A review of the exine structure of extant and fossil gnetophytes and possibly related groups.

Abstract

Background: Gnetophytes are a key group, possessing characters of gymnosperms and angiosperms. In spite of numerous studies, their exact relationship with other plant groups is still uncertain. Extant members of the group are isolated remnants of a formerly diverse group. The range of this diversity, however, is quite inconclusive. More fossil gnetophytes as well as revisions and reviews of existing data are needed.

Scope: This paper reviews accumulated data on the exine ultrastructure of taxa included and/or related to gnetophytes with different degrees of certainty. Here we have divided possible members into four subgroups: extant gnetalean pollen, fossil gnetalean pollen, putative gnetalean pollen and controversial pollen grains considered members of or close to gnetophytes by some authors. We provide short descriptions of the pollen under consideration, both dispersed and in situ, and some general discussion on each subgroup.

Conclusions: Pollen grains of most gnetophytes and possibly related taxa are small or medium-sized, asaccate, inaperturate or with a sulcus or several sulci, a psilate, scabrate, verrucate, polyplicate or microechinate surface, and a granular infratectum of mostly small or medium-sized granules. Rare saccate pollen grains among the taxa possibly related to gnetophytes can have either an alveolate or granular infratectum with sacci that are expressed externally or inconspicuous. A number of peculiar pollen characters in morphology and ultrastructure occur in a mosaic fashion in genera that are rather distant from each other based on their macromorphology or other pollen characters. Possible transformations from an alveolate and granular infratectum to a columellate one and from saccate to asaccate forms can be inferred based on the diversity of the pollen grains considered. Gnetophytes are an extremely diverse group even in their narrow definition, so we need to search for more fossil gnetophytes to understand the evolution of this group.