The classification of tetraploid wheat by phylogenetic and cytogenetic analyses

Tetraploid wheat (Triticum turgidum L.) is an important species within the genus Triticum and harbors many desirable agronomic traits. The classification, origin, and evolution of tetraploid wheat remain confused and controversial, resulting in useless germplasm resources. Two classification systems for tetraploid wheat are widely used: 1) tetraploid wheat comprises two species; 2) all forms of tetraploid wheat are classified as one species. The present study aimed to reassess the classification of tetraploid wheat using phylogenetic analysis of nuclear rDNA internal transcribed spacer region (ITS) sequence data, fluorescence in situ hybridization (FISH) karyotyping, and observation of meiotic pairing behavior in F1 hybrids. Network analysis of ITS sequences indicates that tetraploid wheat was not closely related to other Triticeae species with the exception of Aegilops speltoides and Ae. sharonensis. Phylogenetic analysis of ITS sequences and FISH show that Triticum turgidum and T. timopheevii clustered on distinct branches, and meiotic pairing in F1 hybrids of these species showed a high frequency of univalents. Meiotic behavior of F1 hybrids among forms of T. turgidum revealed a low number of univalents (means < 2) except for T. turgidum ssp. dicoccoides. The significant variation on chromosomes 1A, 2A, 5A, 1B, 2B, 3B, and 6B in the FISH hybridization patterns were observed between T. turgidum ssp. dicoccoides and other T. turgidum accessions. Furthermore, the results of ITS phylogenetic analyses correspond closely with observations of meiotic behavior and FISH karyotyping. The present results indicate that T. turgidum and T. timopheevii are two distantly related species of different origins. Triticum turgidum ssp. dicoccoides should be maintained as a subspecies of T. turgidum whereas other forms of T. turgidum should be reclassified as varieties.