| I. Research Notes Simulation of univalent distribution in a pentaploid wheat hybrid Seiichi TSUJI and Akitsu NAGASAWA Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Khoto 606, Japan The transmission of univalent D-genome chromosomes from a pentaploid hybrid (2n= 5x=35) between common and emmer wheats has been studied by several workers (KIHARA &, WAKAKUWA 1935, ALSTON & JONES 1965, MAKINO 1974, TSUJI & MAAN 1981, and others). KIHARA (1924) reported that seven univalents divided equationally at anaphase I (AI) and the chromatids were distributed at random at AII. As a consequence, the pentaploid produced gametes with different chromosome numbers ranging from 0 to 7 D-genome chromosomes. The data obtained by KIHARA & WAKAKUWA (1935) and by MAKINO (1974) are given in Table 1 in which the relative frequencies of eight kinds of the gametes transmitted from pentaploid hybrids are shown. SEARS (1953) reported from his study of Chinese Spring monosomics that about 75% of the female gametes had only 20 chromosomes. This indicated that about 50% of the univalent chromosomes were lost or eliminated during meiosis. Supposing that random distribution and 50% elimination of each univalent take place in meiosis II of the pentaploid gametogenesis, we can calculate the theoretical frequencies of female gametes with 0 to 7 D-genome chromosomes by the expansion of the binomial (0.75+0.25)7 (Table 1). When compared, however, it is clear that the actual distributions are flatter than the theoretical one. Thus, KIHARA (1975) suggested that the unexpected flatness of the curve might be due to non-random distribution of the univalents in meiosis II, so that seven univalents might go frequently in a mass to one pole. If this is the case, then we can assume that the frequency distribution pattern of female gametes with 0 to 7 D-genome chromosomes is determined mainly by two factors, that is, non-randomness in univalent segregation and certain degrees of elimination of individual univalents. On the basis of this assumption, the computer simulation of univalent behavior in meiosis II was conducted. The result will be presented in this report. A simplified flow diagram for the written computer program is illustrated in Fig. 1. It is assumed here that meiosis I give all cells with the exactly same chromosome constitution because of equational division of the univalents, although SEARS (1952) showed 96% univalent division in meiosis I in mono-5A of Chinese Spring. In this program, the fate of each univalent in meiosis II will be determined by two parameters ER (elimination rate) and DR (distribution rate), and 500 sets of two female gametes having different numbers of transmitted univalents will be produced as a result of meiosis II. The female gametes thus produced will be treated as a population of the gametes ready to fertilization, while in the real situation only one of the four cells formed in a female organ contributes to fertilization. |
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