Microsporogenesis and systematics of Aristolochiaceae

Within Aristolochiaceae, a secretory tapetum and orbicules are ubiquitous, but both simultaneous and successive types of microsporogenesis occur. Simu ltaneous cytokinesis is apparently plesiomorphic within the order Piperales , in which Aristolochiaceae are now placed. Successive microsporogenesis wa s found only in species of Aristolochia confined to a crown clade in the pr oposed phylogeny of this genus. In contrast to many other taxa, within Aris tolochiaceae there is no strict relationship between microsporogenesis type and tetrad configuration, which is strongly influenced by spindle orientat ion, especially during meiosis II. There is also no direct correlation betw een microsporogenesis type and the aperture of mature pollen grains. (C) 20 01 The Linnean Society of London.