Further information on identification of the chromosomes in the A and B genomes

M. OKAMOTO

Curtis Hall, University of Missouri Columbia, Missouri, U. S. A.

Subsequent to the previous report (Okamoto, WIS No. 5) on the identification of certain chromosomes of the A and B genomes, additional data have been obtained, and chromosomes III, X and XII added to the list.

In crosses of the amphidiploid Triticum aegilopoides x Aegilops squarrosa (AADD) with Chinese Spring wheat (AABBDD), in which the F₁'s carried a telocentric for a particular A- or B-genome chrormosome, heteromorphic bivalents were observed as given in the following table.

On the basis of the differences in the frequency of heteromorphic bivalents, chromosomes VI, IX, XI and XII are in the A genome and chromosomes I, III, V, VII, VIII and X are in the B genome. Since it may be assumed that homoeologous chromosomes are in different genomes, XIV and IV must be in genome A.

The above results agree with Larson's hypothesis as concerns chromosomes I and XIV, IV and VIII, and VI and X, but disagree for chromosomes III and XII, V and IX, and VII and XI.

The finding that IX is in the A genome conflicts with the long-standing assumption, for which Matsumura (1947, 1951) has presented corroborative data, that this chromosome is in the B genome. However, although the fact that telo-V forms a heteromorphic bivalent in low frequency is somewhat anomalous, the present results apparently leave no escape from the conclusion that IX is in the A genome. A possible explanation for Matsumura's results, which involved a comparison of AABBDD x AA deficient and non-deficient for chromosome IX, can be found in the reduced pairing caused by chromosome V in AABBDD x AADD (Okamoto, WIS No. 5). Chromosome IX may have had a similar effect in Matsumura's material.



Because of the discovery of Sarkar and Stebbins that the B genome most probably came from Ae. speltoides or one of its close relatives, there is no longer any basis for the belief, expressed by McFadden and Sears (1946), that the free-threshing gene (Mac Key's Q) was obtained from the B-genome ancestor; for no such gene is known in the Ae. speltoides group. Q presumably arose as a mutation in either tetraploid or hexaploid wheat, and may just as well have arisen on a chromosome of either the A or B genomes.

(Received Sept. 1, 1957)