Structural and Transcriptional Polymorphism of Mitochondrial rrn18-trnfM Region in Triticeae Species

The tribe Triticeae provides a vast gene pool for improving the genetic variation of common wheat(Triticum aestivum L.).However,the inter-specific hybridization is commonly occurred under natural and artificial conditions,which increase the difficulty of the molecular polygenetic analysis of Triticeae based on data from nuclear genes.The mitochondrial DNA sequences provide a conserved system,which was confirmed to be useful for the study of the genomic inheritance and evolution.In mitochondrial genome,genes trfM and rrn18,encoding a mitochondrial initator tRNAMet and 18S rRNA,respectively,are tightly linked and the intergenic region exhibits polymorphism in cereal.To determine the variation of mitochondrial rrn18-trnfM region in tribe Triticeae,21 species including 14 diploid and 7 polyploid species were screened by means of PCR,cloned and sequenced.The results showed that two types of the amplified fragments,with 513,515,or 499 bp and 449 or 447 bp,respectively,were obtained in these species.The analysis of the sequences demonstrates that the main difference between the two types of the fragments was in the trnfM region,in which the longer fragments contained trnfM,while the smaller ones lost the part of the trnfM sequence.The facts supported that divergence history of these species is between the barley and wheat/rye lineage.However,the Thinopyrum intermedium presented two types of the fragment simultaneously,indicating the heritage of the paternal mtDNA during its polyploidization.RT-PCR of the Th.intermedium indicated that the smaller fragments were not expressed.It was,therefore,to deduce that the frequent recombination and the selective expression of mitochondrial genome are its unique evolutionary system,differing from the nucleus.Additionally,the relationship between the mitochondrial genome and the evolution among polyploids in tribe triticeae was also discussed.