Classification of miRNAs during pollen development: (A,B) the total number of miRNAs detected during different pollen development stages of Taichung65-2x (A) and Taichung65-4x (B); (C,D) Venn analysis of miRNAs expressed in Taichung65-2x (C) and Taichung65-4x during pollen development (D); and (E,F) specifically up- and down-regulated miRNAs during each adjacent stage of pollen development in Taichung65-2x and Taichung65-4x. PMA, MA and SCP represent pre-meiotic interphase, meiosis and single microspore stage, respectively. “4x” and “2x” represent the autotetraploid and diploid rice, respectively.