Multiple developmental pathways leading to a single morph: Monosulcate pollen (examples from the Asparagales)

Background and Aims Early developmental events in microsporogenesis are known to play a role in pollen morphology: variation in cytokinesis type. cell wall formation. tetrad shape and aperture polarity are responsible for pollen aperture patterning. Despite the existence of other morphologies. motrosulcate pollen is one of the most common aperture types in monocots. and is also considered as the ancestral condition in this group. It is knows to occur from either a successive or a simultaneous cytokinesis. In the present study, the developmental sequence of microsporogenesis is investigated in several species of Asparagales that produce such monosulcate pollen. representing most families of this important monocot clade. Methods The developmental pathway of microsporoeenesis was investigated using light transmission and epifluorescence microscopy for all species studied. Confocal microscopy was used to confirm centripetal cell plat formation. Key Results Microsporogenesis is diverse in Asparaeales. and most variation is generally found between families. It is confirmed that the whole higher Asparaeales clade has a very conserved microsporogenesis. with a successive cytokinesis and centrifugal cell plate formation. Centripetal cell wall formation is described in Tecophilaeaceae and Iridaceae, a feature that had so far only been reported for eudicots. Conclusions Monosulcate pollen can be obtained from several developmental pathways, leading thus to homoplasy in the monosulcate character state. Monosulcate pollen should not therefore be considered as the ancestral state unless it is produced through the ancestral developmental pathway. The question about the ancestral developmental pathway leading to monosulcy remains open. (C) 2004.Annals of Botany Company.