Hybridization and introgression between bread wheat and wild and weedy relatives in North America

Introgression between cultivars and wild relatives is common in several angiosperm taxa including the grass family Poaceae. Bread wheat (Triticum aestivum L.) is a domesticated allohexaploid species (genome formula BBAADD) without any known wild hexaploid relative in the genus Triticum. Bread wheat is also related to the genus A egilops L., which has probably contributed two of the three genomes of bread wheat. A few tetraploid Aegilops species, including Ae. cylindrica Host. and Ae. triuncialis L., occur as weeds both in the Mediterranean basin and in West Asia. Introduced populations of these weeds are also known to occur in North America. These species have been known to introgress occasionally with bread wheat when grown near wheat fields. Similarly, rye (Secale cereale L.), a species from a distant genus, has a potential to introgress with bread wheat. A few natural introgressive hybrids between herbicide resistant wheat and Ae. cylindrica and between wheat and rye have been created or recovered in North America. Natural hybrids between wheat and Ae. triuncialis have not been observed in North America. The available data do not suggest the prevalence of large-scale introgression between bread wheat and its wild relatives in North America. Nevertheless, with modern bread wheat cultivars being developed with novel traits, such as herbicide and disease resistance, an in-depth evaluation of the extent and nature of introgression between weedy Aegilops or Secale species and bread wheat is useful both for assessing potential ecological risks that may be associated with trait presence in hybrids and for formulating strategies to manage gene transfer to hybrids. In this review, we discuss the existing literature on reproductive ecology of bread wheat and on introgression between bread wheat and its wild relatives in the genera Aegilops and Secale that occur in North America. We also discuss the implications of introgression in consideration of the current and possible future development of transgenic wheat.