Evolving specialized pollination, where you eventually develop a complete dependence to a single pollinator species, is both a risky bet and a win-win path to success. That is, dependence is critical to your reproduction, since you will only succeed to make seeds when your pollinating pet lives around. Mind you, this is not only tricky because you need to adjust to its needs and way of life, it’s also that you’ll only be able to expand within its own ecological niche.
If it’s large, then that’s a safe bet. On the other hand, it’s easier to get control over what your pollinating pet will be allowed to take from you. Yeah, pollinators usually don’t pollinate for free. They want a pay back. Pollen, nectar, warmth, maybe breeding sites. In exchange, you get pollinated and you may even pollinate your neighbours. Something you can hardly do alone when you happen to be a plant. Today, we’ll look into a very nice situation, the pollination of the plant Macaranga tanarius by Orius atratus. And guess what? Orius should be a star not only because it is one (or accessorily a much commoner doggy name than one would expect by chance alone), but because it is a bug. A true bug. (Not like you and me). And the study for today is the first report of an apparently mainly if not entirely bug-mediated pollination system.
Macaranga tanarius, a proud member of the Euphorbiaceae family. The genus is diverse, about 260 species (not bad!), apparently all of them are dioecious (Reminder: it means sexes are differentiated, with both male and female plants relying on outcross to mate successfully). Thirty species in the genus are myrmecophytes, that is, provide some ant species with special shelters for nesting. Another one is pollinated by thrips, which is a quite uncommon pollination mutualism (usually thrips do scavenge on plants, really rarily do they do anything good to the plant -just ask any greenhouse staff about “fish & thrips”). Macaranga tanarius has a rather wide and mostly tropical distribution, ranging from Peninsular Malaysia, Borneo, New Guinea, and through southern Japan.
Orius atratus. Anthocoridae, Hemiptera. Despite the flower root in the family name (Anthos), Anthocorids are rather known to be predacious insects, feeding on little hexapodic preys and occasionnally on pollen (though most species can’t be efficiently reared on pollen only). This family is well known (and what’s more, there are lots of pictures on the intertubes), because it is often used as a way to get rid of crop pests via biological control (hum, if your green-house friend says [s]he’s not an astronomer, maybe it’s just she/he never heard of this bug).
The two main insect species found in inflorescences, in Japan, are bugs. Orius atratus, I told you. Plus Decomioides schneirlai, yet another bug. The big difference between them is that only Orius bears pollen grains, be it from staminate-flowered trees or pistillate-flowered ones (which don’t provide the bugs with any pollen). So pollen transfers happens, but only via O. atratus. Movement between the trees still needs to be investigated, but a certain proportion of Orius visitors have what’s needed to fertilize the female trees. Fertilization would hardly be the result of wind pollination since flowers are protected by surrounding bracts and really need help from a beasty friend to receive their pollen load.
The reward is under control.
Macaranga tanarius is nice to its pollinators. First of all, it provides breeding chambers, as the bracts are rounded and enclose the flowers. It offers protected rooms where hemipteran larvae can also develop quietly. But that’s not all: this is a brood-pollination system, that is, there’s a food reward else pollinators would need to leave the shelter. As you’d be guessing, the reward is not pollen else pistillate flowers from female trees wouldn’t be pollinated. And there comes the amazing fact: nectar. Not that it is striking per se, but that the nectar is actually enclosed in special chambers, in the bracts, and completely closed. Therefore, no classical visitor (bee, fly, butterfly…) is allowed to get this ressource for free, unless it bears a mouth able to pierce a hole into the nectaries. Hum, maybe Decomioides schneirlai is just parasitic here, since it takes nectar out without contributing to the plant reproduction, but as long as Orius atratus is playing the game, this is probably an acceptable loss.
O. atratus evolved from a lineage that’s mainly predacious, and all of its cousin species predate small insects on flowers. How did it make it into a vegetarian lifestyle? The study offers a plausible hypothesis: Macaranga hullettii, the tree with a thrips pollination system, is morphologically really similar to M. tanarius, our bug pollinated species. So they both may have evolved from a common ancestor that was pollinated by thrips, and our little now M. tanarius managed to domesticate its pollinator predator and turn it into its new pet. How that? Good question, but we should assume that having too many thrips in the inflorescence was maybe damaging the flowers, so that attracting its predators would allow a greater reproductive success, as long as pollination occured. If true, it would be great to investigate when the tradeof was at its greatest, before the pollinator shift… Maybe we’ll know more soon!
* C. Ishida, M. Kono and S. Sakai (2009). A new pollination system: brood-site pollination by flower bugs in Macaranga (Euphorbiaceae). Annals of Botany 103: 39-44.