Overcoming self-incompatibility in grasses: a pathway to hybrid breeding Javier Do Canto 1,2,* , Bruno Studer 3 and Thomas Lubberstedt 1 1 Department of Agronomy, Iowa State University, 2104 Agronomy Hall, Ames, IA 50011‐1010, USA; 2 National Institute of Agricultural Research, INIA, Route 5 Km 386, Tacuarembo, Uruguay; 3 Forage Crop Genetics, Institute of Agricultural Sciences, ETH Zurich, LFW Building, University Street 2, Zurich, 8092, Switzerland. * Corresponding author, E‐mail: javierd@iastate.edu Abstract Allogamous grasses exhibit an effective two‐locus gametophytic self‐incompatibility (SI) system, limiting the range of breeding techniques applicable for cultivar development. Current breeding methods based on populations are characterized by comparably low genetic gains for important traits such as biomass yield. In order to implement more efficient breeding schemes, the overall understanding of the SI system is crucial as are the mechanisms involved in the breakdown of SI. Self‐fertile variants in outcrossing grasses have been studied and the current level of knowledge includes approximate gene locations, linked molecular markers and first hypotheses on their mode of action. Environmental conditions increasing seed set upon self‐pollination have also been described. Even though some strategies were proposed to take advantage of self‐fertility, there have so far not been changes in the methods applied in cultivar development for allogamous grasses. In this review, we describe the current knowledge about self‐fertility in allogamous grasses and outline strategies to incorporate this trait for implementation in synthetic and hybrid breeding schemes. Introduction Perennial grasses are widely used as forage crops across the globe representing an important component of animal production systems. Moreover, as turf they are essential for sport fields and key components for landscape architecture and home yards. More recently, perennial grasses appeared as an attractive option in the bioenergy market due to their intrinsic properties for biofuels and their ecological advantages over annual crops. As perennials, each evaluation trial takes two or more years since persistence is usually a trait of interest and forage yield varies with the age of the pasture, leading to breeding cycles that easily exceed five years (Wilkins and Humphreys, 2003). It can take as long as 10 to 15 years to develop and release a cultivar (Lee et al., 2012). Among perennial grasses, genetic self‐incompatibility (SI), which promotes cross‐pollination and prevents self‐pollination, is widespread. SI in grasses is controlled gametophytically by at least two multiallelic and independent loci, S and Z. This SI system is assumed to be conserved in grass species, including annual grasses like Italian ryegrass (Lolium multiflorum) and rye (Secale cereale) (reviewed by Baumann et al., 2000; Yang et al., 2008; Klaas et al., 2011). A general tendency in allogamous crops has been to move towards hybrid breeding, which offers opportunities for greater uniformity, higher selection intensities, absolute parental control and maximum exploitation of heterosis. However, cultivars from species exhibiting SI can only be developed as improved populations or synthetic varieties since inbred lines cannot be obtained by continued self‐pollination. Breeding methods applied in grasses varies from restricted recurrent phenotypic selection to half‐sib selection and between‐within family selection among others (Vogel and Pedersen, 1993; Wilkins and Humphreys, 2003; Posselt, 2010). Reported genetic gains