Molecular cytological identification of the gametocidal chromosome discovered by Endo and Tsunewaki (1975) in an alloplasmic wheat

In this issue of Cytologia, a series of manuscripts are dedicated to the late Professor Koichiro Tsunewaki (1930–2022), who has been the mentor of wheat researchers worldwide. We often recognize him the researcher who developed the series of alloplasmic wheat lines that provided great opportunities to study maternal lineage of evolution of Triticum and Aegilops species, to elucidate interactions of nuclei and cytoplasms, and to improve wheat by hybrid wheat breeding. Eventually, his research interest was broader. He found the gametocidal (Gc) genes, nuclear genes causing hybrid sterility, in the backcrossing generations of alloplasmic wheat. Following the first report of the Gc gene by Endo and Tsunewaki (1975), we now know at least five different Gc genes in wheat.

Here, we demonstrate spontaneously modified chromosomes of the first found Gc chromosome 3C^t from Aegilops triuncialis L. (2n=4x=28, genome constitution C^tC^tU^tU^t) in a wheat (Triticum aestivum L., 2n=6x=42, AABBDD) cultivar Chinese Spring. Endo (2007) reported a spontaneous structural rearrangement of chromosome 3C^t, namely del(3CL), that is a telosome with terminal deletion of the long arm of 3C^t (enlarged in the inset at the bottom right of the cover). The del(3CL) chromosome maintains the Gc function, which indicates that the Gc3-C^t1 gene locates in the pericentromeric region of chromosome arm 3C^tL. Currently, the disomic addition of del(3CL) is available with the accession number LPGKU2162 from NBRP-Wheat, Japan (https://shigen.nig.ac.jp/wheat/komugi/).

The cover figure shows a metaphase spread of a cell in the root tip meristem as probed with three repetitive element probes. Wheat has been a good material for cytogenetics because of its large chromosomes size and therefore different molecular-cytogenetic techniques have been adapted. Non-denaturing fluorescence in situ hybridization (ND-FISH) technology of localized repetitive elements is less labor and time intensive than conventional FISH and, more importantly, provide higher signal/noise ratio when adapted to wheat chromosomes. We applied three probes: Oligo-pSc119.2-1 (TAMRA-5′-CCGTT TTGTG GACTA TTACT CACCG CTTTG GGGTC CCATA GCTAT-3′) detecting mainly the B-genome chromosomes (pseudo-colored in blue), Oligo-pTa535-2 (Alexa Fluor 488-5′-GACGA GAACT CATCT GTTAC ATGGG CACTT CAATG TTTTT TAAAC TTATT TGAAC TCCA-3′) localized mainly on the D-genome chromosomes (red), and Oligo-CTT₆ (Alexa Fluor 647-5′-CTTCT TCTTC TTCTT CTT-3′) to distinguish C genome chromosomes (green) (Mirzaghaderi et al. 2014, Tang et al. 2014).

The Gc chromosome del(3CL) carries diagnostic foci of Oligo-pSc119.2-1 and Oligo-CTT₆ in the interstitial region of the truncated long arm (shown in the inset of the cover). The smaller size of the Gc chromosome del(3CL) than normal meta- or submeta-centric wheat chromosomes make us possible to flow-sort them from others and obtain shotgun sequences of the target chromosome (Murata et al. unpublished), which will allow us to identify the Gc3-C^t1 gene on del(3CL) through molecular genomic analyses.

References

• Endo, T. R. 2007. The gametocidal chromosome as a tool for chromosome manipulation in wheat. Chromosome Res. 15: 67–75.
• Endo, T. R. and Tsunewaki, K. 1975. Sterility of common wheat with Aegilops triuncialis cytoplasm. J. Hered. 66: 13–18.
• Mirzaghaderi, G., Houben, A. and Badaeva, E. D. 2014. Molecular-cytogenetic analysis of Aegilops triuncialis and identification of its chromosomes in the background of wheat. Mol. Cytogenet. 7: 91.
• Tang, Z., Yang, Z. and Fu, S. 2014. Oligonucleotides replacing the roles of repetitive sequences pAs1, pSc119.2, pTa-535, pTa71, CCS1, and pAWRC.1 for FISH analysis. J. Appl. Genet. 55: 313–318.