Putative core cell-cycle model Imposed the heterochronic shift during angiosperm spermatogenesis and fertilization. (a) Representative dynamic expression of cell-cycle genes at the time of anthesis of tobacco bicellular pollen as derived from transcriptomic data. The relative abundances between negative and positive regulators gives a snapshot of the cell-cycle status of germ cell and vegetative cell. Increased expression of positive regulators and post-translational inhibition of the repressors signifies the progression of the germ cell cycle to enter PMII for sperm cells production. This progression is in parallel with the expression of germ-cell fate determinants integrated by the NtDUO1-like Myb transcription factor and in synchrony with pollen-tube growth rate through the female pistil, with the effect of synchronizing the gametes cell-cycle progression for successful karyogamy. (b) Depicted model of core cell-cycle regulatory network likely to account for the heterochronic shift in spermatogenesis among flowering plants. The model was derived on the basis of the combined expression analysis presented herein. Key other proteins and family members are also expected to be involved in this network. Note: the hierarchical relationship between R2R3 NtDUO1-like and NtMYBA1 (both activators of CycB1;1 transcription) is yet to be established, together with that of three other R1R2R3 plant Myb transcription factors.