage grass breeding is hampered by their sterility hence a need for vegetative propagation. However, artificial chromosome doubling of triploids to create fertile hexaploids, or seeking ways to propagate them vegetatively at an industrial scale might overcome this limitation.