Abstract
Polyploidization is a fundamental process in plant evolution. One of the biggest challenges faced by a new polyploid is meiosis, particularly discriminating between multiple related chromosomes so that only homologous chromosomes synapse and recombine to ensure regular chromosome segregation and balanced gametes. Despite its large genome size, high DNA repetitive content and similarity between homoeologous chromosomes, hexaploid wheat completes meiosis in a shorter period than diploid species with a much smaller genome. Therefore, during wheat meiosis, mechanisms additional to the classical model based on DNA sequence homology, must facilitate more efficient homologous recognition. One such mechanism could involve exploitation of differences in chromosome structure between homologs and homoeologs at the onset of meiosis. In turn, these chromatin changes, can be expected to be linked to transcriptional gene activity. In this study, we present an extensive analysis of a large RNA-seq data derived from six different genotypes: wheat, wheat–rye hybrids and newly synthesized octoploid triticale, both in the presence and absence of the Ph1 locus. Plant material was collected at early prophase, at the transition leptotene-zygotene, when the telomere bouquet is forming and synapsis between homologs is beginning. The six genotypes exhibit different levels of synapsis and chromatin structure at this stage; therefore, recombination and consequently segregation, are also different. Unexpectedly, our study reveals that neither synapsis, whole genome duplication nor the absence of the Ph1 locus are associated with major changes in gene expression levels during early meiotic prophase. Overall wheat transcription at this meiotic stage is therefore highly resilient to such alterations, even in the presence of major chromatin structural changes. Further studies in wheat and other polyploid species will be required to reveal whether these observations are specific to wheat meiosis.
Highlights
Polyploidization, or whole genome duplication (WGD), has an important role in evolution and speciation, in plants
Ploidy level and changes in chromatin structure associated with Ph1 have any effect on global transcription during early meiotic prophase I, the transcriptome of wheat, wheat–rye hybrid and the corresponding triticale were analyzed by RNA-seq in the presence and absence of the Ph1 locus
We addressed three questions in the study: whether overall wheat transcription was affected by the level of synapsis; whether wheat transcription was reshaped upon genome duplication; and whether wheat transcription was altered in the absence of the Ph1 locus
Summary
Polyploidization, or whole genome duplication (WGD), has an important role in evolution and speciation, in plants. It is clear that all seed plants and angiosperms have experienced multiple rounds of WGD during their evolutionary history and are considered to possess a paleopolyploid ancestry (Renny-Byfield and Wendel, 2014). Polyploidy is traditionally classified into two separate types, autopolyploidy, arising from intraspecies genome duplication, and allopolyploidy, arising from interspecific hybridization. Several studies have reported major changes in transcription in somatic tissues following polyploidization (Renny-Byfield and Wendel, 2014 and references therein; Li et al, 2014; Edger et al, 2017; He et al, 2017; Sun et al, 2017; Lloyd et al, 2018). There have been very few previous reports on the effects of polyploidization on transcription during meiosis, a critical stage in the establishment of a polyploid (Braynen et al, 2017)
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