Interspecific hybridization is considered a ubiquitous process in flowering plants. However, viable natural hybrids are generally rare and highly sterile, with a shrunken endosperm (Matus-Cádiz et al. 2004; Raybould & Gray 1993). Populations rarely persist unless hybrids can backcross with one of the parental lines (Raybould & Gray 1993; Zemetra et al. 1998).
Some successful natural hybridization events have nonetheless been observed between T. aestivum and the other lineages from the Triticeae tribe, with gene flow occurring at low rate (see (Jacot et al. 2004)for review). Traditional mixed cultivation of diploid, tetraploid and hexaploid wheats in the Middle East and Transcaucasia has given rise to new subspecies (Matsuoka 2011). For example, morphological and genetic studies have shown that the endemic Georgian wheat and macha wheat (respectively tetraploid and hexaploid) are issued from a cross between the tetraploid T. turgidum and T. aestivum (Matsuoka 2011; Dvorak et al. 2012). F1 hybrids from T. aestivum x T. turgidum are pentaploids (AABBD), and the segregating progeny is fertile (82.27 % fertility rate) (Wang et al. 2005). Interspecific pollen-mediated gene flow is low, with trace levels (≤0.05 %) observed past 20 m from the pollinator source. No gene flow was detected at or beyond 40 m from the source (Matus-Cádiz et al. 2004). T. aestivum and T. turgidum are grown in overlapping areas in south Qld, northern NSW, western Vic. and south-western SA (Figures 5a and 5b).
9.3 Natural intergeneric crossing
Intergeneric hybridization has been observed in natural conditions. The first report of natural cross between wheat and Aegilops was documented in Europe in 1825 (van Slageren 1994). It was later demonstrated that T. aestivum originated from a cross between T. turgidum and Ae. tauschii (reviewed in (Matsuoka 2011). Because of their common ancestor, wheat and Aegilops species share the same D genome. Thus gene flow between these species is expected to be more likely if genes are located within the D genome (Schoenenberger et al. 2006; Zemetra et al. 1998; Jacot et al. 2004). Spontaneous hybridizations between T. aestivum and Aegilops sp. have been observed on field margins (≤1 m from the field) in Spain, with a spontaneous hybridisation rate of 0.19 % (Loureiro et al. 2006). Average self-fertility for the pentaploid hybrid has been observed ranging from 0 to 3.22 % (Loureiro et al. 2008; Wang et al. 2001). Further restoration of tetra- or hexaploidy is possible, following backcrosses with the parent lines (Zemetra et al. 1998; Schoenenberger et al. 2006).
Ae. cylindrica is the Aegilops species with the most pronounced tendency to weediness. It is considered a noxious weed in winter wheat cropping systems in the western United States, mainly due to its hardiness and its competitiveness (one plant can produce up to 135 tillers) (Wang et al. 2001; Schoenenberger et al. 2006). The introgression of imidazolinone resistance from wheat to Ae. cylindrica in field conditions has been observed as a hybridization rate of 0.1 % and a maximum distance of 16 m (Gaines et al. 2008).
There are no Aegilops species native to Australia. The Australian government has declared Aegilops a pest. Its entry is prohibited, unless the seeds are to be grown under quarantine conditions for wheat breeding (Department of Agriculture and Water Resources). Although some specimens have been collected, presumably originating from seed accidently introduced or straying from that brought in for breeding programs (ALA 2010), no Aegilops species is considered to be naturalised in Australia.