Fate of multicentric and ring chromosomes induced by a new gametocidal factor located on chromosome 4M g of Aegilops geniculata R. G. Kynast 1 , B. Friebe 2  & B. S. Gill 2 1 Department of Agronomy & Plant Genetics, University of Minnesota, 411 Borlaug Hall, St. Paul, MN 55108 - 6026, USA; 2 Wheat Genetics Resource Center, Department ofPlant Pathology, Kansas State University, Throckmorton Plant Sciences Center, Manhattan, KS 66506 - 5502, USA; Tel: (1) 785 5326176; Fax: (1) 785 5325692; E-mail: friebe@ksu.edu  Correspondence Received 28 June 1999; received in revised form and accepted for publication by Pat Heslop-Harrison 20 December 1999 Key words: Aegilops geniculata, BFB cycle, C-banding, ¯uorescence in-situ hybridization, gametocidal factor Abstract A new gametocidal (Gc) factor was identi®ed on chromosome 4M g of Aegilops geniculata Roth. When transferred to Chinese Spring wheat, monosomic and disomic Triticum aestivum±Ae. geniculata chromosome 4M g addition plants undergo regular ®rst and second meiotic divisions. Male gametogenesis in disomic 4M g addition plants also is normal. However, chromosome breakage and anaphase bridges were observed at ana/ telophase of the ®rst (29%) and second (11%) pollen mitosis in monosomic 4M g addition plants. Gc-induced multicentric and ring chromosomes can be transmitted to the offspring and initiate breakage fusion bridge (BFB) cycles in dividing root tip meristem cells of the derived sporophytes. The fate of multicentric and ring chromosomes was analyzed in root meristems at different time intervals after seed germination. The majority of the BFB cycles ceased about 32 days after germination. Broken chromosome ends were healed either by the fusion of a centric and an acentric fragment forming terminal translocation chromosomes or as de®ciencies or telocentric chromosomes. Lack of cytologically detectable telomeric repeats at the stabilized newly broken termini suggests that chromosome healing by addition of telomeric repeats may be a gradual process. Introduction Gametocidal (Gc) factors have been identi®ed in several diploid and polyploid species of the genus Aegilops (for review, see Endo 1990). When trans- ferred to wheat, Triticum aestivum L. (2n  6x  42, AABBDD) or T. turgidum ssp. durum Desf. (2n  4x  28, AABB), disomic Gc chromosome addition plants have regular meiotic divisions and gametogenesis. Meiosis also is normal in plants monosomic for a Gc chromosome. However, chromo- some breakage occurs at the G1 stage of the ®rst postmeiotic interphase mainly in gametophytes lack- ing the Gc chromosome (Finch et al. 1984, King & Laurie 1993, Nasuda et al. 1998). Several Gc systems have been described (Endo 1982, Finch et al. 1984, Tsujimoto & Tsunewaki 1984, 1985, Endo 1985a, 1985b, 1988, Tsujimoto & Tsunewaki 1988, Tsujimoto 1994, 1995). Strong Gc factors induce extensive chromosome breakage which results in non-functional gametes and, thus, in the exclusive transmission of the Gc chromosome to the offspring (Nasuda et al. 1998). A weak Gc factor induces lower levels of chromosome breakage that Chromosome Research 8: 133±139, 2000. # 2000 Kluwer Academic Publishers. Printed in the Netherlands 133