Cluster analysis of expression profiles of altered expressed genes in the QS versus EG. A showed a cluster of ESTs that were down-regulated mainly at squaring stage (SF) when the tetrads were produced and the microsporocyte underwent meiosis. B showed ESTs that were down-regulated especially at full bloom stage (BF). C and D suggested a cluster of ESTs that were down-regulated and up-regulated constitutively during the four developmental stages respectively. The ratio value was log2 transformed for each gene and used for the hierarchical clustering analysis.