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Title: Reassessment of the evolution of wheat chromosomes 4A, 5A, and 7B

item DVORAK, JAN - University Of California, Davis
item WANG, LE - University Of California, Davis
item ZHU, TINGTING - University Of California, Davis
item JORGENSEN, CHAD - University Of California, Davis
item LUO, MING-CHENG - University Of California
item DEAL, KARIN - University Of California, Davis
item Gu, Yong
item GILL, BIKRAM - Kansas State University
item DISTELFELD, ASSAF - Tel Aviv University
item DEVOS, KATRIEN - University Of Georgia
item QI, PENG - University Of Georgia
item MCGUIRE, PATRICK - University Of California, Davis

Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2018
Publication Date: 8/23/2018
Citation: Dvorak, J., Wang, L., Zhu, T., Jorgensen, C.M., Luo, M., Deal, K.R., Gu, Y.Q., Gill, B.S., Distelfeld, A., Devos, K.M., Qi, P., Mcguire, P.E. 2018. Reassessment of the evolution of wheat chromosomes 4A, 5A, and 7B. Theoretical and Applied Genetics. https://doi: 10.1007/s00122-018-3165-8.

Interpretive Summary: The genome of hexaploid bread wheat (Triticum aestivum) consists of three subgenomes designated as A, B, and D, each of which contributes 14 chromosomes to the total of 42. The A subgenome was contributed by wild einkorn wheat Triticum urartu, the B subgenome was contributed by an unknown species closely related to Aegilops speltoides, and the D subgenome was contributed by wild relative Aegilops tauschii (Taush’s goatgrass). In previous research, translocations of large chromosome segments among different subgenomes have been reported in hexaploid wheat. The mechanism underlying these chromosomal rearrangement events is not clear. In this study, the genome of sequenced tetraploid wild emmer wheat (AABB) was aligned to the genome of Aegilops tauschii (DD) to analyze and compare sequences at the proposed translocation breakpoints to determine if the events occurred in the diploid ancestors or after the formation of modern polyploid bread or pasta wheats. Our study suggests that translocation events observed in hexaploid wheat are complex and occurred in a single evolutionary event via multiple and different recombinational processes.

Technical Abstract: Past research suggested that wheat chromosome 4A was subjected to a reciprocal translocation T(4AL;5AL)1 that occurred in the diploid progenitor of the wheat A subgenome and to three major rearrangements that occurred in polyploid wheat: a pericentric inversion, Inv(4AS;4AL)1, a paracentric inversion, Inv(4AL;4AL)1, and a reciprocal translocation, T(4AL;7BS)1. We aligned the pseudomolecules of tetraploid wild emmer wheat (Triticum turgidum ssp. dicoccoides, subgenomes AABB) and diploid Aegilops tauschii (genomes DD) to confirm these rearrangements and to analyze their breakpoints. A reciprocal exchange of distal segments of arms 4AS and 4AL during evolution of the present-day chromosome 4A, indicative of pericentric inversion Inv(4AS;4AL)1, was confirmed. The breakpoints of Inv(4AS;4AL)1 were identified and validated using an optical Bionano genome map. Sequence alignments revealed that both breakpoints contained satellite DNA. Also located and analyzed were the breakpoints of reciprocal translocations T(4AL;5AL)1 and T(4AL;7BS)1. However, breakpoints unique to the putative paracentric inversion Inv(4AL;4AL)1 were not found, confirming our previous failure to find the breakpoints on an ultra-dense genetic map of wild emmer wheat. We therefore reject the hypothesis of evolution of chromosome 4A in tetraploid wheat by sequential inversions and translocations and propose instead that Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 originated simultaneously, resulting in the evolution of the rearranged karyotype of wild emmer wheat in a single step.