Differentiation between homoeologous chromosomes 1A of wheat and 1Am of Triticum monococcum and its recognition by the wheat Ph1 locus.

Abstract

In most allopolyploid plants, only homogenetic chromosome pairing occurs in meiosis, as a result of the recognition of genome differentiation by the genetic system regulating meiotic chromosome pairing. The nature of differentiation between chromosomes of closely related genomes is examined here by investigating recombination between wheat chromosome 1A and the closely related homoeologous chromosome lAm of Triticum monococcum. The recognition of the differentiation between these chromosomes by the Phi locus, which prevents heterogenetic chromosome pairing in wheat, is also investigated. Chromosomes lA and lAm are shown to be colinear, and it is concluded that they are differentiated "substructurally." This substructural differentiation is argued to be recognized by the Phi locus. In the absence of Phi, the distribution and frequencies of crossing over between the 1A and lAm homoeologues were similar to the distribution and frequencies of crossing over between IA homologues. The cytogenetic and evolutionary significance of these findings is discussed.Although polyploid plant species contain two or more related pairs of genomes, their meiosis is usually diploid-like, with almost exclusively homogenetic chromosome pairing. Bread wheat (Triticum aestivum L., 2n = 6x = 42) is a good example of this paradox. Bread wheat originated from hybridization of three diploid species, T. urartu Thum. (A genome) (1-3), an extinct or undiscovered population closely related to T. speltoides (Tausch) Gren. (B genome) (4,5), and T. tauschii (Coss.) Schmalh. (D genome) (6, 7). Although the chromosomes of these species have been subjected to significant differentiation, they are still capable of limited chiasmatic pairing at meta-

Effective chromosome pairing requires chromatin remodeling at the onset of meiosis

“…The chromatin remodeling enables chromosomes to become competent to pair and recombine. Moreover, the Ph1 locus in wheat is also able to block recombination from occurring between similar but distinct chromosome segments located within otherwise identical chromosomes (21,22). Thus, the present study suggests that the lack of recombination between these regions in the presence of Ph1 reflects a failure to remodel the regions at the onset of meiosis.…”

Fine mapping and characterization of Sr21, a temperature-sensitive diploid wheat resistance gene effective against the Puccinia graminis f. sp. tritici Ug99 race group

“…By contrast, McIntosh and Bennett (1979) used a cross between two hexaploid lines, one of which carried the 2A m L introgression segment with Sr21. Previous studies have shown that recombination between the A and A m genome chromosomes is greatly reduced in the presence of the Ph1b gene (Dubcovsky et al 1995;Zhang et al 2010). The absence of restriction in recombination, the large segregating population used in the construction of the high-density map, and the high level of polymorphism found in the PI 272557 × G3116 cross (cultivated x wild type diploid wheat) allowed us to define a relatively small candidate region for Sr21 of only 0.20 cM.…”

“…The gene was mapped 2.4 cM distal to the centromere on the long arm of chromosome 2A in hexaploid wheat by using monosomic analyses and telocentric mapping (McIntosh and Bennett 1979). However, this genetic distance needs to be considered with caution because chromosomes of T. monococcum and T. aestivum are known to recombine poorly in the presence of the Ph1b gene in hexaploid wheat (Dubcovsky et al 1995). The position of Sr21 relative to mapped molecular markers was not known.…”

“…The second objective was to generate a precise map of Sr21 to accelerate its deployment in breeding programs, assist in the reduction of the T. monococcum chromosome segment introgression in hexaploid wheat, and facilitate the cloning of Sr21. We hypothesized that the use of T. monococcum mapping populations will eliminate the distortions in the recombination rates known to occur between T. monococcum and T. aestivum chromosomes in hexaploid wheat (Dubcovsky et al 1995).…”

Cytokinesis: The Art of Partitioning

“…The ph1 mutation proves critical for breeding purposes, because it allows wheat chromosomes to recombine with homeologs from related species (Dubcovsky et al, 1995;Foote et al, 1997). Moore and coworkers generated a series of overlapping deletions of the Ph1 locus in wheat, and identified a syntenous region in rice to within 200 kb.…”