Pollination Ecology of the Royal Irises

Oncocyclus Irises are among the most impressive flowers in the Near East and are all narrowly endemic and endangered species. Most of the species in the Southern Levant share common traits of extremely large and dark flowers, which might have evolved by pollinator mediated selection. Night-sheltering of solitary (mainly Eucera) male bees within the nectarless flowers of Iris species of section Oncocyclus are proposed to be the major selective force for floral colour evolution. The lack of other visitors during daytime and the efficient self-incompatibility suggest that the night sheltering males are the only pollinators.

We used black and white plastic cones, each of two sizes (diameter of 4 & 7 cm) to study the preferences of male bees choosing an overnight shelter. Significantly, more male bees were found in black cones than in white ones, which usually hosted no males. However, similar numbers of males were found in small and big black cones. The results suggest that the male bees prefer dark colour for night shelter, no matter the size.

 


Figure 1 – Mean ( s.d) number of night-sheltering male bees within cones in binary-choice experiments. Significance calculated by Wilcoxon Signed Ranks test.

 


Preferring dark night shelter is probably because the thermal energy reward the sleeping male gets during early morning by from the warmer flowers exposed to the morning sun. Just after sunrise, the temperature in the black cones, as well as in the dark flowers of Oncocyclus Irises, was up to 7C above ambient air temperature. Therefore, males that shelter within dark flowers begin their daily activity earlier than other males. Consequently, they are the first to forage in highly rewarding flowers and the first to search for virgin females or for a nectar-full flowers.

The evolution of flowers is in many cases derived by their pollinators. High specialist flowers, e.g. orchids, are frequently stated as being shaped by the high selective pressure of their pollination system. The flowers of Oncocyclus Irises are subjected to pollinator selection, as indicated the preference of the black tunnels and the dark colour of all the night shelter flowers. Surprisingly, shelter size did not affect bee’s preference, suggesting that the large flowers of the Irises are not the result of pollinator selection. To conclude, the results of this study support the hypothesis of dark coloured flowers as an adaptation for pollination by night-sheltering male bees. The solitary male bees supply pollination services to the Iris flowers, and by that increase their own fitness, as well as the fitness of the Irises.

 

 

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Pollination of the Royal Irises

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