B chromosomes in Aegilops mutica Boiss.

A. MOCHIZUKI

Division of Plant Science, University of Manitoba Winnipeg, Manitoba, Canada.

In 1956 a small closed bivalent in addition to the normal chromosomes was observed in meiotic metaphase of an F₁ plant obtained from a cross between T. Timopheevi and Ae. mutica. A similar association was later observed in the F₁'s of Ae. bicornis x Ae. mutica and Ae. sharonensis x Ae. mutica. The bivalent association was readily distinguishable from normal chromosomes. Somatic chromosome counts of all three F₁'s revealed two more than the expected number¹⁾. Only one mutica plant²⁾ which had been clonally increased was used in these crosses. This evidence suggests that the supernumerary chromosomes could be attributed to the mutica plant used as a pollen parent.

A check of 18 offsprings raised from the original mutica plant showed that 6 had one B chromosome and 12 had three B chromosomes in addition to the 14 chromosomes in somatic cells. The B chromosomes have a median centromere and are shorter than the smallest chromosomes of Ae. mutica. Their staining reaction to acetocarmine and leuco-basic fuchsin was the same as for ordinary chromosomes and there was no variation among the B chromosomes for size or staining reaction. However, it was frequently seen that the number of B chromosomes varies in different parts of the same plant. Usually B chromosomes were found in stem tissues, the integuments and anthers but not in the root tips of the same plant. Even in plants with a B chromosome the proportion of cells with or without the B chromosome in the root tips is quite variable (Table 1).

In contrast to this, no B chromosomes could be found in the root tips of a plant known to have three B chromosomes even in a very young root tip approximately 2 mm long coming from a new tiller. This occurrence requires further investigation for an explanation.

In all plants examined to date, meiosis is abnormal when one B chromosome is present. There is no pairing at first metaphase and all the univalents are distributed to the poles, apparently at random. The next division is equational. In contrast to this meiosis is reasonably normal when three B chromosomes are present. The most frequent figure at first metaphase is 7 bivalents and 1 trivalent. When univalents are observed there is always an even number indicating that the trivalent does not become 3 univalents or a bivalent and 1 univalent. The trivalent, presumably made up of the 3 extra chromosomes, forms a V or O configuration and thus must be made up of closely homologous chromosomes. At anaphase the distribution is 8 to one pole and 9 to the other with no laggards in either the first or second divisions. However, in pollen mitosis non-disjunction of the B chromosome takes place in the first division and both its halves are included in one daughter nucleus.

Since to the writer's knowledge there has been no report of B chromosomes in the genus Aegilops, it would be much appreciated if readers of this article having seed samples of Ae. mutica could share them so that a more complete survey could be made on samples from several sources.

(Received April 18, 1957)