The convergent evolution of exine shields in angiosperm pollen

Abstract

The appearance of the pollen grains (or tetrads) of some taxa of only distantly related angiosperm families Berberidaceae (Mahonia), Bignoniaceae (Catalpa), Euphorbiaceae (Phyllanthus), Iridaceae (Iris sect. Juno), Malpighiaceae (Malpighia, Banisteriopsis, Heteropteris) and Martyniaceae (Ibicella, Proboscidea) is very similar. For this pollen type we propose the term “clypeate” (= covered with shields), i.e. “pollen grains with large, isolated, polygonal or circular (s)exinous shields, separated by grooves”. Despite their similar appearance both their aperture condition and wall stratification are different. Phyllanthus × elongatus and the Malpighia species investigated have (panto-)porate pollen grains, and local intine thickenings are present beneath the pores, which are located between the exine shields in a regular (Phyllanthus) or more irregular manner (Malpighia). This pantoporate condition differs markedly from the aperture condition which is present in all of the other taxa investigated and can best be described as inaperturate. © 1995 Scandinavian University Press.