Hybrid Breeding in Wheat
Abstract Despite promising superior performance, hybrid wheat currently occupies only a niche sector in commercial wheat production. However, with the recent development of practicable hybrid seed production systems, a switch from line to hybrid breeding in wheat seems realistic. Here, we discuss what consequence this may have for wheat breeding programs and provide suggestions on how quantitative genetic analysis can contribute to design optimal selection strategies.
Status Quo of Wheat Hybrid Breeding
Hybrid cultivars with improve yield and other favorable agronomic traits are widely used in plant production. For wheat, one of the most important staple crops, commercial hybrid breeding and seed production is still restricted to a niche sector in comparison to other cereals like maize or rice (Longin et al. 2012; Whitford et al. 2013). Currently, there is a limited number of wheat hybrid cultivars based on chemical hybridization agents (CHAs, gametocytes) registered for the European market (Hybridwheat 2013). In China and India, hybrid wheat is produced based on cytoplasmic male sterility (CMS) systems or photoperiodic sensitivity sterility systems (Longin et al. 2012). Major practical limitations for a more widespread use of hybrid wheat are seed production capacities and costs, but progress has been made to improve the availability and economic competitiveness of hybrid wheat (reviewed by Kempe and Gils 2011; Whitford et al. 2013). Eventually, economically successful broad implementation of hybrid wheat will need the combination of a practicable low cost hybrid seed production system, high performance in traits of interest such as grain yield and yield stability and an efficient breeding scheme for further improvement (Longin et al. 2012).
Hybridization Systems in Wheat
Hybrid seed production requires the enforcement of an efficient cross-pollination between wheat inbred lines that overcomes the naturally autogamous pollination mode of wheat. This is practically achieved by planting male-sterile maternal plants with good pollen recipient properties in close proximity to paternal plants with good pollen shedding properties. Thus, effective male-sterility of the maternal plants is a general requirement that can be based on different mechanisms.
Some major efforts have been made to deploy cytoplasmic male sterility (CMS) for hybrid wheat breeding. While CMS is functional in other important cereals such as rice and rye, it has turned out to be difficult to develop, complex to maintain and marginally reliable for wheat. Although sterility-inducing cytoplasms were identified (e. g. from Triticum timopheevii), reliable restorer genes are not yet available for wheat (Angus 2001) and CMS systems are often sensitive to environmental factors, in particular to temperature and photoperiod (Kaul 1988; Murai et al. 2008). Thus, no wheat CMS system with more than regional application is currently available for hybrid seed production.
Rather, contemporary commercial hybrid wheat production is mainly based on the in-field application of CHA preventing the formation of viable pollen on the maternal crossing partner (Cisar and Cooper 2002). On a production site, the intended maternal line is planted in strips alternating with strips of the intended pollen donor lines and maternal plants are sprayed with CHA, while treatment of paternal plants is strictly avoided. Hybrid seeds are then harvested from the pollinated mother plants to give rise to F1 progeny that then displays the heterosis (hybrid vigor) effect (Kempe and Gils 2011). The plant growth regulator Croisor®100 (Sintofen, former Dupont-Hybrinova, Saaten Union Recherche, France) is currently the only CHA for wheat registered in Europe for commercial production (Hybridwheat 2013). Although modern CHAs are in principle functional for a broad spectrum of genotypes and display relatively low phytotoxicity in wheat, they still have limitations such as compromised seed set on treated plants (Adugna et al. 2004) or variation in field-efficiency depending on the weather conditions at the time of application.
Alternatively, the exploitation of transgene technologies may be promising for the establishment of hybrid wheat production systems (Kempe and Gils 2011; Whitford et al. 2013). As an example, a recessive split-gene transgene system was suggested that utilizes complimentary fragments of barnase to induce male-sterility in maternal plants with simultaneously retaining pollen fertility and thus grain yield in the resulting F1 hybrids (Gils et al. 2008; Kempe et al. 2009). Functional barnase formation in the tapetum layer of anthers and corresponding male-sterility should exclusively be effective in heterozygous plants, which can eventually serve as the mother plants in the hybrid cross. With both barnase gene fragments located on allelic chromosomal positions and thus “linked in repulsion”, hybrid F1 plants inherit only one of the barnase fragments and, as a result, remain fully fertile (Kempe and Gils 2011).
Moreover, the modification of the naturally closed inflorescence structure of wheat adapted to self-pollination to a more open structure allowing more efficient pollen reception as well as shedding will be an important breeding objective (Whitford et al. 2013). With these recent developments in hybrid seed production in mind, it seems now timely to devote some thought on how to develop optimal parental combinations for highest yield and quality improvements in wheat hybrid breeding.