Inter-specific Hybridization to Broaden the Crop Gene Pool
Although the gene pools used in conventional breeding are relatively restricted (Hajjar and Hodgkin 2007), a vast pool of genetic resources is available for breeding. Different gene pools have been defined depending on the difficulty of employing them. The most easy to use are those from the primary gene pool represented by germplasm that share a common genome but which have become isolated from mainstream gene pools such as landraces (as discussed in the section above). The secondary gene pool is represented by closely-related genomes that can be utilized through inter-specific hybridization, and would include the development of socalled “synthetic” or “re-synthesized” wheat, where tetraploid durum wheat has been hybridized with Aegilops tauschii, the ancestral donor of the D-genome, to recreate hexaploid bread wheat (Mujeeb-Kazi et al. 1996). This approach has been successful in introducing disease resistance as well as drought adaptive traits
Fig. 41.2 Yield and biomass of synthetic hexaploid wheat derived lines under two different late sown environments expressed as additional yield (left) or biomass (right) relative to non-stressed conditions, NW Mexico, 2012–2013
(Reynolds et al. 2007b; Trethowan and Mujeeb-Kazi 2008). Many thousands of accessions of both ancestral genomes (AB and D) exist as candidates for interspecific hybridization. Physiological characterization is being used to help select the most promising accessions, as well as to select among the initial products (primary 'synthetics'), for pre-breeding and genetic studies. In parallel to this, genotyping of existing primary synthetics and potential progenitors (AB tetraploid, and Ae. tauschii) is being carried out to target under-utilized genetic diversity.
Analysis of impacts from using synthetic wheat in breeding have shown significant contributions to drought adaptation (Lopes and Reynolds 2011), and it appears that the introduction of alleles from the wild D genome have increased the capacity of roots to partition resources to lower soil profiles when experiencing water stress (Reynolds et al. 2007b). A recent analysis looked at the same lines under heat stress in the hot desert of NW Mexico. To generate moderate and more extreme heat stress in the field, fully irrigated crops were sown later than normal in the months of February and March, respectively (normal sowing occurring in December). Results showed highly significant impacts of synthetic derived (SYN-DER) lines in hot, irrigated environments on both yield and biomass (Fig. 41.2).
In general, and across all three environments (temperate, heat and extreme heat), the elite SYN-DER lines (primary synthetic lines backcrossed to a conventional wheat line) showed superiority in terms of performance over lines derived from single crosses to a conventional line or the conventional line itself. However, yield advantages of elite SYN-DER lines over conventional hexaploids were larger in both heat environments than under temperate environments. Biomass at maturity of the elite-SYN-DER lines was also higher than conventional hexaploid lines. The advantages in aboveground biomass were also observed at heading. The SYN-DER lines also showed larger expression than conventional lines for water soluble carbohydrates in stems and tended to be earlier maturing. These results underline the value of exploring the secondary gene pool of wheat.