Advantages of Hybrids in Comparison to Lines
In several outcrossing crop plant species, hybrid varieties have already largely replaced population varieties (Coors and Pandey 1999). The major benefits of hybrids compared to population varieties are increased yields due to an optimal exploitation of heterosis, higher biotic and abiotic stress resistance, and enhanced yield stability (Hallauer et al. 1988). In wheat as an autogamous species, the amount of midparent heterosis for grain yield is less pronounced, and commercial heterosis, the contrast between hybrids and the best commercially available line variety, is lower in comparison to outcrossing cereal species such as maize (Longin et al. 2012). Concerning yield stability, Léon (1994) concluded that for autogamous crops, findings in hybrids versus lines are contrasting, ranging from higher yield stability of hybrids (Borghi et al. 1988; Oury et al. 2000; Oettler et al. 2005; Gowda et al. 2010) to no differences (Borghi and Perenzin 1990; Peterson et al. 1997; Bruns and Peterson 1998; Koemel et al. 2004).
Recent large scale-phenotyping activities involving extended collections of inbred lines and hybrids, however, revealed a positive commercial heterosis for grain yield in hybrid winter wheat adapted to Central Europe (Gowda et al. 2012; Longin et al. 2013). Thus, a change from line to hybrid varieties has the potential to break the yield barriers in wheat breeding. Using a broad base of data from multilocation field trials, Mühleisen et al. (2013) also re-evaluated grain yield stability in wheat, triticale, and barley hybrids in comparison to lines. The authors observed consistently higher yield stability for hybrids in all three autogamous crops (Fig. 24.1). Moreover, the lack of consistencies of results on yield stability of hybrids versus lines in previous studies is most likely owing to an unbalanced definition of the environmental index caused by exploiting exclusively data from inbred lines but not from hybrids. Thus, in summary, current experimental studies clearly underline the advantages of hybrids in comparison to inbred lines in terms of higher yield and yield stability. Due to the expected climate change with more extreme weather conditions, such improved robustness of performance will become even more important in the future.
Fig. 24.1 Yield stability measured based on the stability variance of wheat, barley, and triticale singlecross hybrids (descending hatching) versus lines (ascending hatching) based on the data published by Mühleisen et al. (2013). Low values of stability variance correspond to high yield stability
Beyond improved grain yield and yield stability, hybrid wheat also outperforms inbred lines in sturdiness to abiotic and biotic stress. Longin et al. (2013) observed a positive midparent (albeit negative better parent) heterosis for frost tolerance as well as resistance against leaf rust, stripe rust, septoria tritici blotch, and powdery mildew for hybrid winter wheat. In two companion genome-wide mapping studies, Zhao et al. (2013a) and Gowda et al. (2013) investigated the degrees of dominance of putative QTL underlying the above outlined abiotic and biotic stress resistances. For most QTL with large effect on the genotypic variation, the degree of dominance was in the range of partial dominance. Thus, if particular traits are in focus, superior genotypes should carry resistance QTL in the homozygous state, favoring breeding of lines instead of hybrids. However, if pyramiding of multiple relevant QTL is desired, hybrid breeding will offer far more flexibility to combine favorable partially dominant alleles originating from either maternal or paternal inbred lines. This advantage of hybrids is illustrated by calculating a stress susceptibility index combining data on frost tolerance and resistance to leaf rust, stripe rust, septoria tritici blotch, and powdery mildew for an extended set of winter wheat inbred lines and hybrids derived from them (Fig. 24.2). Hybrids show overall lower stress susceptibility than related inbred lines, which might also be contribute the higher yield stability observed for hybrids as mentioned above (Mühleisen et al. 2013).
Beside the described advantages of hybrid versus line varieties in terms of yield, yield stability and biotic as well as abiotic stress resistance, it is pivotal to point out that the recurrent selection gain in hybrid breeding is at least as high in hybrid as compared to line breeding. The expected recurrent selection gain depends on quantitative genetic parameters such as the ratio of additive variance which can be exploited in hybrid versus line breeding. Besides quantitative genetic parameters,
Fig. 24.2 Distribution of 1.604 wheat hybrids (grey) and 135 parental lines (red) for a stress susceptibility index combining frost tolerance and resistance to leaf rust, stripe rust, septoria tritici blotch, and powdery mildew. Calculation was done assuming equal weight based on data published by Longin et al. (2013)
the economic framework such as the return in investment is pivotal to determine the selection gain of line versus hybrid breeding. Longin et al. (2012) performed a first rough theoretical comparison of the expected selection gain for line versus hybrid breeding under the simplifying assumption of an equal budget. The model calculations indicate that if the dominance effect is substantial, hybrid breeding can result in a competitive selection gain as compared to line breeding (Longin et al. 2012, 2014). These theoretical considerations are corroborated by one study comparing hybrid versus line breeding using experimental data of official variety tests in hard winter wheat across 20 years in the U.S. (Koemel et al. 2004). The authors observed a higher selection gain across time for hybrids compared to line breeding. These findings are very stimulating but further experimental data are needed to substantiate the long-term selection gain for hybrid versus line breeding in autogamous cereals.
Prediction of Hybrid Wheat Performance
In a traditional wheat breeding program, several thousand inbred lines are produced every year. Superior genotypes are efficiently identified by applying multi-stage selection programs. Selection of superior hybrids, however, is afflicted by the vast number of potential single-cross combinations among available elite parental lines (Zhao et al. 2013a). Consequently, field evaluation of all potential hybrid combinations is unfeasible, leading to a strong demand for hybrid prediction approaches.
For complex traits such as grain yield, midparent performance is only moderately associated with hybrid performance (Longin et al. 2012). Predicting hybrid performance based on general combining ability (GCA) effects of their parents is accurate in situations with predominance of variance due to GCA (σ2
) versus variance due to specific combining ability (SCA) effects (σ2 GCA). A recent large scale experimental study in wheat revealed that σ2 SCAis quite substantial for grain yield in wheat (Longin et al. 2013), which suggests that field testing of single-cross combinations is needed at least in the final stages of a selection program. Therefore, robust methods to predict hybrid wheat performance are urgently required.
The potential of genomic selection to predict hybrid wheat performance has been investigated for grain yield with a small factorial mating design comprising 90 hybrids fingerprinted with a 9k SNP array (Zhao et al. 2013b). The results of this cross validation study point towards a high potential of genomic selection to predict hybrid wheat performance. This finding was further substantiated in further studies predicting hybrid wheat performance with genomic selection focusing on several agronomic traits (Miedaner et al. 2013; Zhao et al. 2013a, 2014). Further empirical data analyses are needed, however, to finally judge the prospects of genomic selection models for predicting hybrid wheat performance.
Upcoming Challenges for Wheat Hybrid Breeding
In conclusion, the most urgent challenges for wheat hybrid breeding will be
(i) development of a stable hybridization system to reduce the costs of hybrid seed production compared to the use of CHA, (ii) identification of the genetic architecture of pollination capability for knowledge-based improvement of cross-pollination among elite lines, (iii) optimum design of hybrid wheat breeding programs including dimensioning of multi-stage selection programs including genome wide prediction approaches and (iv) development of heterotic pools in wheat.