TRITICUM URARTU AND GENOME EVOLUTION IN THE TETRAPLOID WHEATS

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. MANY ATTEMPTS have been made to identify the A and B genomes of the tetraploid wheats (AABB, 2n = 28), each with the genome of a different diploid parent, on the basis of chromosome pairing affinities as deduced from Meiosis I (MI) configurations. Triticum boeoticum Boiss. (2n = 14) is believed to be the donor of the A genome, while B is commonly thought to be derived from Aegilops speltoides Tausch (2n = 14) or one of the other diploids comprising the section Sitopsis Jaub. et Spach. of the genus Aegilops. In crosses with the tetraploid wheats, T. boeoticum gives an average of about five bivalents. But some Aegilops diploids (including Ae. speltoides) show high pairing affinity with T. boeoticum (about 511) and also give about 511 in crosses with the tetraploid wheats (Kihara, 1937; Sears, 1956), suggesting that they also qualify as the A-genome parent. On the other hand, a plausible B-genome parent has never been identified 1 Received for publication 1 March 1978; revision accepted 13 June 1978. This study was supported by grant no. 1059 to B.L.J. from the National Geographic Society and by a D. F. Jones postdoctoral fellowship to H.S.D. 2 Present address: Friedrich Miescher-Institut, Basel, Switzerland. by means of chromosome pairing homology. This has led to speculation that the B donor may be an unknown or extinct species, or that the B genome became modified at the tetraploid level (Sears, 1948; Sarkar and Stebbins, 1956). The decisive factors in the early acceptance of T. boeoticum (Fig. 1) as the A-genome donor were undoubtedly its close morphological similarity to the wild tetraploid wheats (Figs. 9, 10) and the fact that, until recently, it was the only widely known wild diploid wheat. The inference that the B genome was derived from one or another Aegilops species of the section Sitopsis (Fig. 3-8) also rests on the morphological proximity of those species to the genus Triticum. From a comparison of spikelet morphology, Sarkar and Stebbins (1956) inferred that the tetraploid wheats arose as a result of chromosomal rearrangement in an amphiploid between diploid wheat and Ae. speltoides (Fig. 7, 8) or a species very similar to it. Sears (1956) suggested that on the basis of morphology, Ae. bicornis Jaub. et Spach. (Fig. 3) was a more likely donor of the B genome than was Ae. speltoides. However, evidence from electrophoretic patterns of ethanol extracts of crude seed proteins (Johnson. 1972) indicated that both Darents of the