Faculty of Agriculture, Kyushu University, Fukuoka, 812 Japan
A desynaptic gene was located on chromosome 7 by (acro)trisomic analysis. The desynaptic mutant, MM 29, was induced by N-methyl-N-nitrosourea (MNU) treatment of fertilized single egg cells (Yasui et al. 1989). The mutant showed morphological resemblance to the original cultivar Taichung 65 except for short stature and high sterility. Cytological evidence revealed that the high sterility was caused by failure of homologous chromosomes to pair in meiosis. MM 29 was classified as a medium-strong desynapsis type since the mean numbers of bivalents per cell at the first metaphase was 5.16. Genetic data indicated that the
Table 2. Segregation of normal and sterile plants in the F\3\ progenies derived from the disomic F\2\ plants originated from trisomic hybrids produced from the cross between a desynaptic mutant (MM 29) and the original cultivar, Taichung 65 ======================================================================== No. of F\3\ lines Trisomic ========================= X2 valuea X2 valueb hybrid Normal Segregated Total for (1:2) for (1:4) ======================================================================== Triplo 7 29 120 149 12.90*** 0.03 Acro 7 35 72 107 0.02 10.80** ======================================================================== a Normal disomic segregation (X2 value for 1:2) b Critical trisomic segregation (X2 value for 1:4) ***, ** significant at 0.1 and 1% levels, respectively.
The mutant was crossed with the normal genotype and a complete set of the primary trisomics were obtained in F\1\ plants. These trisomic hybrids representing each chromosome were selfed and trisomic analyses were carried out in the progenies. Although Triplo 1, 2, and 3 were not available because of their low fertility, remaining 9 types of primary trisomic plants and one acrotrisomic plant for chromosome 7 produced enough seeds and the segregation of normal and sterile plants was investigated. The results of trisomic analysis are shown in Tables 1 and 2. The ratios from all the trisomic hybrids except for Triplo 7 fitted 3:1. The ratio given by the disomic fraction of Triplo 7 fitted 5:4 and that given by F\3\ progenies derived from normal F\2\ plants showed a 1 normal homozygous : 4 segregating ratio, suggesting that ds3(t) was located on chromosome 7. The ratio from an acrotrisomic plant for chromosome 7 (Acro 7) fitted 3:1 in F\2\ and 1 normal homozygous : 2 segregating in F\3\ derived from normal F\2\ plants. This result revealed that ds3(t) was located on a missing arm or region of the extra chromosome in Acro 7.
The linkage map of chromosome 7 was written as d-6--g-1--spl-5--Rc--v-11-- rfs--ge (Satoh and Iwata 1990). Among these marker genes, d-6, g-1, spl-5 and v-11 gave trisomic ratios in the progeny, but other two genes, rfs and ge did not when the plants with these genes were crossed to Acro 7 (Yasui and Iwata 1992). The present data given by ds3(t) showed disomic segregation when it was crossed to Acro 7. This result suggests that ds3(t) would be located on chromosome 7 between v-11 and the distal end near ge because Acro 7 possesses the region of d-6--g-1--spl-5--Rc--v-11 as the extra chromosome.
Satoh, H. and N. Iwata 1990. Linkage analysis in rice. On three mutant loci for endosperm properties, ge (giant embryo), du-4 (dull endosperm-4) and flo-1 (floury endosperm-1). Jpn. J. Breed. 40 (Suppl.2): 268-269. (in Japanese)
Yasui, H., H. Satoh and N. Iwata 1989. Establishment of a trisomic series in rice by using a desynaptic mutant. RGN 6: 50-51.
Yasui, H. and N. Iwata 1992. Acrotrisomics with chromosome 7 in rice. RGN 9: 45-47.