chapter  14
Pollen Development in Annona cherimola mill. (Annonaceae). Implications for the Evolution of Aggregated Pollen
Pages 23

Angiosperms pollen is most commonly released as single pollen grains or monads [4] which represent the basic angiosperm pollen-unit. Dehiscence of aggregated pollen (mostly in groups of four) is considered a recent apomorphic characteristic [5,6] that has arisen independently several times during evolution primarily in animal-pollinated taxa although, in some cases, monads may have evolved secondarily from groups of four grains [6]. Pollen release as tetrads has been reported in some or all members of 55 dierent angiosperm families and also in some pteridophytes [7]. Blackmore and Crane (1988) [8] put forward that the maintenance of pollen tetrads could be the result of relatively minor ontogenetic changes and, consequently, this could be an excellent example of convergence in situations where the release of pollen as tetrads is an eective reproductive strategy. Interestingly, the dissemination of pollen as tetrads has also been reported in the quartet mutants of Arabidopsis [9,10].