The effect of the wheat Ph1 locus on chromatin organisation and meiotic chromosome pairing analysed by genome painting

Abstract

The Ph1 locus in wheat influences homo(eo)logous chromosome pairing. We have analysed its effect on the behaviour and morphology of two 5RL rye telosomes in a wheat background, by genomic in situ hybridisation (GISH), using rye genomic DNA as a probe. Our main objective was to study the effect of different alleles of the Ph1 locus on the morphology and behaviour of the rye telosomes in interphase nuclei of tapetal cells and in pollen mother cells at early stages of meiosis. The telosomes, easily detectable at all stages, showed a brightly fluorescing chromomere in the distal region and a constriction in the proximal part. These diagnostic markers enabled us to define the centromere and telomere regions of the rye telosomes. In the presence of functional copies of Ph1, the rye telosomes associated at pre-leptotene, disjoined and reorganised their shape at leptotene, and became fully homologously paired at zygotene - pachytene. In plants without functional alleles (ph1bph1b), the rye telosomes displayed an aberrant morphology, their premeiotic associations were clearly disturbed and their pairing during zygotene and pachytene was reduced and irregular. The Ph1 locus also influenced the behaviour of rye telosomes in the interphase nuclei of tapetal cells: in Ph1Ph1 plants, the rye telosomes occupied distinct, parallel-oriented domains, whereas in tapetal nuclei of ph1bph1b plants they were intermingled with wheat chromosomes and showed a heavily distorted morphology. The results shed new light on the effect of Ph1, and suggest that this locus is involved in chromosome condensation and/or scaffold organisation. Our explanation might account for various apparently contradictory and pleiotropic effects of this locus on both premeiotic associations of homologues, the regulation of meiotic homo(eo)logous chromosome pairing and synapsis, the resolution of bivalent interlockings and centromere behaviour.

Terminal Regions of Wheat Chromosomes Select Their Pairing Partners in Meiosis

“…The level of associations in pairs increased in the course of prophase I but the frequencies observed in zygotene and pachytene were lower than the corresponding frequencies in the wild type. We interpret this as being in agreement with a delay in the initiation and development of synapsis in the absence of a functional Ph1 allele, known to exist in wheat (Mikhailova et al 1998;Maestra et al 2002). Homologous centromeres in the 2Brc-2R pair were separated in 97 and 100% of Ph11 and Ph1À cells at leptotene, respectively, and their association level did not increase in the course of prophase I.…”

“…In leptotene, telomeres migrate to form a tight cluster and centromeres appear as compact structures. Chromatin undergoes a conformational change that results in chromosome elongation (Mikhailova et al 1998;Maestra et al 2002), which is apparent in centromere signals at the leptotene-zygotene transition. As synapsis progresses in midzygotene, the telomere bouquet disintegrates (Holm 1986).…”

“…As synapsis progresses in midzygotene, the telomere bouquet disintegrates (Holm 1986). Late zygotene and pachytene are postbouquet stages, with high levels of synapsis or with complete synapsis (Holm 1986;Mikhailova et al 1998), and differ by the degree of chromatin condensation.…”

“…During this stage, chromosomes undergo profound changes in the chromatin conformation and its spatial arrangement. Wheat chromosomes multiply their length fivefold in leptotene relative to premeiotic interphase (Mikhailova et al 1998). Because the size of the nucleus remains the same at the leptotene-zygotene transition or is even reduced (Maestra et al 2002), chromosomes span the entire nucleus.…”

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

“…The main regulator, Ph1, was discovered 50 years ago (Riley and Chapman, 1958;Sears and Okamoto, 1958) and only recently characterized at the molecular level (Griffiths et al, 2006;Al-Kaff et al, 2007). 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).…”