Is There ‘Anther-Anther Interference’ within a Flower? Evidences from One-by-One Stamen Movement in an Insect-Pollinated Plant

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.

Sequential stamen maturation and movement in a protandrous herb: mechanisms increasing pollination efficiency and reducing sexual interference

“…When a flower was found to have an uplifted anther in the floral centre, the stamen was immobilized with a white thread to prevent subsequent deflexion. One end of the thread was bound to the joint between the filament and anther, and the other end was tethered to the pedicel through the space between two opposite petals, according to the manipulations piloted by Ren and Bu (2014) . To minimize possible interference during insect visitation, we camouflaged the thread and petal and kept it outside the path of the floral visitors.…”

A case of behavioural diversification in male floral function – the evolution of thigmonastic pollen presentation

“…Finally, cascade movement mechanisms (whether thigmonastic or not) often occur in combination with subsequent autonomous movements to avoid anther-anther interference during pollination (e.g. Parnassia 43 ; Ruta graveolens 32 ). Moreover, in the latter case all stamens repeat their movement towards the style simultaneously at the end of anthesis to ensure pollination through selfing as a backup mechanism 32 .…”

Parnassia zhengyuana sp. nov. and P. simianshanensis sp. nov.: two new species of Parnassia (Celastraceae) from karst caves and Danxia landform in southwest China

“…Parnassia has two diagnostic traits, i.e. five showy staminodes (Gu and Hultgård , Sandvik and Totland , Wu ) and one‐by‐one stamen movement (Gris 1868, Hultgard , Ren 2010, Armbruster et al , Ren and Bu ). The shape and branching of the staminodes are the most important traits for the taxonomic treatment of Parnassia (Gu , Gu and Hultgård , Wu ).…”

“…The shape and branching of the staminodes are the most important traits for the taxonomic treatment of Parnassia (Gu , Gu and Hultgård , Wu ). The distinctive mode of one‐by‐one stamen movement associated with sequential maturation of the anthers is thought to be an adaptation to increase pollen outputs under low insect visitation rates (Ren and Tang , Armbruster et al , Ren and Bu ).…”