Pollen morphology and ultrastructure of Quercus with focus on Group Ilex (= Quercus subgenus Heterobalanus (Oerst.) Menitsky): implications for oak systematics and evolution

It has been suggested that pollen ornamentation can be used to distinguish infrageneric groups in Quercus. Here, we document pollen morphology of nearly all species of Quercus Group Ilex and show that they share distinctive microrugulate pollen ornamentation. Furthermore, pollen ultrastructure of all six infrageneric groups of Quercus was studied comparatively and evaluated using a phylogenetic framework for Fagaceae. Characteristics of the foot layer, such as its thickness and continuity, and the ratio foot layer to tectum are not stable within infrageneric groups of Quercus. The foot layer in derived groups (Groups Cerris, Quercus, Lobatae) is irregular, discontinuous and thin (type 2 foot layer). Members of Groups Cyclobalanopsis and Ilex either have a regular, continuous, and comparatively thick foot layer (type 1), or a type 2 foot layer. Only Group Protobalanus consistently has a type 1 foot layer and is similar to Trigonobalanus and Colombobalanus among trigonobalanoids, and to castanoids. Because (I) all except for one species of Castanea and Nothofagus have a type 1 foot layer and (II) basal groups in Quercus may have a type 1 foot layer, we suggest that this is the ancestral state in Fagaceae, and that the type 2 foot layer of Fagus is derived.