Loss of heterozygosity and accelerated genotype fixation in rice hybrids

Abstract

Loss of heterozygosity is reported in rice hybrids of a particular heritage. Hybrids derived from a plant selected from the Chinese rice cultivar 'Zhongxin No. 1' exhibited somatic variations as evidenced by having both segregating and uniform panicle rows in F2progenies. F3plants from uniform F2rows were found to be homozygous for all 14 RAPD (random amplified polymorphic DNA) markers, of which two co-dominant markers were located on chromosome 2 and five other markers were on five different chromosomes. RAPD markers unique to either parent were present or absent in all F2plants within 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 findings support the hypothesis that somatic chromosome pairing and recombination lead to loss of heterozygosity and non-identical daughter cells following mitosis. Sequential mitotic assortment of chromosome homologues of a plant's genome can lead to homozygous or nearly homozygous somatic cells that eventually develop into reproductive cells. As a result of this unique mechanism in rice hybrids derived from Zhongxin No. 1, uniform or less-segregating progenies can be identified from F2or F3panicle rows at a much higher frequency 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 rice and effectively transferred into other crops.Key words: LOH, in vivo somatic variation, homozygous F2, RAPD, apomixis.