Allohexaploidization Of Common Wheat And Its Application In Genetics And Breeding

Polyploidy has been found to be very common in plants. Polyploids can be formed viathe duplication of genomes, either of the same genomes (autopolyploid) or of divergedgenomes with homoeologous relationships (allopolyploid). Bread or common wheat(Triticum aestivum L., 2n=42) is a good example of allopolyploid made up of three diploidgenomes A, B and D. Bread wheat has undergone two polyploidizations during itsevolution. T. turgidum L. (2n=28, AABB) was formed in the first intercrossingbetween T. urartu and Aegilops speltoides. Then bread wheat was formed bysecond polyploidization after the intercrossing between T. turgidum (maternal) andAe. tauschii followed by chromosome doubling. By the mimic of common wheatevolution, many synthetic hexaploid wheats have been produced. The syntheticwheat is very useful for genetic improvement of modern wheat. Moreover,common wheat has many distinctive scientific characteristics which make it aninteresting model for the study of the organization and evolution of plant genomes.Allopolyploidization generates two "shocks". One is hybridity, by which twodiverged genomes are joined together to form one nucleus. The other is polyploidy,resulting in duplicated genomes. Allohexaploidization of wheat also include thetwo events, hybridization between T. turgidum and Ae. tauschii andautoduptication of the hybrid. However, some details about the events are stillunclear. In this study, we studied the allohexaploidization by analyzing the processof artificially synthetic hexaploid wheat with emphasis on its application in genetics and breeding. The results were as follows:1. Two experiments to investigate the crossability of Triticum turgidum with Aegilopstauschii are described. In the first, 372 wide hybridization combinations were done bycrossing 196 T. turgidum lines belonging to seven subspecies with 13 Ae. tauschiiaccessions. From the 66220 florets pollinated, 3713 seeds were obtained, with a meancrossability percentages of 5.61% ranged from 0 to 75%. A lot of hybrid seeds couldgerminate and produce plants. Out of 372 combinations, 272 (73.12%) showed a very lowcrossability lower than 5%, 87 (23.39%) showed the crossability of 5-30%, ten (2.69%)showed the crossability of 30-50%, three (0.81%) showed hitch crossability more than 50%,respectively. All the crossability percentages more than 30% were obtained from thecrossing of ssp. dicoccoides or dicoccon with Ae. tauschii. Among the seven T. turgidumsubspecies, there were significant differences in crossability. The ssp. dicoccoides anddicoccon showed the highest crossability, while polonicum showed the lowest. Among the13 Ae. tauschii accessions, AS2405 and AS2404 showed a crossability more than10%,while AS65, AS77 and AS82 showed a crossability less than 2%, respectively. Thegenetics of crossability was investigated using T. turgidum ssp. durum cultivar Langdonand the D-genome chromosome substitution lines of Langdon. The higher crossabilitiescompared with the control in lines 7D(7A) and 4D(4B) suggested that 7A and 4B intetraploid wheat cv. Langdon carried dominant crossability alleles inhibiting crossabilitywith Ae. tauschii. The relationships of present results with previously reported crossabilitygenes of wheat are discussed.2. Highly fertile F₁ hybrids were made between Triticum turgidum L. ssp. turgidum(2n=28, AABB) and Aegilops tauschii Coss. (2n=14, DD) without embryo rescue andhormone treatment. The F₁ plants had an average seedset of 25%. Approximately 96% ofthe F₂ seeds were able to germinate normally and about 67% of the F₂ plants werespontaneous amphidiploid (2n=42, AABBDD). Cytological analysis of malegametogenesis of the F₁ plants showed that meiotic restitution is responsible for the highfertility. It seems that a mitosis-like meiosis led to meiotic restitution at either of the twomeiotic divisions resulting in unreduced gametes. Test crosses of the T. t. turgidum-Ae. tauschii amphidiploid with Ae. variabilis and rye suggested that the mitosis-like meiosis iscontrolled by nuclear gene(s) that are functional in the derived lines. This discoveryimplicates a potential application of such genes in production of double haploids.3. The production of functional gametes in the triploid F₁ hybrids between Triticumturgidum L. (2n=28, AABB) and Aegilops tauschii Coss. (2n=14, DD) was a significantbiological step that led to the emergence of bread wheat. The meiotic restitution at either offirst-division restitution (FDR) or single equational division at the first division (SDM) isthe cytological mechanism responsible for the production of functional gametes. In thisstudy, highly fertile F₁ hybrids were made between T. turgidum L. ssp. durum cultivarLangdon and its disomic substitution 1D(1B) and Ae. tauschii without embryo rescue andhormone treatment. Observation of male gametogenesis and prediction of female gameteof the F₁ plants showed that the production of unreduced gametes was responsible for thehigh fertility. SDM was the major meiotic pathways for the production of unreducedgametes. Environments or genotypes, or both affected the production of unreducedgametes. Besides euhaploids, SDM produced a lot of aneuhaploid gametes. Therelationships between FDR and SDM and the implications of present results for the originstatus of bread wheat in cytology were discussed.4. It was suggested that the rapid changes of DNA sequence and gene expressionoccurred at the early stages of allopolyploid formation. In this study, we revealed themicrosatellite (SSR) differences between newly formed allopolyploids and their donorparents by using 21 primer sets specific for D genome of wheat. It was indicated that rapidchanges had occurred in the "shock" process of the allopolyploid formation betweentetraploid wheat and Aegilops tauschii. The changes of SSR flanking sequence resulted inappearance of novel bands or disappearance of parental bands. The disappearance of theparental bands showed much higher frequencies in comparison with that of appearance ofnovel bands. Disappearance of the parental bands was not random. The frequency ofdisappearance in tetraploid wheat was much higher than in Ae. tauschii, i. e. thedisappearance frequency in AABB genome was much higher than in D genome. Changesof SSR flanking sequence occurred at the early stage of F₁ hybrid or just after chromosomedoubling. From the above results, it can be inferred that SSR flanking sequence region was very active and was amenable to change in the process of polyploidization. This suggestedthat SSR flanking sequence probably had special biological function at the early stage ofpolyploidization. The rapid and directional changes at the early stage of polyploidizationmight contribute to the rapid evolution of the newly formed allopolyploid and allow thedivergent genomes to act in harmony.5. Microsatellites or SSRs as powerful genetic markers have widely been used ingenetics and evolutionary biology in common wheat. Because of the high polymorphism,newly synthesized hexaploid wheat has been used in the construction of geneticsegregation-population for SSR markers. However, data on the evolution of microsatellitesduring the polyploidization event of hexaploid wheat are limited.In this study, 66 pairs of primers specific to A/B genome SSR patterns among newlysynthesized hexaploid wheat, the donor tetraploid wheat and Ae. tauschii were compared.The results indicated that most SSR markers were conserved during the polyploidizationevents of newly synthetic hexaploid wheat, from T. turgidum and Ae. tauschii. Over 70%A/B genome specific SSR markers could amplify the SSR sequences from the D genomeof Ae. tauschii. Most amplified fragments from Ae. tauschii were detected in synthetichexaploid at corresponding positions with the same sizes and patterns as in its parental Ae.tauschii. This suggested that these SSR markers, specific for A/B genome in commonwheat, could amplify SSR products of D genome besides A/B gen0me in the newlysynthesized hexaploid wheat, that is, these SSR primers specific for A/B genome incommon wheat were nonspecific for the A/B genome in the synthetic hexaploid wheat. Inaddition, one amplified Ae. tauschii product was not detected in the newly synthetichexaploid wheat. An extra-amplified product was found in the newly synthetic hexaploidwheat. These results suggested that caution should be taken when using SSR marker togenotype newly synthetic hexaptoid wheat.6. New synthetic hexaptoid wheats were obtained from crosses of five T. turgidum L.lines with Ae. tauschii, which were formed by chromosome autoduplication throughunreduced gametes. Identifications were made by morphological and cytologicalobservation. Two hundred and thirty-one new synthetic wheat lines were coded by "Syn-SAU-N-X-Y". Besides the values in wheat improvement, they are desirable materials for study of allohexaploidization due to without the using of chemical materials, such asembryo rescue and colchicine treatment during the synthetic process.7. Six new nullisomic-tetrasomic lines (Syn-SAU N1AT1D, Syn-SAU N1BT1D, Syn-SAU N2BT2D, SYn-SAU N3AT3D, SYn-SAU N4BT4D and Syn-SAU N7AT7D) wereobtained from crosses of Langdon D-genome substitution lines with Ae. tauschii, whichwere formed by chromosome autoduplication through unreduced gametes. Identificationswere made by morphological, cytological and molecular analysis. They are different fromprevious Chinese Spring nullisomic-tetrasomic lines. The main differences were as follows:(1) for each of tetrasomics, there were four D chromosomes, two from Ae. tauschii and twofrom Chinese Spring; (2) they were obtained by manner of synthetic wheat, other geneticbackgrounds were from T. turgidum L. and Ae. tauschii except that their two Dchromosomes of the corresponding tetrasomics were from Chinese Spring.8. By colchicine treatment of the hybrid plants between Triticum turgidum andAegilops tauschii, a fertile wheat plant (SHW-L2) carrying 56 chromosomes wasartificially synthesized. At metaphaseâ… of pollen mother cells, the 56 chromosomes of newwheat SHW-L2 showed a pairing configuration of 2.82 univalents, 6.18 rod bivalents,19.39 ring bivalents, 0.5 trivalents and 0.14 quadrivalents. Cytological analysis suggestedthat SHW-L2 had additional 7 pairs of chromosomes from A and D genome besides the 42chromosomes as common wheat has. The special chromosome constitute of SHW-L2 maybe derived from the chromosome doubling by colchicine treatment for seedlings and thenspontaneous doubling for gametes. Present results were discussed with reference to specialvalues at both the theoretical and applied levels.9. The primary utilization of synthetic wheat SHW-L1 between T. turgidum ssp.turgidum line AS2255 and Ae. tauschii AS60 on bread wheat improvement indicated thepotential usefulness of synthetic wheat for yield characters, 1000-grain weight and spikeletnumber.