Genome evolution in allopolyploid triticale

Polyploidy, especially allopolyploidy, contributes significantly to speciation in plants. Recently, many studies have suggested that the evolution of allopolyploids is a process of cytological and genetic diploidization, but the genome evolutionary behavior of allopolyploids remains poorly understood. The present studies were intended to provide novel insights into the evolution of allopolyploids using the recently-synthesized species, triticale (X Triticosecale Wittmack), which is an inter-generic polyploid hybrid between wheat (Triticum spp.) and rye (Secale cereale L.).; The overall genome variation in triticale was investigated using AFLP and RFLP analyses. The AFLP analyses were implemented with both Eco RI/MseI (E/M) and PstI/ MseI (P/M) primer combinations, which, because of their relative differences in sensitivity to cytosine methylation, primarily amplify repetitive and low-copy sequences, respectively. The results revealed or suggested that (i) a great degree of genome variation occurred in triticale, and most of the variation was caused by sequence elimination because the frequency of parental band loss was much higher than the frequency of gaining novel bands; (ii) genome variation in triticale was highly directed between parents and was reproducible among sister lines; (iii) genome variation in the rye parental genome was much faster and greater than that in the wheat genome, and variation in hexaploid triticales was much higher than that in octoploid triticales; (iv) genome variation was more frequent in repetitive sequences than that in low-copy sequences, which was also higher than that in expressed sequences; (v) a considerable degree of the variation occurred immediately after inter-generic hybridization before chromosome doubling, especially for the rye parental genome (about 74.5% and 68.4% for E/M and P/M primers, respectively); (vi) genome variation after chromosome doubling was a slow, continuous modification process, but variation in about the first five generations was significantly higher than that in later generations; (vii) concerted evolution probably occurs in the genomic sequences of triticale; and (viii) cytoplasm and the relationship between parents of an allopolyploid are the key factors determining the direction, amount, timing, and rate of sequence variation, especially sequence elimination, in allopolyploids.; Those observations provided significant novel insights into the evolution of allopolyploid species, thus facilities the activities of crop improvement.