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

Abstract

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 midzygotene 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.

Genetic Regulation of Meiotic Cross-Overs between Related Genomes inBrassica napusHaploids and Hybrids

“…However, the very peculiar nature of the Ph1 locus, which consists of a cluster of cyclin-dependant kinases (cdk-like genes) and a segment of subtelomeric heterochromatin, does not readily explain the multiple cytological effects attributed to Ph1 (Feldman, 1993;Mikhailova et al, 1998;Martinez-Perez et al, 2003;Prieto et al, 2005;Corredor et al, 2007). On the other hand, Ph2, another suppressor of homoeologous associations (Upadhya and Swaminathan, 1967;MelloSampayo, 1971), was shown to affect synaptic progression (Martinez et al, 2001) but the genes responsible for the phenotype are still to be identified (Sutton et al, 2003).…”

Synaptic behaviour of hexaploid wheat haploids with different effectiveness of the diploidizing mechanism

“…Two sister anthers of the same floret at meiotic prophase were prepared for the observations of the synaptonemal complexes (SCs) according to the technique described in Martinez et al (2001b). Surface-spread preparations were silver stained by Loidl's method (1984).…”

“…Martinez et al (2001b) demonstrated that the Ph1 locus in Chinese Spring corrects homoeologous associations throughout the meiotic prophase I and suppresses crossing-over between the remaining homoeologous regions at pachytene. In Chinese Spring haploids the non-strictly homologous associations formed during prophase I could be quickly dissolved, thereby avoiding an extensive synapsis at late prophase I.…”

Terminal Regions of Wheat Chromosomes Select Their Pairing Partners in Meiosis

“…Functionally equivalent regions of different chromosomes, such as telomeres or centromeres, provide excellent starting points for homology recognition that avoids scanning of the entire genome, a complicated task in large genomes. However, because wheat chromosomes are bi-armed and very large-in mid-zygotene they reach an average length of 112 mm (Martínez et al 2001b)-it is difficult to envisage how a mechanism of chromosome recognition operating at the centromeres may trigger the initiation of synapsis at the telomeres.…”