Pollen‐connecting threads inGymnocalycium(Cactaceae): Their origin, function, and systematic relevance

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Abstract
In dehisced anthers of all Gymnocalycium species (Cactaceae, subfam. Cactoideae, tribe Trichocereae) investigated so far, a small percentage of pollen grains are aggregated by only a few threads. These threads are located at the margin of the dehiscing zone of stomium and septum. They are not part of the pollen grain wall, and they are not resistant to acetolysis, thus indicating the absence of sporopollenin. The threads originate in part from modified stomium/septum cells (split cell walls, cytoplasmic remnants and lipid droplets) and in part from pollenkitt produced by the—now desintegrated—tapetal cells. This highly viscous, gum‐like substance along the modified stomium and septum cell walls may assume an irregular thread‐like habit before or during dehiscence. Pollen aggregates may be formed frequently by different sticky or non‐sticky agents. A detailed conspectus on origin, nature, significance and function of various types of pollen‐connecting agents forming threads is presented. Pollen‐connecting threads are normally efficient pollination vehicles and are restricted to flowers pollinated by large insects or vertebrates. The genus Gymnocalycium is probably exclusively bee‐pollinated, with most pollen connected by pollenkitt. The functional role of the threads in Gymnocalycium is uncertain: since only a few threads connect a small number of pollen grains per pollen‐sac, they hardly play a significant role in pollination ecology. The threads most probably represent a character restricted to the genus Gymnocalycium as they have not been observed in closely related genera.