Microsporogenesis and pollen morphology in dioscoreales and allied taxa

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Abstract
Analyses of new morphological and molecular data have resulted in a major recircumscription of Dioscoreales. However crucial characters such as microsporogenesis type and pollen morphology are not known for some taxa, and these are investigated here, particularly in view of the revised systematics of the order. Pollen morphological characters and microsporogenesis were examined using both electron microscopy (scanning and transmission) and light microscopy (including differential interference contrast). Microsporogenesis and pollen morphology were found to provide taxonomically useful characters, particularly at the genus level. Successive microsporogenesis is plesiomorphic in monocotyledons, and simultaneous microsporogenesis in Dioscorea, Rajania, Tamus and Tacca represents a probable synapomorphy for these taxa. Simultaneous microsporogenesis also supports a close sister group relationship between Japonolirion and Petrosavia but from other characters (including molecular data), their relationship with Dioscoreales is less clear. Pollen tetrads in Stenomeris are elongated, and are probably formed by a modified and unusual type of simultaneous microsporogenesis. Pollen morphology indicates a close relationship between the hermaphrodite members of Dioscoreaceae, and also between Avetra and Trichopus.

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