Understanding the system of a basic eukaryotic cellular mechanism like meiosis is of fundamental importance in plant biology. Moreover, it is also of great strategic interest in plant breeding since unzipping the mechanism of chromosome specificity/pairing during meiosis will allow its manipulation to introduce genetic variability from related species into a crop. The success of meiosis in a polyploid like wheat strongly depends on regular pairing of homologous (identical) chromosomes and recombination, processes mainly controlled by the Ph1 locus. This means that pairing and recombination of related chromosomes rarely occur in the presence of this locus, making difficult wheat breeding trough the incorporation of genetic variability from related species. In this work, we show that wild and cultivated barley chromosomes associate in the wheat background even in the presence of the Ph1 locus. We have developed double monosomic wheat lines carrying two chromosomes from two barley species for the same and different homoeology chromosome group, respectively. Genetic in situ hybridization revealed that homoeologous Hordeum chromosomes recognize each other and pair during early meiosis in wheat. However, crossing over does not occur at any time and they remained always as univalents during meiosis metaphase I. Our results suggest that the Ph1 locus does not prevent chromosome recognition and pairing but crossing over between homoeologous. The role of subtelomeres in chromosome recognition is also discussed.

Intensive breeding has led to a narrowing in the genetic base of our major crops. In wheat, access to the extensive gene pool residing in its many and varied relatives (some cultivated, others wild) is hampered by the block on recombination imposed by the Ph1 (Pairing homoeologous 1) gene. Here, the ph1b mutant has been exploited to induced allosyndesis between wheat chromosomes and those of both Hordeum vulgare (cultivated barley) and H. chilense (a wild barley). A number of single chromosome Hordeum sp. substitution and addition lines in wheat were crossed and backcrossed to the ph1b mutant to produce plants in which pairing between the wheat and the non-wheat chromosomes was not suppressed by the presence of Ph1. Genomic in situ hybridization was applied to almost 500 BC1F2 progeny as a screen for allosyndetic recombinants. Chromosome rearrangements were detected affecting H. chilense chromosomes 4Hch, 5Hch, 6Hch, and 7Hch and H. vulgare chromosomes 4Hv, 6Hv, and 7Hv. Two of these were clearly the product of a recombination event involving chromosome 4Hch and a wheat chromosome.

Bread wheat is an allohexaploid that behaves as a diploid during meiosis, the cell division process to produce the gametes occurring in organisms with sexual reproduction. Knowledge of the mechanisms implicated in meiosis can contribute to facilitating the transfer of desirable traits from related species into a crop like wheat in the framework of breeding. It is particularly interesting to shed light on the mechanisms controlling correct pairing between homologous (equivalent) chromosomes and recombination, even more in polyploid species. The Ph1 (Pairing homoeologous 1) locus is implicated in recombination. In this work, we aimed to study whether homoeologous (equivalent chromosomes from different genomes) Hordeum chilense (wild barley) and H. vulgare (cultivated barley) chromosomes can associate and recombine during meiosis in the wheat background in the absence of the Ph1 locus. For this, we have developed H. chilense and H. vulgare double monosomic addition lines for the same and for different homoeology group in wheat in the ph1b mutant background. Using genomic in situ hybridization, we visualized the two (wild and cultivated) barley chromosomes during meiosis and we studied the processes of recognition, association, and recombination between homoeologous chromosomes in the absence of the Ph1 locus. Our results showed that the Ph1 locus does not prevent homoeologous chromosome pairing but it can regulate recombination.