| Summary: | Meiosis drives reciprocal genetic exchanges and produces gametes with halved chromosome number, which is important for the genetic diversity, plant viability, and ploidy consistency of flowering plants. Alterations in chromosome dynamics and/or cytokinesis during meiosis may lead to meiotic restitution and the formation of unreduced microspores. In this study, we isolated an <i>Arabidopsis</i> mutant <i>male meiotic restitution 1</i> (<i>mmr1</i>), which produces a small subpopulation of diploid or polyploid pollen grains. Cytological analysis revealed that <i>mmr1</i> produces dyads, triads, and monads indicative of male meiotic restitution. Both homologous chromosomes and sister chromatids in <i>mmr1</i> are separated normally, but chromosome condensation at metaphase I is slightly affected. The <i>mmr1</i> mutant displayed incomplete meiotic cytokinesis. Supportively, immunostaining of the microtubular cytoskeleton showed that the spindle organization at anaphase II and mini-phragmoplast formation at telophase II are aberrant. The causative mutation in <i>mmr1</i> was mapped to chromosome 1 at the chromatin regulator <i>Male Meiocyte Death 1</i> (<i>MMD1</i>/<i>DUET</i>) locus. <i>mmr1</i> contains a C-to-T transition at the third exon of <i>MMD1</i>/<i>DUET</i> at the genomic position 2168 bp from the start codon, which causes an amino acid change G618D that locates in the conserved PHD-finger domain of histone binding proteins. The F1 progenies of <i>mmr1</i> crossing with knockout <i>mmd1</i>/<i>duet</i> mutant exhibited same meiotic defects and similar meiotic restitution rate as <i>mmr1</i>. Taken together, we here report a hypomorphic <i>mmd1</i>/<i>duet</i> allele that typically shows defects in microtubule organization and cytokinesis.
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