Fine Mapping and Marker Development for the Crossability Gene SKr on Chromosome 5BS of Hexaploid Wheat (Triticum aestivum L.)

Most elite wheat varieties cannot be crossed with related species thereby restricting greatly the germplasm that can be used for alien introgression in breeding programs. Inhibition to crossability is controlled genetically and a number of QTL have been identified to date, including the major gene Kr1 on 5BL and SKr, a strong QTL affecting crossability between wheat and rye on chromosome 5BS. In this study, we used a recombinant SSD population originating from a cross between the poorly crossable cultivar Courtot (Ct) and the crossable line MP98 to characterize the major dominant effect of SKr and map the gene at the distal end of the chromosome near the 5B homeologous GSP locus. Colinearity with barley and rice was used to saturate the SKr region with new markers and establish orthologous relationships with a 54-kb region on rice chromosome 12. In total, five markers were mapped within a genetic interval of 0.3 cM and 400 kb of BAC contigs were established on both sides of the gene to lay the foundation for map-based cloning of SKr. Two SSR markers completely linked to SKr were used to evaluate a collection of crossable wheat progenies originating from primary triticale breeding programs. The results confirm the major effect of SKr on crossability and the usefulness of the two markers for the efficient introgression of crossability in elite wheat varieties.DURING domestication and selection of a number of important crop species, diversity has eroded resulting in increased vulnerability to biotic and abiotic stresses while also jeopardizing the potential for sustained genetic improvement of elite cultivars over the long term (Tanksley and McCouch 1997; Fu and Somers 2009). The reintroduction of the remarkable diversity present in the different gene pools into elite varieties through intra- and interspecific crosses (primary and secondary gene pools) and intergeneric crosses (tertiary gene pools) has been practiced for decades in cereals (Feuillet et al. 2008). Despite some highly significant successes, including the incorporation of dwarfing and disease-resistance genes that fueled the Green Revolution, introgression remains laborious and, for complex characters, largely unfulfilled. Wheat (Triticum aestivum L.) has been crossed with a wide range of related species from the Triticeae tribe (Jiang et al. 1994), such as Aegilops, Agropyron, Haynaldia, Secale, and Hordeum, which represent a reservoir of interesting alleles for improving wheat resistance to biotic (diseases, insects) and abiotic stresses (cold, salinity, and drought) as well as for quality traits such as grain protein content (Fedak 1985). Intergeneric crosses have resulted in the transfer of desirable rye (Secale cereale L.) characteristics into wheat (Florell 1931) with one of the best examples being the 1BL/1RS chromosomal translocation that provided novel race-specific resistance to rust diseases, improved adaptation and stress tolerance, superior aerial biomass, and higher kernel weight to wheat varieties (Zarco-Hernandez et al. 2005). However, most of the adapted wheat germplasm is not crossable with alien species thereby restricting the panel of lines that can be used for alien introgression in wheat breeding (Krolow 1970) or for the production of primary triticale, a man-made wheat–rye hybrid.Beginning in the early 1900s, researchers were producing experimental crosses between bread wheat, T. aestivum L. (2n = 6x = 42) as a recipient, and rye, S. cereale L. (2n =14) as the pollen donor (Backhouse 1916). Genetic studies conducted by Lein (1943) showed that dominant alleles of two genes, named Kr1 and Kr2, are responsible for the poor crossability between bread wheat and rye. Kr1 and Kr2 genes were localized roughly on chromosome 5B and 5A, respectively (Riley and Chapman 1967) and subsequently located more precisely on the long arms of these two chromosomes (Lange and Riley 1973; Sitch et al. 1985). Further studies indicated that the dominant alleles driving incompatibility of crossing wheat with rye act by actively inhibiting the production of intergeneric hybrids (Riley and Chapman 1967; Lange and Wojciechowska 1976; Jalani and Moss 1980, 1981; Cameron and Reger 1991 ). Other crossability genes, such as Kr3 on chromosome 5D (Krolow 1970) and Kr4 on chromosome 1A (Zheng et al. 1992), were identified later. Finally, a study elucidated that chromosome 1A, derived from the Chinese (Sichuan) tetraploid wheat T. turgidum L. cv. Ailanmai, carries a recessive allele for high crossability with rye (Liu et al. 1999). Genetic studies also indicated that Kr genes have different effects on wheat–rye crossability. For example, by testing the cultivars Chinese Spring (CS), Hope, and the substitution lines CS/Hope 5B and CS/Hope 5A, Riley and Chapman (1967) demonstrated that Kr1 has a stronger effect than Kr2, whereas Kr3 seemed weaker than the two other genes (Krolow 1970).To further explore the mechanisms controlling crossability in wheat, Snape et al. (1979) performed crosses between the wild barley Hordeum bulbosum and the wheat cultivars Chinese Spring and Hope as well as 21 substitution lines carrying individual chromosomes of Hope in the background of Chinese Spring. The results revealed that the Kr1 and Kr2 genes on chromosomes 5B and 5A that govern crossability between wheat and rye also are involved in controlling crossability between wheat and barley, although the percentage of crossability observed was significantly lower than with rye. Using crosses between Chinese Spring, Hope, and the entire series of substitution lines with the cultivated barley (H. vulgare L.) cv. Betzes, Fedak and Jui (1982) also suggested that the homeologous alleles of the Kr genes on chromosome group 5 of Chinese Spring (5A, 5B, and 5D) favor crossability with additive effects.In 1998, a new locus, named SKr, controlling crossability between wheat and rye was detected using a mapping population of 187 double haploid (DH) lines produced by anther culture from F₁ hybrids of a cross between the noncrossable (NC) French wheat cv. Courtot (Ct) and the Chinese crossable (C) cv. Chinese Spring (Tixier et al. 1998). SKr was identified as a major QTL located on the distal end of the short arm of chromosome 5B within a confidence interval ranging from 8.7 to 20.9 cM (Lamoureux et al. 2002). In this population, the effect of SKr was stronger (22.1% of heritability) than the one of a QTL identified on 5BL (supposedly Kr1, 5.5% of heritability), whereas no significant effect was detected on 5AL (for Kr2). Moreover, the results indicated a 95% crossability rate for cv. Chinese Spring and ∼10% for Courtot, suggesting that the Courtot genotype is Kr1Kr1/kr2kr2, whereas Chinese Spring would be kr1kr1/kr2kr2.Here, we report the construction of a high resolution genetic map at the SKr locus using a single seed descent (SSD) population derived from a cross between Courtot and a crossable DH line (MP98) that allowed us to assess the crossability phenotype as a single “Mendelian factor” and to demonstrate the major dominant effect of SKr on crossability. Synteny between wheat, barley, and rice was used to increase the density of markers and reduce the genetic interval around the SKr gene to 0.3 cM. BAC contigs from Chinese Spring were established with closely linked and cosegregating markers to lay the foundation for positional cloning of the gene. Finally, a SSR marker cosegregating with SKr was developed and its value for the exploitation of SKr in breeding was assessed in a collection of crossable lines used to produce primary triticale.