Reproductive morphology in the Gnetum cuspidatum group (Gnetales) and its implications for pollination biology in the Gnetales

• Background and aims - The Gnetales include the extant genera Gnetum, Ephedra and Welwitschia. They are usually functionally dioecious, but male cones often have sterile (but pollination drop-producing) ovules in addition to male units. There are, however, exceptions, i.e. most species of Ephedra and African species of Gnetum. Furthermore, the literature contains conflicting information on the Asian Gnetum cuspidatum. One study states that sterile ovules are present in this species; another that they are absent. The latter also claims that male cones secrete nectar instead, which is interesting because nectar has only been suggested to be present in four gymnosperm species. Here we aim to elucidate whether or not sterile ovules are present in male cones of G. cuspidatum and related taxa, evaluate evidence for nectar being present in gymnosperms and discuss implications for pollination biology. Methods - Male cones from relevant taxa were examined using a dissecting microscope and scanning electron microscopy. Key results - Sterile ovules are present in G. cuspidatum and the related G. macrostachyum, G. microcarpum, G. diminutum and G. loerzingii, but they are minute, hidden among hairs, and easily overlooked. No indications of nectar or nectaries were found and their presence in Asian species of Gnetum is questioned. Conclusions - Insect pollination is probably ancestral in the Gnetales. Like most species of Gnetum, members of the G. cuspidatum group have sterile ovules in male cones, and they can thus attract pollinators to both male and female plants using sweet pollination drops. Although it is possible that these species, in addition, produce extraovular reward for pollinators, we find no such evidence. Instead, it seems plausible that pollination drops have been mistaken for (extraovular) nectar. However, African species of Gnetum have unisexual male cones. Have they developed another means of pollinator reward in male plants or are they wind-pollinated as are their ephedran analogues?

Ämnesord

NATURVETENSKAP  -- Biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences (hsv//eng)

Nyckelord

Gymnosperms

entomophily

evolution

nectar

pollination drops

taxonomy

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