8. A secondary-trisomic like plant found in rice

Osamu IDETA, Atsushi YOSHIMURA and Nobuo IWATA

Faculty of Agriculture, Kyushu University, Fukuoka, 812 Japan

Studying chromosomal assignment of RFLP linkage groups (Ideta et al. 1990), we found a variant in the F1 plants derived from a cross between primary trisomics triplo 8 originated from a Japonica variety Nipponbare and an Indica variety IR24. The variant resembled the hybrid triplo 8 morphologically, but differed in some respects. The variant showed a low plant height, short panicles, slightly slender grains and a very low seed fertility. We examined chromosome configurations at metaphase I of the variant and compared with those in the hybrid triplo 8 (Table 1). A trivalent or a univalent was observed in each cell of the variant. The frequency of trivalent in the variant was lower than that in the hybrid triplo 8. These morphological and microscopic observations showed that the variant was a kind of trisomics related to triplo 8 with some chromosomal alternation in the extra chromosome.

Table 1. Chromosome configuration at metaphase I in the hybrid primary
trisomics and a related variant derived from the cross of Japonica primary
trisomics triplo 8/IR 24
             Chromosome configuration at metaphase I       Number of cells
            =========================================      observed
                    11 II +1 III       12 II + 1 I       
Hybrid triplo 8          27                 25                     52
Variant                  13                 28                     41

Fig. 1A and B. RFLP gene dosage analysis of the variant by using 6 clones mapped on chromosome 8. A) Summary of the RFLP analysis. Boxes reveal the loci of the RFLP clones which showed more increased dosage effect. Map distance (cM) and RFLP loci mapped on chromosome 8 (Saito et al. 1991) are shown in the left and right, respectively. B) Autoradiograms derived from probing Npb 126 and 68. P1: triplo 8, P2: IR24, lane 1: hybrid disomics, lane 2: hybrid triplo 8, lane 3: variant.

In order to detect the rearrangement of the extra chromosome, we performed RFLP gene dosage analysis using 6 RFLP clones mapped on chromosome 8 following the methods given by McCouch et al. (1988) and Ideta et al. (1990). The RFLP clones were due to Saito et al. (1991). The result of the RFLP analysis is summarized in Fig.1A with two photos in Fig. 1B. When probed with RFLP clones which are located on the extra chromosome, the gene dosage effect in hybrid primary trisomics is expected as follows: The ratio of signal intensities of trisomic fragment (P1 band in Fig. 1B) to disomic fragment (P2 band in Fig. 1B) is 2:1 theoretically. For RFLP clones, Npb 56, 126 (upper photograph in Fig. 1B) and 398, this was observed in the hybrid triplo 8, but was not observed in the variant. While for Npb 41, 38 and 68 (lower photograph in Fig. 1B), the hybrid triplo 8 showed the expected gene dosage effect but the variant showed a more increased gene dosage effect. The ratio of signal intensities of the variant fragments appeared to be 3:1. These results suggested that the chromosomal region from XNpb 68-1 to 41 on RFLP linkage map of chromosome 8 was duplicated in the extra chromosome of the variant.

Secondary trisomics accidentally arise in the progenies of primary trisomics (Khush 1973). The phenomena observed in the variant agreed with those in secondary trisomics. Therefore, the variant appeared to be secondary trisomics. Since the position of the centromere on the RFLP linkage map is uncertain, we consider the variant as a secondary-trisomic-like-plant.


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