The Ph1 and Ph2 loci play different roles in the synaptic behaviour of hexaploid wheat Triticum aestivum

Triticum aestivum is an allohexaploid wheat (AABBDD) that shows diploid-like behaviour at metaphase-I. This behaviour is influenced by the action of several loci, Ph1 and Ph2 being the main loci involved. To study the effect of these two loci on chromosome pairing in T. aestivum we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late-zygotene, and at pachytene, of three different genotypes of cv Chinese Spring: standard line, ph1b and ph2b mutants. The analysis of the synaptic progression showed that only a few nuclei accomplish synapsis in the ph2b genotype, whereas most nuclei completed synapsis in the standard and ph1b genotypes. This result indicates that the Ph2 locus affects synaptic progression. 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 and showed similar values in the three genotypes. These values decreased progressively between mid-zygotene and pachytene in the genotypes with the Ph1 locus because of the transformation of multivalents into bivalents. In the ph1b genotype, this value only decreased between late-zygotene and pachytene. Therefore, multivalent correction was more efficient in the presence than in the absence of the Ph1 locus. It is concluded that the Ph1 and Ph2 loci bring about diploidization of allohexaploid wheat via a different mechanism.