Chromosome healing by addition of telomeric repeats in wheat occurs duringthe first mitotic divisions of the sporophyte and is a gradual process

Alien gametocidal chromosomes cause extensive chromosome breakage prior to S-phase in the first mitotic division of gametophytes lacking the alien chr omosome. The broken chromosomes may be healed either by addition of telomer ic repeats in the gametophyte or undergo fusions to form dicentric or trans location chromosomes. We show that dicentric chromosomes undergo breakage-f usion-bridge (BFB) cycles in the first few mitotic divisions of the sporoph yte, are partially healed before the germ line differentiation regimen, and are healed completely in the ensuing gametophytic stage. The gametocidal f actor on chromosome 4Mg of Aegilops geniculata was used to induce dicentric s involving the satellite chromosomes 1B and 6B of wheat, Triticm aestivum. The dicentrics 1BS . 1BL-2AL . 2AS and 6BS . 6BL-4BL . 4BS initiated BFB c ycles that ceased 2 to 4 weeks after seed germination. At the end of the BF B cycles, we observed deficient 1B and 6B chromosomes with breakpoints in p roximal regions of the 1BL and 6BL arms. The process of chromosome healing was analyzed in root tip meristems, at meiotic metaphase I, and in the deri ved progenies by fluorescence in-situ hybridization analysis using a telome ric probe pAtT4. The results show that chromosome healing in wheat occurs d uring very early mitotic divisions in the sporophyte by de-novo addition of telomeric repeats and is a gradual process. Broken chromosome ends have to pass through several cell divisions in the sporophyte to acquire the full telomeric repeat length.