Yield potential of modern European Plus-Hybrids and relevance of genetic diversity for xenia in maize (Zea mays L.)

Maize (Zea mays L.), grown for both human and animal consumption, is of major importance worldwide.Since the beginning of maize domestication, grain yields have increased constantly, especially since the introduction of single-cross hybrids around 1960.Our Plus-Hybrid system is a promising approach to increase the maize grain yield even further.In a Plus-Hybrid, a cytoplasmic male-sterile (CMS) hybrid is combined with an unrelated male-fertile hybrid, the latter acting as the pollen donor for the whole field.In addition to the heterosis effect on both hybrids, two other phenomena can improve the grain yield of the CMS hybrid seed-plant: the CMS effect, corresponding to the effect of male sterility, and the xenia effect, the direct effect of allo-pollination.Cytoplasmic male sterility in maize is a natural trait due to a mutation of the mitochondrial DNA affecting sporogenesis.CMS plants do not produce or release functional pollen.Three main types of male-sterile cytoplasm (T, C and S) have been identified according to the specific nuclear restorer-of-fertility genes (rf genes) that are able to countermand male sterility and restore fertility.Breeders have made use of this maternally inherited trait since the 1950s to facilitate the production of hybrid seeds and minimize costs.Even though CMS-based systems for seed production were abandoned after the epidemic of Southern Corn Leaf Blight (caused by Bipolaris maydis race T, especially virulent on maize with T cytoplasm), which devastated the US Corn Belt in the early 1970s, European and US breeders are now becoming interested again in this convenient and cost-effective tool.However, they implement mainly C and S CMS systems.CMS could benefit not only breeders but also farmers by the cultivation of our Plus-Hybrids.In regular maize stands, huge amounts of pollen are produced and the harvested grains consist of an F 2 population, i.e. the first inbreeding generation, resulting from the sib-pollination of single-cross hybrids.In contrast, maize Plus-Hybrids, in which a CMS hybrid (counting for 80% of the field) is pollinated by an unrelated male-fertile hybrid (20%) benefits from the effects of CMS and xenia, which can positively impact grain yield.Most of the studies on these two effects and the way, in which they affect yield components, are outdated and, thus, irrelevant for predicting whether and how modern Plus-Hybrids could boost grain yield.