Genomic Evolution In Allopolyploid Plants Of Aegilops

Aegilops, as an important genus in Triticeae of Poaceae, is composed of 24 species. In this genus, 12 species are diploids and others are allotetraploids or allotexaploids formed by hybridization and polyploidization of different diploids. Aegilops is a model system for studying genomic evolution in polyploids.Firstly, Genomic evolutionary changes were characterized by using 31 ISSR (inter-simple sequence repeat) primers among 23 species in the genus Aegilops. In total of 288 ISSR amplified fragments, only 8 fragments were shared by all 23 species. The results indicated that allopolyploid species had changed greatly during evolutionary history compared with their ancestral diploid species. Relatively, genome U was the dominant genome and showed little alterations in U-containing allopolyploids, while others changed largely since the allopolyploids formed. It illustrated that genome U showed strong assimilation effect coexisted with other genomes, and the other genomes showed different evolutionary directions and changes. In addition, D genome was also modified to varying degrees. Some conclusions could be gained based on these results:1. The hypothesis "dominant genomes" was inferred from the evolutionarypatterns of genome U and genome D, and it described a dynamic and ongoing genomic evolutionary process of polyploids that could be divided into three periods i.e. (i) interaction between different genomes; (ii) the stable period of dominant genome; (iii) the differential period of the dominant genome.2. Three tetraploid species carrying UM genome were clustered into different groups and showed different evolutionary directions and changes.3. Some maternal genomes also displayed more alterations in allopolyploids. Secondly, 108 Au retrotransposable element fragments (a novel kind of plantSINE) were amplified and sequenced in 10 species of Aegilops, which were divided into three different polyploid complexes (three groups). The sequences of these members were heterogonous in di-, tetra-, and hexaploids. Similarity analysis revealed that the sequences of Au element members changed to varied degrees along with increasing of ploidy levels in polyploid complexes of Aegilops. Furthermore, the Au element dendrograms obtained from phylogenetic analysis were very complex and could be clustered into many families in three polyploid complexes of Aegilops respectively. A general evolutionary mode of Au elements was proposed anddiscussed:1. The high possibility was that Au element arose early in a distant ancestral genome of the genus Aegilops in the long evolutionary history. Subsequent to a speciation event that gave rises to daughter taxa containing descendants of Au element.2. Au element inserted into many different chromosomal loci of diploid genomesporadically. Subsequent to this process, different levels of divergence would occur inthe nucleotide sequence of Au element under the different selective stress of new chromosomal and genomic environment. Thus, many members of Au element were generated, and they had reached their current wide dispersed distribution by many times of insertion under the transposition-selection balance or mutation-selection equilibrium.3. During the great majority of this large timescale Au element members' variants with enhanced transposition rates were favored and the relatively large number of retrotranspositions resulted in sequence evolution. Polyploids have their capacity to tolerate more insertional events, and provide "playground" for retrotranspositions of Au element. It seemed that Au element members would probably changed largely under above mechanisms in polyploids, but in fact, Au element members' diversity would probably be decreased and many higher heterogeneous members would be eliminated or homogenized under some other evolutionary mechanisms of repeat sequences such as transposition-selection balance or mutation-selection equilibrium, sequence-elimination and concerted evolution. It seemed there should be complex evolutionary relationships between Au element and polyploids evolution, and these relationships were different in three polyploid complexes of Aegilops. The evolutionary variants between retrotransposons and their host species with varied ploidy levels would probably be the plausible reason.