Tell me who’s going to pollinate you…

Flowers have to attract pollinators. They cannot escape this basic necessity. Something has to bring pollen grains onto stigma anyway, to fertilize ovules and let them produce seeds. In most plant species, this is done by insects…

Some insects are more or less efficient. Coleoptera for example are generally not really interested into packaging pollen and prefer eating it directly. They are therefore poor pollinators, even if some plant species just go with it.

Other are probably more efficient but really are amateur when it comes to carrying it and tend to fly around. So are butterflies… Of course the most marvellous pollinators are pollen foraging hymenoptera like bees and bumblesames, which usually do take the time for many visitations and never really forget a single flower… To the point that they finally collect a lot of pollen grains for themselves, but the price is not that high with regard to the pollination service (and even if some hymenoptera species don’t really look for pollen but this is another story).

Actually plants have to attract efficient pollinators; efficient meaning efficient to pollinate them. They really have no interest to send pollen onto stigma of yet another species: their pollen have to find their way onto stigma of flowers of the very same species. We may also note that they don’t any interest into sending their pollen on their own other flowers, because this would translate into selfing, which is the highest form of consanguinity, and consanguinity in general translates into a decreased fitness (through inbreeding depression effects). Some plant families actually evolved specific adaptations to avoid this to occur…

The issue here is thus that plants should attract the right pollinator, which has to be able to take and carry on pollen grains, and has to deposit its load into the right place (best is another flower of the very same species). It seems easily done, but that’s a real challenge… Let’s not forget nevertheless that plants have been selected for doing this anyway, since those that do not simply didn’t reproduce… It’s thus very simple and powerfull…

We nevertheless have yet a very approximate knowledge concerning pollinating relative efficiency for insects visiting plants… A recent study on wild radish (hum this picture is about a wild mustard species but hey, they are relatives from the same family, and their flower morphology are very close so I just illustrate the post easily…), allows us to know more about this question [1].

 The experiment is simple but offers interesting results: experimental plants were exposed in an artificial radish population and taken back (into the greenhouse) as soon as after a single visit to a focal flower. Pollinator species was identified up the Genus level. Finally, when a fruit was produced and matured, seeds were simply counted…

 And here it is: first the relative visitation frequency of the diverse pollinators species, then is the efficiency to fertilize the flower (mean number of seeds produced after this single visit), and finally the weighted relative importance of those pollinators with regard to the plant pollination. We observe that one species is of particular interest for this particular plant species, even if a great number of species actually do visit radish flowers… This is thus probably this bee species that is responsible for the flower morphology! Visitation has its word to put on, even when compared to pollination efficiency…

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[1] - Sahli H.F., and J.K. Conner. 2007. Visitation, effectiveness, and efficiency of 15 genera of visitors to wild radish, Raphanus raphanismum (Brassicaceae). American Journal of Botany 94 (2): 203-209.