Loss of heterozygosity and accelerated genotype fixation in rice hybrids

Loss of heterozygosity is reported in rice hybrids of a particular heritage . Hybrids derived from a plant selected from the Chinese rice cultivar 'Zho ngxin No. 1' exhibited somatic variations as evidenced by having both segre gating and uniform panicle rows in F-2 progenies. F-3 plants from uniform F -2 rows were found to be homozygous for all 14 RAPD (random amplified polym orphic DNA) markers, of which two co-dominant markers were located on chrom osome 2 and five other markers were on five different chromosomes. RAPD mar kers unique to either parent were present or absent in all F-2 plants withi n some panicle rows, yet segregated in a Mendelian manner in other panicle rows. The molecular data suggest that somatic cells in these hybrids do not always contain both parental homologues of some chromosomes. These finding s support the hypothesis that somatic chromosome pairing and recombination lead to loss of heterozygosity and non-identical daughter cells following m itosis. Sequential mitotic assortment of chromosome homologues of a plant's genome can lead to homozygous or nearly homozygous somatic cells that even tually develop into reproductive cells. As a result of this unique mechanis m in rice hybrids derived from Zhongxin No. 1, uniform or less-segregating progenies can be identified from F-2 or F-3 panicle rows at a much higher f requency than normally expected. This phenomenon can be utilized to shorten the breeding cycle of rice, or other crops when plants containing gene(s) for mitotic pairing are identified, or when the genes are isolated from ric e and effectively transferred into other crops.