Synaptic behaviour of the tetraploid wheat Triticum timopheevii.

Triticum timopheevii (2n=4x=A(t)A(t)GG) is an allotetraploid wheat which shows a diploid-like behaviour at metaphase-I. The synaptic process was analyzed in fully traced spread nuclei at mid-zygotene, late-zygotene and pachytene. The length and type of synaptonemal complexes, as well as the number of bivalent and multivalent associations, were determined in each nucleus. A high number of bivalents per nucleus was detected at all three stages. Nuclei at pachytene showed a lower frequency of multivalents than did zygotene nuclei, which suggests the existence of a pairing correction mechanism. At metaphase-I only homologous bivalents and, rarely, one pair of univalents were observed. Similarities between the diploidization mechanism of T. timopheevii and that of allohexaploid wheat, controlled by chromosome 5B, are discussed.