Required for normal spindle orientation at male meiosis II and normal formation of tetrad of microspores. Not involved in female meiosis. Diploid pollen grains (dyads of spores instead of tetrads) due to defective meiosis II. Abnormal spindle orientation at meiosis II. Diploid male spores in sp1 mutants give rise to viable diploid pollen grains and spontaneous triploid plants in the next generation.