TRITICUM URARTU AND GENOME EVOLUTION IN THE TETRAPLOID WHEATS

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Abstract
The A genome of the tetraploid wheats (AABB, 2n = 28) shows 5‐6 bivalents in crosses with Triticum boeoticum (2n = 14) and various Aegilops diploids (2n = 14). The B genome has never been similarly identified with any species, and is commonly thought to have been modified at the tetraploid level. Triticum boeoticum was presumably accepted as the A‐genome donor because of its morphological similarity to the wild tetraploids and because it was formerly the only known wild diploid wheat. The B donor has been thought to be Ae. speltoides or another species of the Sitopsis section of Aegilops, but these diploids show pairing affinity with A rather than B. More recently, another diploid wheat, T. urartu, was found to be sympatric with T. boeoticum throughout the natural range of the tetraploids. The synthetic boeoticum‐urartu amphiploid was virtually identical morphologically with the wild tetraploid wheats, whereas various boeoticum‐Sitopsis amphiploids were markedly different. But the urartu genome, like those of T. boeoticum and Sitopsis, paired with A and not with B. However, cytological evidence also shows (1) that the genomes of any plausible parental combination pair with one another, (2) that the A and B genomes of the tetraploid wheats pair with one another in the absence of the gene Ph, and (3) that homoeologous chromosomes of the tetraploids have differentiated further, presumably as a result of diploidization. Consequently, chromosome pairing at Meiosis I can be expected to give ambiguous evidence regarding the identity of the tetraploid genomes with their parental prototypes. A hypothesis regarding the expected pairing affinities between tetraploid homoeologues that have differentiated from closely related parental chromosomes is advanced to explain the anomalous pairing behavior of the A and B genomes. Triticum boeoticum and T. urartu are inferred to be the parents of the tetraploid wheats.