The synaptic behaviour of Triticum turgidum with variable doses of the Ph1 locus

Cultivated wheat Triticum turgidum is an allotetraploid (AABB) with diploid-like behaviour at metaphase I. This behaviour is mainly influenced by the action of the Phl locus. To study the effect of Phl on chromosome pairing in T. turgidum we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late-zygotene and at pachytene of three different genotypes: a standard line, ph1c mutant and a duplication mutant, with zero, two and four doses of Phl, respectively. The number of synaptonemal complex (SC) bivalents and of the different SC multivalent associations were determined in each nucleus. The mean number of lateral elements involved in SC multivalent associations (LEm) at mid-zygotene was relatively high in all lines and was similar in two and zero doses of Phl. These means changed little with the progression of zygotene but decreased at pachytene because of the transformation of multivalents into bivalents. Multivalent correction was more efficient in the presence than in the absence of Phl. The four doses of Phl genotype showed a higher number of SC bivalents at mid-zygotene, and the frequency of multivalents decreased progressively throughout zygotene and pachytene. The results suggest that the main action of the Ph1 locus on the diploidisation mechanism would be related to a process of checking for homology operating during prophase I.