Cell Division, Mitosis and Meiosis Cytogenet Genome Res 109:210–214 (2005) DOI: 10.1159/000082402 Synaptic behaviour of hexaploid wheat haploids with different effectiveness of the diploidizing mechanism M. Martinez, a C. Cuadrado, a D.A. Laurie b and C. Romero a a Departamento de Genética, Facultad de Ciencias Biolo ´ gicas, Universidad Complutense, Madrid (Spain); b John Innes Centre, Colney, Norwich (UK) Supported in part by a grant awarded by the Spanish Ministry of Education and Sciences to C.R. Received 16 October 2003; manuscript accepted 15 April 2004. Request reprints from C. Romero, Departamento de Genética Facultad de Ciencias Biolo ´ gicas, Universidad Complutense 28040 Madrid (Spain); fax: 34-1-3944844 e mail: romeroc@bio.ucm.es ABC Fax + 41 61 306 12 34 E-mail karger@karger.ch www.karger.com © 2005 S. Karger AG, Basel 0301–0171/05/1093–0210$22.00/0 Accessible online at: www.karger.com/cgr Abstract. Haploids of three cultivars of Triticum aestivum (Thatcher, Chris, and Chinese Spring) were obtained from crosses with Zea mays. The level of chromosome pairing at metaphase I and the synaptic behaviour at prophase I was stud- ied. There were differences in the meiotic behaviour of the haploids from different cultivars. Thatcher and Chris haploids had significantly higher levels of pairing at metaphase I than Chinese Spring haploids. This metaphase I pairing was corre- lated with higher levels of synapsis achieved in the Thatcher and Chris prophase I nuclei than in the Chinese Spring nuclei. Variation in the effectiveness of the diploidizing mechanism among cultivars of wheat is proposed to have a genetic origin and the role of the Ph1 locus in the different haploids is dis- cussed. Copyright © 2005 S. Karger AG, Basel Bread wheat, Triticum aestivum L., is an allohexaploid spe- cies with genomes A, B and D (2n = 42, AABBDD). These dip- loid genomes evolved from a common ancestral genome, and, therefore, have partially homologous (homoeologous) chromo- somes. Despite similarities, homoeologous chromosomes do not pair at metaphase I, therefore wheat shows strictly diploid- like behaviour at meiosis. On the basis of chromosome pairing at metaphase I in wheat euploids, haploids or hybrids lacking individual chromosomes or chromosome arms, it was demon- strated that the most important locus involved in the suppres- sion of homoeologous pairing was located on the long arm of chromosome 5B (Sears and Okamoto, 1958; Riley and Chap- man, 1958). This locus was designated Ph1. A second locus, Ph2, with a lower effect on the suppression of homoeologous pairing was localized on the short arm of chromosome 3D (Mello-Sampayo and Canas, 1973). Additionally, less effec- tive suppressors were discovered on several other chromo- somes (Sears, 1976; Cuadrado et al., 1991). Studies of chromosome synapsis at prophase I in plants with and without the 5B chromosome led Holm and Wang (1988) to suggest that the Ph1 locus regulates the stringency of synapsis at early zygotene and suppresses crossing-over between the re- gions homoeologously synapsed at pachytene. Additional evi- dence in support of the crossing-over restriction was provided by Gillies (1987), Dubcovsky et al. (1995) and Luo et al. (1996). On the contrary, Feldman (1966) suggested that the effect of the Ph1 locus is exerted at the premeiotic stages, affecting the alignment of homologous and homoeologous chromosomes. This hypothesis has been supported by fluorescence in situ DNA hybridization experiments using telomere and centro- mere probes, and by the genomic in situ hybridization in wheat or wheat-barley and wheat-rye disomic addition lines (Moore, 2002) and rejected by Maestra et al. (2002). Using a spreading technique, Martinez et al. (2001a, b) demonstrated that the most important action of the Ph1 locus in tetraploid and hexa- ploid wheats was related to the correction of multivalent synap- sis throughout prophase I. Recently, the role of the Ph1 locus in the correction of non homologous associations during chromo- some pairing has received additional support from the “in situ” DNA hybridization experiments (Martı ´nez-Pérez et al., 2003). Most of the knowledge about the action of the diploidizing mechanism arises from the studies on wheat cultivar Chinese