Wheat, Artificial And Natural Polyploid Genome Variation

Wide hybridization followed by genome doubling is one of the prominent models of speciation in higher plants. Nevertheless, little is known regarding early events of coordination or changes that are required to ensure compatibility for the co-existence of two or more divergent genomes derived from different species in a single nucleus. Recent studies demonstrated that allopolyploid formation is often accompanied by genomic changes that cannot be readily explained by Mendelian genetic principles. Common or bread wheat (Triticum aestivum L.) is one of the most important food crop worldwide, and also a classic example of speciation via allopolyploidy. Although numerous genetic studies have been conducted on common wheat, it is not well understood on nature and extent of genomic changes, nor the identity or characteristics of the genomic regions undergoing the changes.In the present dissertation, we undertook a comparative study on wheat genome evolution by using the standard laboratory genotype (Chinese Spring) of natural common wheat and a synthetic hexaploid allopolyploid wheat (symbol, XX340) between tetraploid wild emmer wheat, T. dicoccoides var. dicoccoides, accession TTD09 and a diploid goat grass, Aegilops tauschii Coss var. typical, as experimental materials. We employed various DNA fingerprinting methods, including AFLP, MSAP, ISSR and Southern blotting, to evaluate genomic additivity in natural and synthetic allopolyploid wheats vs. their respective parents at the genome-wide level. Results indicate that genetic and epigenetic changes abound in both synthetic and natural allopolyploid wheat. Some of the changes are apparently non-random, as being reflected by identical changing patterns among randomly selected individuals and concordance between synthetic and natural wheats. In contrast, some of the changes are random, as being exhibited by polymorphism among the individuals in synthetic wheat, as well as between synthetic and natural wheat.Of the types of genomic changes, loss of fragment is the most frequent, with appearance of new fragments being occasionally observed. Southern blot analysis showed that the main underlying causes for loss of fragments are alterations in sequence or methylation status of the restriction sites, and no evidence for bonafide sequence elimination. Of the methylation alterations,most showed increase in methylation level. Sequence analysis indicated that among clones that manifested significant homology to known-function genes are predominantly transposons or retrotransposons, as well as genes encoding for metabolic enzymes. This later result suggest that during the early stages of allopolyploid formation, there might be changes in gene expression and innovations in phenotypic traits, and the later might be subjective to natural selection, which in turn, may have facilitated stabilization and establishment of nascent allopolyploid as new species.