Successive microsporogenesis in eudicots, with particular reference to Berberidaceae (Ranunculales)

The eudicot clade of angiosperms is characterised by simultaneous microsporogenesis and tricolpate pollen apertures. Successive microsporogenesis, where a distinct dyad stage occurs after the first meiotic division, is relatively rare in eudicots although it occurs in many early branching angiosperms including monocots. An extensive literature survey shows that successive microsporogenesis has arisen independently at least six times in eudicots, in five different orders, including Berberidaceae (Ranunculales). Microsporogenesis and pollen apertures were examined here using light and transmission electron microscopy in eleven species representing six genera of Berberidaceae. Successive microsporogenesis is a synapomorphy for the sister taxa Berberis and Mahonia (and possibly also Ranzania), the remaining genera are simultaneous. Callose wall formation in Berberis and Mahonia is achieved by centripetal furrowing, though centrifugal cell plates are more usual for this microsporogenesis type. This discrepancy could reflect the fact that the successive type in Berberidaceae is derived from the simultaneous type, and centripetal furrowing has been retained. Eudicots with successive microsporogenesis usually produce tetragonal or decussate tetrads, though occasional tetrahedral or irregular tetrads in Berberis and Mahonia indicate that the switch from simultaneous to successive division is incomplete or “leaky”. In contrast, linear tetrads produced by successive microsporogenesis in Asclepiadoideae (Apocynaceae s.l.) are the result of a highly specialised developmental pathway leading to the production of pollinia. Pollen in successive eudicots is dispersed as monads, dyads, tetrads, and as single grains in pollinia. Apertures are diverse, and patterns include spiraperturate, clypeate, irregular, monocolpate, diporate and inaperturate. It is possible that successive microsporogenesis, although rare, potentially occurs in other eudicots, for example, in species where pollen is inaperturate.