Turning Meiosis into Mitosis

The mutation of as few as three genes in a sexual plant transforms meiosis into mitosis and results in diploid gametes that are genetically identical to the mother plant. This phenotype resembles apomeiosis, which is a major step in apomixis

Mutations in AtPS1 (Arabidopsis thaliana Parallel Spindle 1) Lead to the Production of Diploid Pollen Grains

Polyploidy has had a considerable impact on the evolution of many eukaryotes, especially angiosperms. Indeed, most—if not all—angiosperms have experienced at least one round of polyploidy during the course of their evolution, and many important crop plants are current polyploids. The occurrence of 2n gametes (diplogametes) in diploid populations is widely recognised as the major source of polyploid formation. However, limited information is available on the genetic control of diplogamete production. Here, we describe the isolation and characterisation of the first gene, AtPS1 (Arabidopsis thaliana Parallel Spindle 1), implicated in the formation of a high frequency of diplogametes in plants. Atps1 mutants produce diploid male spores, diploid pollen grains, and spontaneous triploid plants in the next generation. Female meiosis is not affected in the mutant. We demonstrated that abnormal spindle orientation at male meiosis II leads to diplogamete formation. Most of the parent's heterozygosity is therefore conserved in the Atps1 diploid gametes, which is a key issue for plant breeding. The AtPS1 protein is conserved throughout the plant kingdom and carries domains suggestive of a regulatory function. The isolation of a gene involved in diplogamete production opens the way for new strategies in plant breeding programmes and progress in evolutionary studies

The CYCLIN-A CYCA1;2/TAM Is Required for the Meiosis I to Meiosis II Transition and Cooperates with OSD1 for the Prophase to First Meiotic Division Transition

Meiosis halves the chromosome number because its two divisions follow a single round of DNA replication. This process involves two cell transitions, the transition from prophase to the first meiotic division (meiosis I) and the unique meiosis I to meiosis II transition. We show here that the A-type cyclin CYCA1;2/TAM plays a major role in both transitions in Arabidopsis. A series of tam mutants failed to enter meiosis II and thus produced diploid spores and functional diploid gametes. These diploid gametes had a recombined genotype produced through the single meiosis I division. In addition, by combining the tam-2 mutation with AtSpo11-1 and Atrec8, we obtained plants producing diploid gametes through a mitotic-like division that were genetically identical to their parents. Thus tam alleles displayed phenotypes very similar to that of the previously described osd1 mutant. Combining tam and osd1 mutations leads to a failure in the prophase to meiosis I transition during male meiosis and to the production of tetraploid spores and gametes. This suggests that TAM and OSD1 are involved in the control of both meiotic transitions

Efficient transposition of the Tnt1 tobacco retrotransposon in the model legume Medicago truncatula

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Mitosis-like divisions instead of meiosis in <i>MiMe</i> plants.

<p>(A) Male metaphase I (B) Male anaphase I. The vignette shows a dyad in <i>MiMe</i>. (C) Female metaphase I. (D) Female anaphase I. Scale bar = 10 µm.</p

Primers for mutant genotyping.

<p>Primers for mutant genotyping.</p

<i>osd1</i> mutants skip meiosis II.

<p>(A and B) Male meiotic products stained with toluidine blue. (A) A wild-type tetrad. (B) A dyad in the <i>osd1-1</i> mutant. (C–D) Male meiosis in <i>osd1</i> is indistinguishable from wild type until telophase I (compare to <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000124#pbio-1000124-g004" target="_blank">figure 4</a>), but no figures characteristic of a second division were observed. (C) Pachytene. (D) Diakinesis. (E) Metaphase I. (F) Anaphase I. (G) Telophase I. Two nuclei separated by a band of mitochondria are observed. (H) Metaphase I of female meiosis in <i>osd1</i>.</p

Schematic summary of the main results.

<p>During mitosis in diploid cells, chromosomes replicate and sister chromatids segregate to generate daughter cells that are diploid and genetically identical to the initial cell. During meiosis, two rounds of chromosome segregation follow a single round of replication. At division one, homologous chromosomes recombine and are separated. Meiosis II is more similar to mitosis, resulting in equal distribution of sister chromatids. The obtained spores are thus haploid and carry recombined genetic information. In the <i>osd1</i> mutant (this study), meiosis II is skipped giving rise to diploid spores and gametes with recombined genetic information. The double <i>Atspo11-1/Atrec8</i> mutant undergoes a mitotic-like division instead of a normal first meiotic division, followed by an unbalanced second division leading to unbalanced spores and sterility <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000124#pbio.1000124-Chelysheva1" target="_blank">[9]</a>. In the triple <i>osd1/Atspo11-1/Atrec8</i> mutant (<i>MiMe</i>, this study), the presence of the <i>Atspo11-1</i> and <i>Atrec8</i> mutations leads to a mitotic-like first meiotic division, and the presence of the <i>osd1</i> mutation prevents the second meiotic division from occurring. Thus meiosis is replaced by a mitotic-like division. The obtained spores and gametes are genetically identical to the initial cell.</p