In order to elucidate the mechanisms of the selective retention of the Ae. caudata chromosome in JF, some 2n=43 plants were crossed with JF and/or were self pollinated. Their seed set by crossing or selfing was very sporadical in all cases; the crossed seed fertility was 17.0% on the average (17 seeds from 100 florets pollinated) and in the selfed individual spikes none to 14 seeds were set. Some 2n=43 plants had about 40% pollen fertility, and some were completely pollen sterile. The reduction in fertility of both sexes was probably caused by the monosomic Ae. caudata chromosome which has been retained in JF.

Breeding behavior of the Ae. caudata chromosome was studied using four 2n=43 plants with partial pollen fertility, which had been proved to be the real monosomic addition type forming 21"+1' at MI, i.e., the univalent must have been the Ae. caudata chromosome. The result is shown in Table 1. First, transmission of the Ae. caudata chromosome through egg cells was studied in a cross between one of the 21"+1' plants as female and normal JF. Six of the eight plants from this cross had 2n=43 chromosomes including one chromosome with a subterminal centromere. One seedling had the critical chromosome among the 42 chromosomes, and the other 2n=42 seedling did not have such a chromosome. This means that the univalent Ae. caudata chromosome is preferentially transmitted through the egg cell. Next, chromosome constitutions of selfed progenies from the four 21"+1' plants were studied. Of the 44 seedlings examined 41 had 2n=44 chromosomes including a pair of chromosomes with a subterminal centromere. Plants of 2n=42 with no and 2n=43 with one critical chromosome were not found at all, but two plants had 2n=43 including two critical chromosomes, and one had 2n=42 including one critical and two fragment chromosomes. This in turn indicated the exclusively preferential transmission of the Ae. caudata chromosome through pollen as was found in the egg cell.

Accordingly, the exclusively preferential transmission of the Ae. caudata chromosome can be attributed to the selective gametocidal action of this chromosome in JF. Namely, only gametes with the Ae. caudata chromosome can function normally, but those without it can not function, which results in reduced fertility of the plants having the monosomic Ae. caudata chromosome and at the same time causes its selective retention.

Such being the case, it becomes difficult to produce the chromosome addition series of JF which would have the respective chromosomes of Ae. caudata.

Since there are three pairs of chromosomes with subterminal centromeres in the karyotype of Ae. caudata, it is not confirmed yet whether or not the Ae. caudata chromosomes retained through the three backcrosses are the same. Apart from this question, they are similar in morphology and selective gametocidal action to the selectively retained chromosomes of the natural and synthetic strains of Ae. triuncialis, one of whose parental species is Ae. caudata (KIHARA, 1940; KIHARA and KONDO, 1943). Therefore, the present findings of an Ae. caudata chromosome selectively retained in common wheat strongly suggests that the critical chromosome of Ae. triuncialis originated from Ae. caudata.

Literature Cited

ENDO, T. Ryu 1978. On the Aegilops chromosomes having gametocidal action on common wheat. Proc. 5th Internat. Wheat Genet. Symp. (in press)

ENDO, T. Ryu and TSUNEWAKI, K. 1975. Sterility of common wheat with Aegilops triuncialis cytoplasm. J. Hered. 66: 13-18

KIHARA, H. 1940., Anwendung der Genomanalyse fur die Systematik von Triticum und Aegilops. Japan. J. Genet. 16: 309-320

KIHARA, H. and KONDO, N. 1943. Studies on amphidiploids of Aegilops caudata x Ae. umbellulata induced by colchicine. Seiken Ziho No. 2: 24-42

MAAN, S.S. 1975. Exclusive preferential transmission of an alien chromosome in common wheat. Crop Sci. 15: 287-292