Haploid Meiosis in Arabidopsis: Double-Strand Breaks are Formed and Repaired but without Synapsis and Crossovers

Meiosis plays an essential role in evolution by creating new allelic combinations, and hence genetic diversity of species. This specialized cell division helps to ensure fertility by the production of balanced gametes while also allowing stable transmission of genomes over generations.

Two hallmark features of meiosis are i) the formation of crossovers (COs) between homologous chromosomes and ii) the production of genetically-unique haploid spores that will fuse to restore the normal ploidy level upon fertilization. With the aim of understanding how meiosis progresses without a homolog, we analyzed meiosis in haploid Arabidopsis thaliana plants and a range of haploid meiotic mutants. No COs and no synapsis were formed in wild-type haploids. The resulting univalents segregated randomly, producing unbalanced gametes. Double strand breaks were formed but were repaired in a RAD51-dependant manner, presumably using the sister chromatid. Turning meiosis into mitosis in haploids resulted in the production of balanced haploid gametes and restoration of fertility. Interestingly, these plants produced homogeneous populations of diploid Mime plants. Our findings provide new insights into the mechanisms underlying meiosis.