| Morphological evidence for the origin of the B genome
in wheat P. SARKAR and G.L. STEBBINS Department of Botany, University of Manitoba, Winnipeg, Canada, and Department of Genetics, University of California, Davis, California, U.S.A. Morphological comparison between both cultivated and wild members of the T. monococcum (Einkorn) group of wheats on the one hand and the tetraploid Emmer wheats on the other, followed by the use of Anderson's method of extrapolation, indicates that the Emmer wheats arose as amphiploids between the diploid Einkorns and another species or species group having the following characteristics: rachis internodes long and narrow; spikelets with 3 or more florets; glumes with one inconspicuous keel, with more than 6 veins, with the tip 1-toothed, and with a thick margin; lemmas all long awned, the shoulder inconspicuously dentate or blunt; the palea not splitting at maturity; the grain large, thick, and grooved. These characters are all approached or equalled by Aegilops spelioides var. ligustica. The hypothesis is therefore advanced that the tetraploid Eimmer wheats evolved from amphiploids between T. monococcum and its relatives on the one hand and various diploids similar to Ae. speltoides on the other. The amphiploidy is believed to have occurred several times, and to have been followed by evolution on the tetraploid level which consisted of hybridization between different raw amphiploids, accompanied by chromosome rearrangement and gene mutation. In this way, it is postulated that the monococcum type A and the speltoides type S genomes became modified to form the A and B genomes of the modern Emmer as well as the hexaploid wheats. Discussion in Correspondence Yamashita: I am very much interested in the fact that you consider one of the ancesters of Emmer wheat as Ae. speltoides. Under the guidance of Dr. Kihara, Mr. Tanaka obtained an amphidiploid, Ae. longissima x T. aegilopoides. The former parent is related to Ae. speltoides. According to the genome analysis, Ae. longissima has Sl- and Ae. speltoides S-genome. In the hybrid between the amphidiploid and Emmer wheat, the chromosome pairing was not too good. Stebbins: We did not consider the evidence of Mr. Tanaka on Ae. longissima x T. aegilopoides as critical, since Sears found little pairing in the cross between Ae. longissima and Ae. speltoides var. ligustica. Sarker is continuing the work by studying the relation between T. monococcum x Ae. speltoides amphidiploid and tetraploid wheat. |