Abstract
The selective pressure imposed by maximizing male fitness (pollen dispersal) in shaping floral structures is increasingly recognized and emphasized in current plant sciences. To maximize male fitness, many flowers bear a group of stamens with temporally separated anther dehiscence that prolongs presentation of pollen grains. Such an advantage, however, may come with a cost resulting from interference of pollen removal by the dehisced anthers. This interference between dehisced and dehiscing anthers has received little attention and few experimental tests to date. Here, using one-by-one stamen movement in the generalist-pollinated Parnassia palustris, we test this hypothesis by manipulation experiments in two years. Under natural conditions, the five fertile stamens in P. palustris flowers elongate their filaments individually, and anthers dehisce successively one-by-one. More importantly, the anther-dehisced stamen bends out of the floral center by filament deflexion before the next stamen's anther dehiscence. Experimental manipulations show that flowers with dehisced anther remaining at the floral center experience shorter (1/3–1/2 less) visit durations by pollen-collecting insects (mainly hoverflies and wasps) because these ‘hungry’ insects are discouraged by the scant and non-fresh pollen in the dehisced anther. Furthermore, the dehisced anther blocks the dehiscing anther's access to floral visitors, resulting in a nearly one third decrease in their contact frequency. As a result, pollen removal of the dehiscing anther decreases dramatically. These results provide the first direct experimental evidence that anther-anther interference is possible in a flower, and that the selection to reduce such interferences can be a strong force in floral evolution. We also propose that some other floral traits, usually interpreted as pollen dispensing mechanisms, may function, at least partially, as mechanisms to promote pollen dispersal by reducing interferences between dehisced and dehiscing anthers.
Highlights
In the current resurgence of pollination biology, the role of male reproductive successes in floral adaptation and evolution has been frequently emphasized in both theoretical [1,2,3] and experimental studies [4,5,6]
We examined two main predictions of this hypothesis: (i) the one-by-one filament elongation and anther dehiscence is a special type of ‘pollen dispensing mechanism’ that prolongs the male phase of the flower; (ii) preventing the dehisced anther from bending away from the floral center will negatively affect pollen removal from the dehiscing anther
This study suggests that one-by-one stamen movement in Parnassia palustris reflects an adaptation to decrease interference between dehisced and dehiscing anthers, which probably could promote pollen exports
Summary
In the current resurgence of pollination biology, the role of male reproductive successes (pollen transfer to the conspecific stigmas) in floral adaptation and evolution has been frequently emphasized in both theoretical [1,2,3] and experimental studies [4,5,6]. When the previous anther has finished its pollen presentation, it could block the contact of the dehiscing anthers from pollinators since the optimal spatial position for pollen precise placement on pollinator’s body is always restricted [8,10]. Such interference between dehisced and dehiscing anthers has received little attention in current pollination biology, the ‘anther-stigma interference’ We examined two main predictions of this hypothesis: (i) the one-by-one filament elongation and anther dehiscence is a special type of ‘pollen dispensing mechanism’ that prolongs the male phase of the flower; (ii) preventing the dehisced anther from bending away from the floral center will negatively affect pollen removal from the dehiscing anther
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