Strategic Crossing to Achieve Cumulative Gene Action
Trait selection has made continual progress in wheat breeding through incorporating agronomic traits such as height and flowering time, resistance to a spectrum of prevalent diseases, quality parameters determined by end use, and yield based on multi-location trials (Braun et al. 2010). However, current genetic gains are not adequate to meet future demand while climate change threatens to erode these further. Breeding more specifically for heat and drought adaption can help to increase genetic gains (Reynolds et al. 2009). The main objective of strategic trait-based crossing is to accumulate traits that will be complementary for a given target environment (target-traits). Under water-limited situations, traits that improve water
Fig. 41.4 Traits used in strategic crossing to combine complementary drought adaptive traits at CIMMYT since 2005
uptake, water use efficiency and partitioning to yield, respectively, are likely to work synergistically to maximize productivity (Richards 2006). Achieving this in a practical breeding context involves the following interventions, expressed chronologically:
(i) Target traits are identified using conceptual models as described above.
(ii) Genetic resources are screened to provide potential sources of target traits.
(iii) Candidate parents are characterized for traits that may contribute to genetic gains in target environments (Fig. 41.4).
(iv) ) Crosses are made such that traits expressed by respective parents encompass as many of the target-traits as possible. Top crosses may be used to facilitate the accumulation of traits.
(v) Early generation bulks are screened for integrative traits such as canopy temperature (CT) in relevant environments; families with warm canopies -compared to checksare mainly discarded.
The focus of much physiological research in the past has been to identify traits for early generation selection (EGS) that can be used to screen progeny for favorable trait expression. There is consensus that the more feasible traits are either genetically relatively simple, or integrative traits that explain a reasonable degree of the variation in yield and are relatively quick and easy to measure (Richards 2006). In fact very few traits meet these criteria and have found application. A successful application in wheat was the use of carbon isotope discrimination (CID) to select progeny with high transpiration efficiency conferring a conservative rate of water use that is associated with increased yield when the soil water profile is restricted (Richards 2006). Another trait that has found application in EGS under a range of environments is canopy temperature (CT). Under drought, selection for cooler CT permits genetic gains for yield (Olivares-Villegas et al. 2007) and genotypes with cooler canopies have been shown to extract more water from deeper soil profiles (Reynolds et al. 2007b). The trait is measured using infra-red thermometry and is ideal for high throughput screening for the following reasons (i) it is quick and easy to measure, (ii) the technology is inexpensive (currently US$200 for a hand held instrument) and is amenable to airborne approaches as described above, (iii) CT shows robust association with performance; trait expression shows low interaction with growth stage or time of day. The trait is used routinely by CIMMYT's wheat breeding program for rainfed environments to enrich for alleles associated with dehydration resistance. For example, F3 and F4 bulks are screened for CT under drought; a larger number of plants -also expressing favourable agronomic traitsare selected from cooler CT families while bulks with warmer CT (compared to checks) are discarded. The development of relatively easy to use spectral radiometers offers another high throughput screening approach for comparing spectral reflectance indices (SRIs) of genotypes.
The result of investment in physiological trait (PT) based crossing has generated advanced lines distributed by CIMMYT as part of the 23rd Semi Arid Wheat Screening Nursery –SAWSNand the 17th. Semi Arid Wheat Yield Trial (17th SAWYT). Of the 205 candidate genotypes for SAWSN in 2009, 48 (23 %) were derived from PT crossing and a similar proportion were represented in the SAWYT. The PT lines performed well in 2010 with the average yield of the PT lines beating the group of conventionally bred lines at 75 % of the international sites. The most recent products of physiological breeding have shown spectacular results, with 70 % of newest PT (PT-SA) lines beating the drought adapted check Vorobey under drought in Mexico in 2012, some by as much as 30 %. The underlying assumption for the PT strategy is that crosses between parents with different but potentially complementary PT expression will realize cumulative gene action in selected progeny. This has been borne out in previous analyses (Reynolds et al. 2009) and was shown again in the newest PT-SA lines which have landraces, re-synthesized hexaploids, and early PT lines in their pedigrees (Fig. 41.5).
Phenotyping for expression of physiological traits is an efficient way to select among genetic resources for promising candidate parents that can be used to accumulate stress adaptive traits and alleles. High throughput phenotyping approaches can facilitate the process of screening and progeny selection as well as the characterization of mapping populations. Genetic analysis of the latter will ultimately lead to identification of molecular markers that can be used in breeding and mining genetic resources for allelic diversity.
Fig. 41.5 New PT-SA line SOKOLL/3/PASTOR//HXL7573/2*BAU/4/WBLL4//OAX93.24.35/
WBLL1 containing PT + synthetic (Sokoll) (PT + Syn) and PT + Mexican landrace (OAX93.24.35) (PT + Land) in its pedigree grown under two distinct drought environments: (a) yield and biomass under gravity and drip irrigation simulating post monsoon stored soil moisture, and Mediterranean drought environments, respectively. (b) Improved water relations in new PT line showing canopy temperature and residual soil moisture at harvest. Traits are compared with the drought adapted check Vorobey (also with synthetic in its background)