Systematic analysis of bivalent configurations and chiasma localisation at metaphase I in different varieties and ploidy levels of wheats.

Algopishi, Uthman (2023). Systematic analysis of bivalent configurations and chiasma localisation at metaphase I in different varieties and ploidy levels of wheats. University of Birmingham. Ph.D.

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Abstract

Meiosis is a specialized cell division in eukaryotes which consists of one round of DNA replication followed by two rounds of cell division. During meiosis crossovers (COs) occur which are the genetic exchange that take place between paternal and maternal homologous chromosomes. COs are very important for genetic variation, and they ensure proper chromosome segregation during the first meiotic division (reductional division) by holding the homologous chromosomes (bivalents) by the chiasmata. Depending on where COs have occurred along the homologues different bivalent configurations can be observed at metaphase I. In cereals, specifically in wheat, COs seemed to preferentially occur around telomeric (distal) regions rather than close to the centromeric (proximal) regions. As a result, it restricts the genetic variation that could be available to plant breeders. Thus, analysing the different factors underlining the distal localisation of COs in wheat might provide new insights and tools to plant breeders to manipulate CO localisation and unrestrain the genetic variation available in natural varieties.

In this study, we have systematically analysed the different bivalent configurations observed at metaphase I in a selection of wheat varieties in hexaploid (6X-AABBDD), tetraploid (4X-AABB) and diploid wild relatives of wheat. We have classified the different bivalent configurations in rods and rings and defined the different localizations where the chiasmata are more likely present along the chromosome arm length: distal (d), interstitial 1 (i\(^{1}\)), interstitial 2 (i\(^{2}\)), interstitial 3 (i\(^{3}\)), and proximal (p) regions. This analysis has allowed us to compare the mean chiasma frequency per bivalent in different varieties and different ploidy level species including hexaploid wheat (AABBDD) varieties (Chinese spring, Cadenza, Paragon and Fielder), tetraploid wheat (AABB) varieties (Kronos, Cappelli and Langdon) and three diploid ancestral relatives of wheat (\(\textit{Trititicum monoccocum}\) (AA), \(\textit{Aegilops speltoides}\) (BB) and \(\textit{Aegilops tauschii}\) (DD)). Furthermore, Fluorescence in situ hybridisation (FISH) using 45S and 5S rDNA probes allowed us to differentiate specific chromosomes among these materials: 5A, 1B, 5B, 6B, 1D and 5D. Our results have shown important differences among varieties, among ploidy level and among different chromosomes in chiasma frequency and localisation. Furthermore, the spatio-temporal polarization of the meiotic progress has been analysed and no differences were observed between diploid, tetraploid and hexaploid wheat species. Nevertheless, time course analysis showed that meiosis was completed faster in diploid species (39h) than in tetraploids (41h) and hexaploids (43h).

Wheat \(\textit{ph1}\) mutants (\(\textit{ph1b}\) and \(\textit{ph1c}\)) and a Langdon substitution line 5D(5B) were analysed. The analysis showed a significant reduction on the mean chiasma frequency, a delay in the meiotic progression and even changes in chiasmata localization in specific chromosomes. Finally, chemical treatments including hydroxyurea (HU) and suberoylanilide hydroxamic acid (SAHA) were applied to hexaploid wheat cultivar Cadenza to manipulate chiasma localization in wheat. Our result show that, the mean chiasma frequency of Cadenza-HU and Cadenza-SAHA were significantly lower compared to the untreated material. Nevertheless, immunolocalization of class l COs using Hei10 antibodies showed a significant increase in the number of Hei10 foci in HU treated Cadenza. Furthermore, treatments with HU and SAHA showed new chiasmata localizations that did not exist in untreated Cadenza.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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