An Update of Recent Use of Aegilops Species in Wheat Breeding

Aegilops species have significantly contributed to wheat breeding despite the difficulties involved in the handling of wild species, such as crossability and incompatibility. A number of biotic resistance genes have been identified and incorporated into wheat varieties from Aegilops species, and this genus is also contributing toward improvement of complex traits such as yield and abiotic tolerance for drought and heat. The D genome diploid species of Aegilops tauschii has been utilized most often in wheat breeding programs. Other Aegilops species are more difficult to utilize in the breeding because of lower meiotic recombination frequencies; generally they can be utilized only after extensive and time-consuming procedures in the form of translocation/introgression lines. After the emergence of Ug99 stem rust and wheat blast threats, Aegilops species gathered more attention as a form of new resistance sources. This article aims to update recent progress on Aegilops species, as well as to cover new topics around their use in wheat breeding.

An Aegilops ventricosa Translocation Confers Resistance Against Root-knot Nematodes to Common Wheat

Root knot nematodes (RKN; Meloidogyne spp.) cause severe losses worldwide to a wide range of crops. Crop rotations with resistant hosts can be used to control losses, but the wide host range of RKN limits this option. In this study, we found that the wheat cultivar Lassik is resistant to several isolates of the RKN species M. incognita and M. javanica, including those that can reproduce on tomato with the resistance gene Mi-1. Comparison of near-isogenic lines of wheat showed that the wheat resistance gene(s) is localized within a segment of the short arm of chromosome 2N from Aegilops ventricosa (Zhuk.) Chennav translocated into common wheat (Triticum aestivum L.) chromosome arm 2AS and is associated with a highly significant decrease in RKN eggs in the roots. This RKN resistance gene has been assigned the name Rkn3. While wheat itself is tolerant of RKN infection, a microplot experiment coupled with tomato bioassays showed less RKN root galling in the tomato samples grown in soil from the previous microplots including RKN resistant wheat varieties than in those including a susceptible wheat isogenic line. This result suggests that rotation with Rkn3 resistant wheat cultivars has the potential to be a valuable component of nematode management for crops that are highly susceptible to nematode damage and for which alternative strategies are limited.

Sequence evidence for sporadic intergeneric DNA introgression from wheat into a wild Aegilops species

Introgressive hybridization has played a crucial role in the evolution of many plant species, especially polyploids. The duplicated genetic material and wide geographical distribution facilitate hybridization and introgression among polyploid species having either homologous or homoeologous genomes. Such introgression may lead to the production of recombinant genomes that are more difficult to form at the diploid level. Crop genes that have introgressed into wild relatives can increase the capability of the wild relatives to adapt to agricultural environments and compete with crops or to compete with other wild species. Although the transfer of genes from crops into their conspecific immediate wild progenitors has been reported, little is known about spontaneous gene movement from crops to more distantly related species. We describe recent spontaneous DNA introgression from domesticated polyploid wheat into distantly related, wild tetraploid Aegilops peregrina (syn. Aegilops variabilis) and the stabilization of this sequence in wild populations despite not having homologous chromosomes. Our results show that DNA can spontaneously introgress between homoeologous genomes of species of the tribe Triticeae and, in the case of crop-wild relatives, possibly enrich the wild population. These results also emphasize the need for fail-safe mechanisms in transgenic crops to prevent gene flow where there may be ecological risks.