Z.K. Cheng1, H.X. Yu2, H.H. Yang1, M.H. Gu2 and L.H. ZHU’

1) Institute of Genetics, Chinese Academy of Sciences, Beijing 100101, China

2) Agricultural College, Yangzhou University, Yangzhou 225009, China

     The rice trisomics, including primary trisomics, secondary trisomics and teletrisomics, are very useful in rice genetic studies. We have so far developed a whole set of primary and telotrisomics, as well as 14 secondary trisomics in the genetic background of an indica rice variety, Zhongxian 3037 (Cheng eta!. 1996). As the extra chromosomes in these trisomics may behave abnormally during meiotic division, it is quite possible to find new types of variants from their prdgenies (Khush 1973). We report here a new kind of cytological variant with chromosome 4 double ditelosomic, in which one pair of chromosome 4 is replaced by two pairs of telocentric chromosomes.
     From the progeny of a telotrisomic with an extra long arm of chromosome 4, a 25-chromosome plant with 23 normal chromosomes plus two telocentric arms of chromosome 4 was selected, which was morphologically similar to Zhongxian 3037 but had lower spikelet fertility (57.4%). From the selfed progeny of this plant, we further isolated several 26-chromosome offsprings. In each of their somatic chromosome preparations, we could easily identify two short and two long arms of chromosome 4 according to their characteristic condensation patterns besides 22 normal chromosomes (Fig. ib). This chromosome constitution was further verified by pachytene analysis. In each PMC of the variant plants 13 bivalents could be found, i.e., 11 bivalents of the whole chromosome complement excluding chromosome 4; one bivalent of the short arm of chromosome 4 (short in length with some darkly stained chromomeres) and one bivalent of the long arm of chromosome 4, having four darkly stained chromomeres near centromere (Fig. id). So the variants should be 2n+22+2.4S+2.4L. Since the genetic dosage in the variant is actually the same as that in the normal diploid in spite of the change in chromosome number, it is difficult to distinguish such variant plants from the normal diploids based on their morphology. This is the first report of a double ditelosomic in rice. It is expected that more double ditelosomics for other chromosomes would be selected from the progenies of other aneuploids of Zhongxian 3037.