| Meiotic abnormalities observed in some varieties of
common wheat and occurrence of aneuploids Y. WATANABE Tohoku National Agricultural Experiment Station, Morioka, Japan When the "Meiotic Indices" were examined with over 200 varieties grown on our Experimental Farm for the purpose of contributing data to the wheat breeding program of the future, five types of anomalies were distinguished in the course of meiosis of some varieties which showed a low meiotic index of less than 95%. "Meiotic Index", so designated by LOVE (1949), is the percentage of normal pollen tetrads used as an index of the regularity of meiotic chromosome behavior. The first type of these anomalies is characterized by the occurrence of univalents, the second type by the occurrence of multivalents, the third type by chromosome fragmentation - fusion, the fourth type chromosome - mosaics, and the last type by the formation of microcytes. From a population of 50 plants of Shirahada, 5 plants were selected on the basis of seed - fertility and morphological characters as follows: One plant with the highest and 2 with low fertility, and 2 with pubescent glumes. From the self-pollinated progenies of these 5 plants, 5 plants from each one, i.e., 25 plants altogether were studied cytologically. The fertility of the parental 5 plants and the 2n chromosome number in their self-pollinated progenies are shown in Table 1. During the course of a series of studies dealing with "Meiotic Index", several aneuploids were found among natural populations of Akasabi - Shirazu No. 1, Hitsumi-Komugi, Shirahada, Yushoki, Tohoku No. 98, Norin No. 10, Norin No. 33 and Norin No. 42. Most of them were monosomics, but trisomics were also found in Akasabi-Shirazu No. 1 and Norin No. 10. Out of these varieties showing aneuploids, Norin No. 42 attracted the author's attention especially on account of monosomics occurring in comparatively high frequency. Therefore, of a population consisting of 35 plants, 2n chromosome number of every plant was examined. As shown in Table 3, nullisomics as well as monosormics were found. Since disomic plants of this variety ptoduce 2 to 6 univalents in 10 to 26% of the cells observed, there is no doubt that aneuploids occur in their progenies as a result of the formation of aneuploid gametes. However, in other varieties forming as many univalents as Norin No. 42, the occurrence of monosomics is not so frequent as in Norin No. 42, and the occurrence of nullisomics is much less. Therefore, the cause of the frequent occurrence of aneuploids in this variety seems to be due to repeated careless sampling of seeds for the preservation of genes from a monosomic plant which occurred spontaneously in this variety one or several generations before. In common wheat, it is difficult to distinguish monosomics or trisomics from disomics by simple observing the morphological characters, though nullisomics can be easily distinguished because of their characteristic dwarfishness. Accordingly, most of monosomics or trisomics that spontaneously occurred in the fields are apt to be overlooked. As described above failure of chromosome pairing, formation of multivalent chromosomes, or other types of anomalies have been often observed in common wheat. This is because common wheat is an allopolyploid, exhibiting semihomologous relationships between its three different genomes and moreover, showing sometimes structural differentiation among the chromosomes. If a gene or genes controlling meiotic abnormalities happen to occur on these chromosomes by mutation, the meiotic chromosome behavior will be further disturbed. Thus, in the progenies, aneuploids will necessarily occur. |