29. Isolation of thirteen-paired rice
        Hideshi Yasui, Noriko Ohta and Nobuo Iwata
        Faculty of Agriculture, Kyushu University, Fukuoka, 812 Japan

    Aneuploids are useful for research in genetics, breeding, and evolution in plants. In barley various forms of new karyotypes with chromosome numbers other than 14 have been synthesized through trisomics. Eight-paired barley (2n=16) were isolated by Tsuchiya (1969), Wiebe (1975), and Ries (1982). Furthermore, Hang and Tsuchiya (1992) reported nine-paired barley (2n=18) from the cross between two lines with 2n=16 chromosomes. In this study, thirteen-paired rice (2n=26) carrying a pair of homologous chromosomes in addition to the normal chromosome complement was isolated through a trisomic plant derived from a desynaptic mutant crossed with rice cultivar, Taichung 65.
    A trisomic plant selected from the cross between a desynaptic mutant described by Yasui et al. (1993) and a normal disomic was backcrossed and subsequently selfed. In a BC1F3 progeny, a few plants which showed late flowering and dark green leaves were isolated in addition to parental trisomic and normal disomic segregants. The plants isolated were subsequently selfed and mitotic and meiotic chromosomes of BC1F5 individuals obtained from two kinds of BC1F6 aneuploids, were examined. For cytological studies, enzyme maceration method following Giemsa stain and acetocarmine squash method were employed for root tip cells and pollen mother cells (PMCs), respectively. Morphological and reproductive features and transmission rates of the extra chromosomes were investigated in the same population.
    Amongst the BC1F6 individuals, the plants with three kins of chromosome numbers, 24, 25, and 26, were isolated (Table 1), During meiosis in the plant with 24 chromosomes, normal chromosome associations (12 II) were observed at metaphase I in

Table 1. Stability of the extra chromosome(s) of the plants with 25 and 26
chromosomes isolated from a trisomics derived from a desynaptic mutant
 

Parental plant in BC1F5
with 2n=		 Number of plants in BC1F6 generation with 2n= Transmission rates of the extra
chromosome(s)
24  25 26
25 
26		 14
2		 14
2 		 1 
24		 26.3%
93.3%

all the PMCs. PMCs observed in the plant with 25 chromosomes showed 1211+1 I or II 11+1 III at metaphase I. The result indicated that the extra chromosome of the plant with 25 chromosomes was often associated with a normal homologous chromosome pair. On the other hand, during meiosis in the plant with 26 chromosomes, extra chromosomes were associated with each other in most of the PMCs. At diakinesis, most of the PMCs showed 13 II (Fig. la). At metaphase I, mostly 13 II (Fig. lb) and sometimes 1 1 11+1 1+1 III (Fig. lc) configurations were observed. At anaphase I, normal 13:13 segregation (Fig,1d) was usually observed although lagging chromosomes were sometimes present.
    The plant with 26 chromosomes flowered later than the plants with 24 and 25 chromosomes. The plant height was a little shorter than disomics and characterized by dark green leaves. The plants with 25 and 26 chromosomes were fertile and produced good progenies. The stability of extra chromosomes in those plants through selfing is shown in Table 1. The transmission rate of the extra chromosome(s) in the plant with 26 chromosomes was quite high (93.3%) compared with the plant with 25 chromosomes (26.3%), suggesting stable transmission of the extra chromosome(s) through both female and male gamates. As no fertile tetrasomic plants have been isolated although Wang and Iwata (1991) reported several tetrasomic plants in rice, the plants with 26 chromosomes studied in this paper may carry a modified homologous pair of chromosomes resulting in 13 bivalents at meiosis.

Fig. 1. Meiotic chromosomes in a rice plant with 26 chromosomes, (a) Diakinesis, 1311 (b) Metaphase 1, 1311 (c) Metaphase 1, 1111+11+1111. Narrow and bold arrows indicate a univalent and a trivalent, respectively, (d) Anaphase I showing 13:13 segregation. Bars=5 microm. References

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