Abstract
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.
Highlights
Meiosis is a central feature in the reproductive program of all sexually reproducing eukaryotes
We identified genes that control the entry into the first and the second meiotic division in the model plant Arabidopsis thaliana
The tam-2 and tam-3 mutations are in the Columbia (Col-0) background, whereas the tam-4 mutation is in the Landsberg erecta (Ler) background (Figure 1A)
Summary
Meiosis is a central feature in the reproductive program of all sexually reproducing eukaryotes. (i) Homologous chromosomes pair and closely associate along a proteinaceous structure called the synaptonemal complex (SC). This process culminates at a substage of prophase called pachytene. (ii) Crossovers occur between homologs during prophase. (iii) Homologous chromosomes separate at anaphase of the first division. To ensure accurate chromosome segregation at anaphase I each homolog must remain connected to the other through metaphase I. Since the SC disappears before the end of prophase, it cannot ensure linkage of homologs at metaphase I. This connection is maintained until anaphase I by chiasmata, the cytological manifestation of crossovers
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