Os10g0405500

The rice PAIR3 plays a crucial role in homologous chromosome pairing and synapsis in meiosis, which is important for eukaryotic sexual reproduction.

Annotated Information

Figure 1. Chromosome behavior in Wild type.(from reference) .

Figure 2. Chromosome behavior in Pair3 mutants (from reference) .

Function

Meiosis is essential for eukaryotic sexual reproduction and important for genetic diversity among individuals. PAIR3(HOMOLOGOUS PAIRING ABERRATION IN RICE MEIOSIS 3) localize to the chromosome core during prophase I and associated with both unsynapsed axial elements and synapsed lateral elements .PAIR3 plays a crucial role in homologous chromosome pairing and synapsis in meiosis.PAIR3 functions in male meiosis. The cells of the parietal layer including the tapetum appeared to be normal, indicating that PAIR3 did not function in anther wall development and that the male sterile mutant phenotype was not due to defects in anther wall development. This is different from the situation frequently reported in the literature inwhich male sterility is often associated with defects in tapetum development. PAIR3 may not function in tapetal degeneration. If it is mutation during meiosis in rice, rice could not form bivalents and result in sterility in both male and female gametes. PAIR3 was essential for bouquet formation, homologous pairing and normal recombination, and synaptonemal complex assembly.It is required for normal development of the resulting microspores and embryo sac formation.

Wild Type VS. Mutant

The researh by Kejian Wang et al. described the difference of chromosome behavior between PAIR3 mutants and wild type in 2011:

• In the wild type, chromosomes began to condense and form thin lines at leptotene(Figure 1A). In zygotene, pairing and synapsis occurred between homologous chromosomes(Figure 1B). The synapsis was completed, and 12 fully synapsed chromosomes were clearly visible at pachytene(Figure 1C). With further chromosome condensation occurring in diplotene and diakinesis, SCs were gradually disassembled and the paired homologous chromosomes were held together by chiasmata(Figure 1D). At metaphase I, all bivalents were aligned along the equatorial plate(Figure 1E). Subsequently, homologous chromosomes were separated from each other and moved to the opposite poles of the cell at anaphase I(Figure 1F). The second meiotic division was very similar to mitosis and finally produced tetrad spores(Figure 1, G–I).
• In the PAIR3 mutant, A single nucleotide deletion at position 1044 in the coding region was found in the pair3-1 mutant, which resulted in a frame shift mutation. We also obtained one other mutant showing the same defects as pair3-1 from Nipponbare. Immunostaining with an antibody against PAIR3 failed to find any PAIR3 protein in the mutant. Thus, sequencing analysis was carried out and revealed that the mutant also contains a single nucleotide deletion at position 1538 in the coding region, and it was named pair3-2. Both mutations resulted in the loss of predicted coiled-coil motifs. There was no obvious difference in pair3-1 during leptotene, compared with the wild type(Figure 2A ).However, pairing between homologous chromosomes was not observed from zygotene to pachytene (Figure 2, B and C ). During diakinesis and metaphase I, many randomly distributed univalents were found (Figure 2D ). In telophase I and prophase II, an uneven number of chromosomes could be seen in the two related cells (Figure 2, F and G ). After the second division, tetrads and polyads with different numbers of chromosomes were formed (Figure 2, H–L ).In addition, multiple micronuclei were frequently observed. These results implied that correct chromosome disjunction and cytokinesis were also affected by the mutation in the PAIR3 gene. Such abnormality was also found in the pair3-2 mutants but not in the previously reported pair3 mutants.We suspected that the mutants identified in the present study contain alleles of pair3 that result in more severe defects.

Expression

RT-PCR analyses by Wenya Yuan et al. shows that PAIR3 was expressed at a very low level in vegetative organs(root, stem, shoots and leaf). It was preferentially expressed in the meiocytes, especially during the male and female meiosis stages.The PAIR3 gene was preferentially expressed in the meiocytes, especially during the male and female meiosis stages. The expression pattern of PAIR3 together with the phenotype observed in the loss-of-function mutants indicated that the PAIR3 protein works in the meiocytes and plays crucial roles in homologous pairing and synapsis. Meanwhile data report that expression of PAIR1, PAIR2, MEL1, OsDMC1 and OsRad21-4 genes are likely to be independent of PAIR3function in controlling chromosome pairing during meiosis. To recover the internal sequence of the transcripts, RT-PCR was performed by using DNase I and SuperScript II (Invitrogen) according to the manufacturer’s instructions. Primers (F78RT1 to F78RT4) were designed according to the sequences within the boundaries defined by the exon ends for PCR amplification of the reverse-transcription products.The primer sequences of InDel markers were as follows:

• P1 (5’-TACGTGTCCTTGTGCCTGA-3’and 5’-ACAAATGAACTGAGAACGTG-3’)
• P2 (5’-CTTCCGATTAATTTCCTTTC-3’and 5’-GGGTCTAGCTATTTGCTCT-3’)
• P3 (5’-TTCTACGAGCTTCCTACG-3’and 5’-CTTAGACTCATTATAGTAC-3’)
• P4 (5’-CATATAGTGTCATATGCACC-3’and 5’-GTCGTGTGTTTGCCAACAAT-3’)
• P5 (5’-CACTTCAGTGCTGGGTGTGC-3’and 5’-TCAAAGGGCAAGTTAACGAC-3’)
• P6 (5’-GCGCATCCATGCATATCCAA-3’and 5’-GACAAGGTGTTGCCCAAGAA-3’)
• P7 (5’-GTCAATGTGAATGGGAGACC-3’and 5’-AGCAGCTCAAGGCAGCGA-3’)
• P8 (5’-TTTCAGTGGCTCTGTTCGG-3’and 5’-CAAAAGGAAATGAGTGAGGCT-3’)

Evolution

As PAIR3 shows no strong similarity with known proteins in Arabidopsis and other model organisms, it is likely to be a newly evolved meiotic protein in rice. The only motif found in PAIR3 is coiled-coil domain.It shows that proteins with poor homology at the amino acid sequence level, but with similar structure, perform similar functions in other model organisms. The PAIR3 protein contains putative coiled-coil motifs in its Cterminal domain. The PAIR3 protein contains putative coiled-coil motifs in its Cterminal domain. Similarly, the SYCP3/COR1 protein also has a predicted coiled-coil motif in the C-terminal half. In addition, both PAIR3 and SYCP3/COR1 assemble and disassemble in the same time as the SCs and label the AE/LE in a continuous manner. Mutation of either gene results in the defects in pairing, synapsis, and chiasma formation. However, SYCP3 is essential for the formation of LEs, while PAIR3 is not. Several lines of evidence suggest that the AE/LE seen by staining is only a small component of the axial structure by both morphological and functional criteria. In addition, it is likely that axial components may well occur in multiple (partially overlapping) layers. Therefore, it is also possible that PAIR3 is an AE/LE component and localizes on one layer of the AE/LE.

Knowledge Extension

How haploid gametes are generated from diploid somatic cells is a interesting question in cytogenetics. Two important procedures are involed in it: Mitosis and Meiosis.

• The term mitosis was first introduced by Walther Flemming, who made detailed observations of chromosomes during cell division in Salamandra maculosa. His results were published in 1878 and then in the seminal book Zellsubstanz, Kern und Zelltheilung (1882; Cell substance, nucleus and cell division).During mitosis, one round of DNA replication is followed by a single round of chromosome segregation, thus generating two genetically identical daughter cells).
• The term meiosis was coined by Farmer and Moore meaning reduction in chromosome number. It is achieved by one round of DNA replication being followed by two rounds of chromosome segregation with no intervening round of DNA replication. Fusion of two gametes during sexual reproduction restores the diploid complement of chromosomes in the zygote that gives rise to a new individual. Meiosis was devided into several stages including Meiosis I, Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis, Synchronous processes, Metaphase I, Anaphase I, Telophase I and Meiosis II.

Labs working on this gene

• 1.National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan 430070, China
• 2.Department of Cell and Developmental Biology, John Innes Centre, Norwich NR4 7UH, UK
• 3.State Key Laboratory of Plant Genomics and Center for Plant Gene Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China
• 4.National Key Facility for Crop Gene Resources and Genetic Improvement, Institute of Crop Science, Chinese Academy of Agricultural Sciences, Beijing 100081, China
• 5. Department of Plant and Resources College of Bioresources, Nihon University, Fujisawa, Kanagawa 252-8510, Japan
• 6. Key Laboratory of Crop Genetics and Physiology of Jiangsu Province/Key Laboratory of Plant Functional Genomics of Ministry of Education, Yangzhou University