From acaveate to caveate: evolution of pollen types in the Rhaponticum group (Asteraceae, Centaureinae) related to extreme conditions

12 p.A survey of pollen morphology and ultrastructure within the Rhaponticum group, using both scanning and transmission electron microscopy, revealed two pollen types. Most representatives of the Rhaponticum group exhibited Serratula-type pollen. The exceptions were the genus Oligochaeta, with Centaurea jacea-type pollen, and the genus Myopordon, with both types of pollen. Phylogenetic inference within the group indicated that Jacea-type pollen is derived from Serratula-type pollen. The fact that Myopordon exhibited a series of pollen types, from Serratula type to Jacea type, suggests that a transition from one form to another has occurred in the genus. Therefore, the factor which has produced this evolutionary tendency should be sought within Myopordon. Jacea-type pollen is characterized by a decrease in size, ornamentation, the absence of a columellar infratectum and the presence of a cavea. A comparative survey of similar tendencies in other groups, and their correlation with potential functional implications, prompted the hypothesis that the critical factor involved in such evolution of pollen could be habitats with extreme conditions.Peer reviewe