Persistence of Genetic Effects of Ae. caudata Plasmon on Wheat Phenotypes
Kihara crossed in 1949 Ae. caudata as female to a common wheat, Tve, and the F1 hybrid and its progenies were successively backcrossed with the pollen of Tve until SB16 (Kihara 1959; unpubl.). I continued the backcrosses with the same pollen parent, up to the SB60 generation in 2013. Meiotic pairing was normal, forming 21 bivalents, in an SB56 plant.
Selfed and backcrossed seed fertilities of the (caudata)-Tve were observed in the entire backcross program. The backcrossed seed fertility (%) was highly variable, with the mean and S. E. of 68.10 ± 16.43. The linear regression to the backcrossed generation calculated using the records of the SB3 to SB60 generations to grasp the tendency of improvement or depression of the backcrossed seed fertility, Y, with progression of the backcross generations, X, turned out to be Y = 0.181X + 62.3 (%).
The regression coefficient of 0.181 was non-significant at the 5 % level of probability, meaning the female fertility of (caudata)-Tve did not change consistently during 60 years of backcrossing. On the contrary, the selfed seed fertility was completely zero in most backcrossed generations. These results indicated that the caudata plasmon induced complete male sterility in Tve wheat, which did not show any sign of recovery by repeated backcrossing with the wheat pollen for 60 generations, proving persistence of the plasmon effect on male sterility induction.
Another prominent effect of the caudata plasmon on the wheat phenotype was production of germless grains, which was first noticed in the SB15 generation of (caudata)-Tve. Since then, occurrence of germless grains in (caudata)-Tve was examined in each backcross generation. Its frequency in every ten SB generations was 15.3 % (SB15-SB20), 14.7 % (-SB30), 28.5 % (-SB40), 14.2 % (-SB50) and 5.2 %
(-SB60). Overall frequency of the germless grains was 11.8 % in (caudata)-Tve, as compared to 0.022 % in normal Tve, indicating more than 500 times increase in its frequency by the caudata plasmon. Genetic effect of the caudata plasmon to produce germless grains in Tve continued, at least, for 46 generations, from SB15 to SB60. These results demonstrated that the genetic effects of the caudata plasmon on male sterility induction and germless grain production to 'Tve' persistently expressed during 60 generations of backcrossing with the Tve pollen.
Reconstitution of Ae. caudata from Its Genome and Plasmon Separated for Half a Century and Paternal mtDNA Transmission in Wheat
Ae. caudata (genome, CC) was reconstructed from the genome of its native strain and the plasmon originated from it and coexisted with the genomes of Tve wheat (AABBDD) for 50 generations of repeated backcrosses (details of the procedure and the results will be published elsewhere). The reconstructed caudata plants resembled those of the caudata accession that provided the plasmon to (caudata)Tve SB50 in general morphology and showed normal vigor and fertility. We analyzed 8 chloroplast and 5 mitochondrial simple sequence repeat (abbrev. SSR) loci out of 45 loci reported in wheat (Ishii et al. 2001, 2006) (Yotsumoto et al. unpubl.). The tentative results indicated that the reconstructed caudata had the same band patterns in most SSR loci, as those of the original caudata accession and (caudata)Tve SB50. These results suggested that the caudata plasmon remained unchanged during coexistence with the wheat genomes for 50 generations.
Following the report of Laser et al. (1997) with triticale, Tsukamoto et al. (2000), Hattori et al. (2002), Kitagawa et al. (2002) and Kawaura et al. (2011) obtained evidence on the paternal transmission of mtDNA in alloplasmic wheats, which included coding regions of, at least, ten mitochondrial genes. Laser et al. (1997) and Kawaura et al. (2011) showed that the paternally transmitted mtDNAs were rarely transcribed in the heteroplasmic plants. This is compatible with the persistence of phenotypic effects of the caudata plasmon during the repeated backcrosses with wheat pollen. The constancy of mitochondrial SSR loci in the alloplasmon during the repeated backcrosses proved here, however, is hardly compatible with the paternal transmission of mtDNAs, because the majority of mtDNA molecules of an alloplasmic line at an advanced backcross generation is assumed to be the paternal (wheat) molecules, but this is not the case. Further studies are needed to clarify the cause of this discrepancy.
Acknowledgments I wish to thank Naoki Mori, Shigeo Takumi and Tatsuya Yotsumoto of the Kobe University, for their kind help and support for the present work.