Cytological staging of pollen mother cells (PMCs). (a) Acetocarmine-stained chromosomes in PMCs in anthers of various sizes. PMCs in both fertile and mutant anthers at 1.0 mm are mostly at interphase. PMCs in 1.5 mm fertile anthers are at the leptotene or zygotene stage of meiotic prophase I. PMCs in both 1.5 mm and 2.0 mm am1-489 anthers show unsynchronized, mitotic characteristics with 20 univalents; am1-praI central cells start meiosis successfully when anthers reach ~2.0 mm but arrest at the L/Z transition. (b) Developmental timing of meiotic stages at the corresponding anther lengths in fertile anthers.