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This paper presents literature data regarding molecular structure and function of Ph1 locus. In this review, we focus on the current understanding of the Ph1 influence on subtelomeric and centromeric chromatin remodeling, required for meiotic homologous chromosome pairing in wheat. Wheat is an allopolyploid having two or three different sets of chromosomes (allotetraploid wheat 28 chromosomes, AABB genome and allohexaploid wheat 42 chromosomes, AABBDD genome). During the course of meiosis homologous chromosomes have to somehow recognize each other and do not pair with homoeologous chromosomes. Homologous and homoeologous chromosomes contain the same genes but the differences between them concern repeated sequences of DNA. Although the genomes of the tetraploid and hexaploid wheat are extremely complex 1A chromosome pairs with 1A, 1B with 1B and 1D with 1D, this rule refers to all seven pairs of chromosomes of each genome, indicating that the plant behaves like diploid and the inheritance is disomic. The process of diploidization of alloploid wheat, beside other factors, is controlled by Ph1 locus which is localized on a long arm of 5B chromosome. Detailed studies by means of comparative genomics and deletions mapping was performed. The results showed that Ph1 locus is a cluster of seven Cdk-like genes (CDKL2) and inside of this cluster a part of subtelomeric heterochromatin is inserted. This dominant gene arose as a consequence of a tandem duplication during the process of polyploidization. Locus on 5B chromosome has a suppressing effect on equivalent loci on 5A and 5D. Cdk-like genes revealed their close homology to mammalian Cdk2 and budding yeast Ime2. Cdk2 has an influence on DNA replication and also affects chromatin remodeling and recombination. At the onset of meiosis, before pairing of homologous chromosomes occurs, in the presence of Ph1 locus subtelomeric and centromeric heterochromatin under- goes remodeling, changing the binding properties of HP1 protein, and as a result heterochromatin elongate. Remodeling is synchronous and the pairing is complete in homologous chromosomes with the same size of heterochromatin regions. If slight differences in subtelomeric heterochromatin amount occur, remodeling is not synchronous. In this case, instead of complete pairing, pegging and zipping takes place and chromosomes pair in chain type manner. However, if large portion of subtelomeric heterochromatin shows differences, synchronous remodeling would not occur and most of the meiocytes would contain unpaired chromosomes. Elucidation of Ph1 locus molecular structure is of great importance in wheat breeding. In perspective the high homology of Ph1 locus to mammalian Cdk2 gene together with the fact that chemical factors turning off Cdk2 activity are known make possible switching off the Ph1 locus in crosses between wheat and its wild relatives. This allows for homoeological pairing and homoeological recombination. This could enable introducing new, beneficial genes from wild species to bread wheat.
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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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