Studies on the origin and evolution of tetraploid wheats based on the internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA
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In this study, the internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA in the tetraploid wheats, Triticum turgidum (AABB) and Triticum timopheevii (AAGG), their possible diploid donors, i.e., Triticum monococcum (AA), Triticum urartu (AA), and five species in Aegilops sect. Sitopsis (SS genome), and a related species Aegilops tauschii were cloned and sequenced. ITS1 and ITS2 regions of 24 clones from the above species were compared. Phylogenetic analysis demonstrated that Aegilops speltoides was distinct from other species in Aegilops sect. Sitopsis and was the most-likely donor of the B and G genomes to tetraploid wheats. Two types of ITS repeats were cloned from Triticum turgidum ssp. dicoccoides, one markedly similar to that from T. monococcum ssp. boeoticum (AA), and the other to that from Ae. speltoides (SS). The former might have resulted from a recent integression event. The results also indicated that T. turgidum and T. timopheevii might have simultaneously originated from a common ancestral tetraploid species or be derived from two hybridization events but within a very short interval time. ITS paralogues in tetraploid wheats have not been uniformly homogenized by concerted evolution, and high heterogeneity has been found among repeats within individuals of tetraploid wheats. In some tetraploid wheats, the observed heterogeneity originated from the same genome (B or G). Three kinds of ITS repeats from the G genome of an individual of T. timopheevii ssp. araraticum were more divergent than that from inter-specific taxa. This study also demonstrated that hybridization and polyploidization might accelerate the evolution rate of ITS repeats in tetraploid wheats.
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