| In plant kingdom, approximately 70% of flowering plants have experienced polyploidization. Polyploidization has a meaningful role in plant evolutionary history, and is a driving force for evolution and variation in plants, and is a pathway for plants to maintain genetic diversification and genomic stability. Therefore, to explore the effects of polyploidy on plant sexual reproduction and development can undoubtedly contribute to our understandings of tremendous events of polyploidization in plants. In the present study, the model plant Arabidopsis was used for its fast growth and development to artificially synthesize polyploid pants including autotetraplod A.thaliana and allotetraploid A.suecica (by crossing between A.thaliana and A.arenosa). The meiotic progression of newly synthesized allotetraploid A.suecica was analysed. In combination with 3 meiosis-related genes spo11-1, dmc1 and asy1, the effects of polyploidy on meiosis and transmission of sexually reproductive gametes were also exploited. Besides, aided with a pair of cis-linked transgenic fluorescent marks G-R, the frequency of meiotic recombination was investigated as well. The results were presented as follows:1. In diploid A.thaliana, 3 mutant genes were investigated during meiosis. Spo11-1 was involved in initiation of double-strand breaks (DSBs), which further affected the meiotic recombination. Dmc1 played a key role in repairing DSBs by contributing to fixing the protruded 3'single DNA strands. Asy1 was a key component controlling synapsis and pairing of homologous chromatins (chromosomes). The results showed that, although the 3 genes examined here have distinct functions during meiosis respectively, the cytological observations of meiotic chromosomes were very similar. Mutation of the 3 genes can result in the failure of synapsis and pairing of homologous chromosomes, and chromosomes displayed as a unique formation of univalents in spo11-1 and dmc1, whereas in asy1 the rare formation of bivalents was infrequently observed at 1.26 per cell. Besides, the alignment of amino acid sequences of spo11-1, dmc1 and asy1 from other systems such as human, yeast, wheat and rice showed that 3 genes are highly conserved in sequence similarity which indicated an important role performed by these 3 genes in plant sexual life.2. After polyploidization, the autotetraploid Arabidopsis was obtained in the mutants dmc1 and asy1. The effects of polyploidization on meiotic progression in tetraploid dmc1 and asy1 were observed. The results demonstrated that the defects caused by mutation of dmc1 was more severe than that of asy1, and no improved meiosis was observed in tetraploid dmc1, but an increasing number of bivalents, even quadrivalents were frequently observed. These results indicated that polyploidy of plants can buffer the defectiveness brought about by specific mutations.3. The transmission of aneuploids was investigated in autotetraploid asy1, and the results showed that chromosome number in the produced gametes determines the pathway of aneuploid transmission. Transmission of hypoploids was mainly dependent on male gametes, whereas hyperploids have a dual pathway of transmission, through male or female. And chromosome number and relative ratio balance of genomic contents between male and female determine on the fate of development after fertilization.4. The statistical model on estimation of frequency of meiotic recombination based on a pair of cis-linked markers GFP and RFP was established on condition that the linked markers were closely located on a chromosme (5 Mb). The model indicated that the formation of multivalents in autotetraploid contributed little to the frequency change of meiotic recombination, which was proved further with our solid data.5. In diploid Arabidopsis, the recombination frequency of the cis-linked markers was estimated on chromosome 3 and chromosome 5 respectively. The results showed that on chromosome 3, the averaged frequency of homologous recombination was 16.8%, and the frequency in male was 20.2% and 7.4% in female. However, on chromosome 5, the averaged frequency of homologous recombination was 14.9%, and the frequency in male was 18.1% and 14.0% in female. These data indicated that recombination frequency was affected by a series of factors including chromosome of genes, locus of gene on chromosome. Besides, sex-related difference of recombination was generally observed with a higher frequency in male than female.6. After polyploidization including autopolyploidization and allopolyploidization, the recombination frequency of the same cis-linked markers was investigated in autotetraploid and allotetraploid Arabidopsis. The data indicated that in contrast with diploid Arabidopsis, autotetraploid has averaged 23.2% of recombination frequency, 28.0% in male and 15.0% in female. Comparatively, in allotetraploid, the averaged frequency of recombination was 28.2%, and 29.8% in male, and 13.0% in female. These results indicate that polyploidization contributes much to the increased frequency of meiotic recombination, and sex-related difference of recombination existed as well in polyploid plants.7. Aided with fluorescence in situ hybridization (FISH), the newly formed allotetraploid Arabidopsis was analyzed during meiosis. The observations showed that normal meiotic process was guaranteed with possible presence of wheat Ph1-like gene albeit the genomic shock of dirrerent chromosome resources. The synapsis and pairing of homologous chromosomes progress without interference of homoeologous chromosomes, which provided the opportunity for recombination and a basis for production of functional gametes after meiosis.8. Cytogenetic analysis demonstrated that formation of multivalents in polyploid plants can not account for the increased frequency of recombination, thus the further work was needed for unraveling the mechanism of increased recombination frequency brought about by polyploidy associated effects.
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