Academic journal article Genetics

Phenotypic Selection for Dormancy Introduced a Set of Adaptive Haplotypes from Weedy into Cultivated Rice

Academic journal article Genetics

Phenotypic Selection for Dormancy Introduced a Set of Adaptive Haplotypes from Weedy into Cultivated Rice

Article excerpt

ABSTRACT

Association of seed dormancy with shattering, awn, and black hull and red pericarp colors enhances survival of wild and weedy species, but challenges the use of dormancy genes in breeding varieties resistant to preharvest sprouting. A phenotypic selection and recurrent backcrossing technique was used to introduce dormancy genes from a wild-like weedy rice to a breeding line to determine their effects and linkage with the other traits. Five generations of phenotypic selection alone for low germination extremes simultaneously retained dormancy alleles at five independent QTL, including qSD12 (R^sup 2^ > 50%), as determined by genome-wide scanning for their main and/or epistatic effects in two BC^sub 4^F^sub 2^ populations. Four dormancy loci with moderate to small effects colocated with QTL/genes for one to three of the associated traits. Multilocus response to the selection suggests that these dormancy genes are cumulative in effect, as well as networked by epistases, and that the network may have played a "sheltering" role in maintaining intact adaptive haplotypes during the evolution of weeds. Tight linkage may prevent the dormancy genes from being used in breeding programs. The major effect of qSD12 makes it an ideal target for map-based cloning and the best candidate for imparting resistance to preharvest sprouting.

WEDS, plants adapted to human disturbances (HARLAN and DE WET 1965), compete with crops for environmental resources and occasionally interbreed with conspecific cultivars (LADIZINSKY 1985; LANGEVIN et al. 1990). Understanding the genetic basis underlying adaptation is crucial to devising integrated strategies for weed control or to preventing the rise of "superweeds," that is, weeds with transgenes derived from transgenic crops (GRESSEL 1999; BASU et al. 2004; LÉGÈRE 2005). Conversely, to meet breeding objectives for sustainable agriculture, conspecific weeds, as part of the primary gene pool (HARLAN et al. 1973), are valuable to regain some genetic diversity that was eliminated during domestication (TANKSLEY and McCoucH 1997). Weedy rice, including red rice, accompanies cultivated rice worldwide (ÏêÁ 1988). Research on weedy rice has been done for its origin, classification, control, and risk assessment related to transgenic cultivars (Sun et al. 1997; TANG and MORISHIMA 1997; BAKER et al. 2000; VAUGHAN et al. 2001; GEALY et al. 2003; CHEN et al. 2004), but rarely on the genetics of adaptive traits (BRES-PATRY et al. 2001) or for identification of beneficial genes for rice improvement (AHN et al. 2002).

Seed dormancy optimizes timing of germination for wild and weedy plants and provides resistance to preharvest sprouting (PHS) for cereal crops (BEWLEY and BLACK 1982). Both dormancy and PHS are complex traits controlled by many genes (CHANG and TAGUMPAY 1973) or quantitative trait loci (QTL) such as in Arabidopsis (ALONSO-BLANCO et al. 2003), barley (UBERTHUR et al. 1995; Li et al. 2003; PRADA et al. 2004; ZHANG et al. 2005), rice (LiN et al. 1998; CAI and MORISHIMA 2000; DONG et al. 2003; Gu et al. 2004), sorghum (LIJAVETZKY et al. 2000), and wheat (ANDERSON et al. 1993; KATO et al. 2001; MARES and MRVA 2001; GROOS et al. 2002; OSA et al. 2003; KULWAL et al. 2004). Dormancy alleles at a few QTL have been introduced into the nondormant genetic background to validate their effects or to explore their potential in breeding programs (HAN et al. 1999; GAO et al. 2003; TAKEUCHI et al. 2003). Validated QTL may be cloned to characterize molecular mechanisms directly regulating germination and dormancy (KOORNNEEF et dl. 2002).

Utilization of dormancy genes from wild and weedy germplasm to control PHS in cereal crops may be hampered by linkage with some traits that may have an adaptive value under natural conditions but are undesirable for modern cultivars. Dormancy association with red grain color in wheat (NiLSSON-EHLE 1914) has prevented the use of these dormancy genes in the development of white grain-colored cultivars with resistance to PHS (FLINTHAM 2000). …

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