A remarkable case of pollinator/orchid coevolution and specificity

September 29, 2023 • 9:45 am

A science paper at last! Truth be told, I don’t come across many science papers that are both of general interest and that I can explain easily. But I do have several more in the queue.

A colleague sent me an old paper (from 2006), but its age does not diminish how spectacular the results are. And in short, the results are these: a group of 15 phenotypically similar (but probably not closely related) orchids in SW South Africa are pollinated by females of a single species of bee, which collects oil produced by the flowers and feeds it to their offspring.

This poses a problem, because orchids are pollinated by affixing sticky pollinia (sacs of pollen) collected from a flower of one species to the next flower of the same species. (The orchids in this group do not self-fertilize). With pollen sacs from 15 different orchid species sticking to a bee, how can a plant be sure that its own pollen gets transferred to another individual of the same species, rather than to another individual of a different species, in which case cross-species pollination would produce either inviable or maladapted hybrids?

The bees and orchids have solved this in a very clever way.

But let’s back up: the paper, from the American Journal of Botany, can be seen for free by clicking on the screenshot below, and the pdf is here.

The reason the author, Anton Pauw, gives for his 8-year investigation is that, he says, the “conventional wisdom” in botany is that it’s not adaptive for a bunch of flowers to depend on a single species of pollinator. That’s because if some environmental fluctuation or other contingency makes the pollinator rare (or even drives it extinct), the flowers wouldn’t get pollinated. This would imply that flowers should evolve to attract several species of pollinator, for those flowers that are generalists in this way are less likely to become rare or extinct themselves.

But this doesn’t seem to be the case in this group of 15 orchids, which, according to Pauw’s observation, come from three different genera (molecular phylogeny also suggests that they’re not each other’s closest relatives, though they look remarkably similar). Yet all are pollinated by a single bee, Rediviva peringueyi. This is in a genus called “long-legged oil bees.”

The flowers, as I said, look like each other, all produce oil that the bee collects, and all live in the same area, as well as flowering at the same time. As the author says,

Subgroups of similar plant species can be recognized within the extensive oil-bee pollination system. The one examined here includes 15 oil-secreting orchids that share the following syndrome of floral features: pale yellow-green flowers without extensive black markings; secretion of floral oil as a pollinator reward; characteristic pungent scent; flowering period 15 August to 25 October peaking in September; flower depth 5–8 mm (Fig. 1a–n). The species occur in close association with one another in the lowlands of the Cape Floral Region and include members of three genera (PterygodiumCorycium, and Disperis). According to the pollination syndrome concept, the similar floral features of this group indicate a shared pollinator. My aim was to test this prediction through extensive field work.

Figure 1 below (click to enlarge; caption from paper) shows how similar the flowers are. The pollinating bee (R. poeringueyi, which I’ll henceforth call “the bee”) is shown in the middle. The arrows show where the pollinia of each orchid species gets attached:

Figure 1. The Rediviva peringueyi pollination guild. Center, the oil-collecting bee R. peringueyi, arrows indicate pollinarium attachment sites of orchid species. (a) Pterygodium catholicum. (b) P. alatum. (c) P. caffrum. (d) P. volucris. (e) Corycium orobanchoides. (f) Disperis bolusiana subsp. bolusiana. (g) D. villosa. (h) D. cucullata. (i) D. circumflexa subsp. circumflexa. (j) P. inversum. (k) P. hallii. (l) P. platypetalum. (m) D. ×duckittiae. (n) P. cruciferum. (o) D. capensis var. capensis. Attachment sites f–i after Steiner. Pollinarium attachment sites are confirmed in a–g. Pollination and/or pollinarium attachment are predicted in h–o on the basis of floral features. R. peringueyi 5× life size, orchids 2× life size. Images e, h, k by Bill Liltved.

The bees also collect pollen and nectar, too, but not from these orchids.  From these 15 orchids they take only flower oil (I had no idea it even existed), and do so by, as you can see in the photo below, gripping the plant with the bee’s middle and hindlegs and collecting the oil with modified forelegs. In the process (and of course this is why the flower produces oil and scent to attract the bee). During oil collection, the pollinia of the orchid, which is sticky, attaches to the bee’s body. That’s also shown in the photo below.

This, of course, raises the problem noted above. If a pollen sac from one of the 15 orchid species is stuck to the bee’s body, how can it be guaranteed to pollinate the same species of orchid, for there’s no guarantee that the next flower the bee visits will be from the same species. (All the orchids are, after all, flowering at the same time.)

The answer is the cool part of the story. Each orchid has evolved to stick its pollen to a different part of the bee’s body. And each orchid has its female parts placed so that the pollinia from its own species, stuck to a specific place on the bee’s body, will contact it’s own species-specific style (the female bit that gets the pollen for fertilization). Thus cross-pollination is prevented by the specificity of where the pollinia stick to the bee and bu the specific position of the female part of each orchid, which has evolved so, that when the bee collects oil, the right pollen will land on the right stigma.

Paux found this out by identifying the different pollina of the flowers (they have different shapes), and trapping wild bees to see where the pollinia of each species was stuck to the body. That’s what’s shown in the figure above: each letter corresponds to the orchids depicted around the edges, and the arrows show where on the bee’s body the pollinia from each species are stuck. Notice that they’re all different. Except for two, that is: the pollinia from orchids b and c, which both stick to the foretarsi of the bee’s middle legs.

Does this mean there’s cross-pollination between orchids b and c, which would be bad? No, because the pollinia of these two species are of different length, and the stigmas of the two orchids are placed so that each will get the pollen from the right species.

This is a remarkable example of specificity in pollen placement; I know of nothing similar! You can see below, in “b” and “c” of Fig. 3, that the pollen are stuck to very specific parts of the body. In “b”, the pollen of the flower Pterogodium cathlocium get attached to the bee’s “basistarsi” on the middle legs (the most distal part of the large leg tarsi), while the pollinia of the orchid Pterygodium volucris get attached to the ventral surface of the bee’s abdomen. The pollen sacs on the flowers have to be in very different places to accomplish this, and the bee has to collect oil in a specific position to get the pollinia stuck to the right spot.

(From paper): Fig. 3. Rediviva peringueyi pollination mechanism. (a) Female R. peringueyi collecting floral oil from the apex of the lip appendage of Pterygodium alatum with a rapid rubbing motion of the front tarsi. The bee hangs onto the lip appendage with the middle tarsi, onto which the pollinaria (visible) become attached. Bar: 3 mm. (b) Several pollinaria of P. catholicum attached precisely to the basitarsi of the middle legs of R. peringueyivia the sticky viscidia. Bar: 1 mm. (c) Pollinaria of Pterygodium volucris attached to the ventral surface of the last abdominal segment of R. peringueyi. Bar: 3 mm.

Note that several types of evolution appear to be involved in this phenomenon:

a.) Convergent evolution of the different, unrelated orchids so that they develop a common scent, appearance, and “lip” that allows the bees to hang on while collecting oil.

b.) Divergent evolution of the orchids so that each evolves a lip and pollinia position that will stick its pollen to a previously uncolonized part of the bee’s body

c.) Possible evolution of the bee’s behavior so that it “knows” how to hold onto each species of flower to collect oil (this might not involve genetic evolution, but simply be due to learning).

So this is the cool way that fifteen different species of orchids can pollinate members of their own species, even if they’re all serviced by the same species of pollinator.  According to Pauw, though, this doesn’t solve the problem raised at the beginning: such specificity makes the whole system precarious—liable to collapse if anything happens to the pollinator. And indeed, he says that the degree of pollination of the orchid species vary strongly from year to year. So it goes.

Another aspect of this system is the possible extinction of the bee. In a sad ending, Pauw notes that the habitat for both orchid and bee is disappearing:

The biggest challenge in this study was the scarcity of suitable study sites. About 80% of lowland vegetation has already been transformed by urbanization and agriculture (Heijnis et al., 1999). What remains are scattered fragments of natural habitat, mostly less than 1 ha in size. In many of these fragments, the absence of R. peringueyi and repeated pollination failure in the entire guild was recorded. We have probably already lost the chance to understand the intriguing flowers of species such as P. cruciferum, which persists in fewer than five remnants of natural vegetation where they seldom, if ever, receive pollinator visits. In contrast with the pollination systems of the north temperate regions, which almost invariably involve several ecologically equivalent pollinator species (Waser et al., 1996Fenster et al., 2004), the pollination system described here is dependent on a single insect species. This presents a challenge for conservation because of the low level of ecological redundancy means that the loss of R. peringueyi may trigger linked extinctions amongst the plants in the R. peringueyi pollination guild. It seems unlikely that the R. peringueyi pollination guild will persist in a modern, cultural landscape without unique conservation planning.

If the bee goes extinct, so will every one of these orchid species, for their reproduction depends on the insects. There’s a lot more to study here, and I’m hoping that they’re trying to save some habitat for both plant and insect.  Since pollination itself has been observed in only about five of these orchids, there’s a lot more observational work to be done. Further, the DNA analysis of the orchids, indicating that they are not a “monophyletic group” (i.e., not each other’s closest relatives) was rather crude, and that needs to be done using more modern methods. If they are not each other’s closest relatives, then we have a new and solid case of “convergent evolution” (unrelated species developing very similar traits).

h/t: Martim

22 thoughts on “A remarkable case of pollinator/orchid coevolution and specificity

  1. Amazing adaptations! And an amazing piece of work by Pauw. What a patient and acute observer— positively Darwinian in acuity if you asked me. The specificity of these adaptations is incredible. Those who claim that natural selection is not “creative” or that it cannot produce complex structures ought to study this case.

    And, as illustrated here, natural selection isn’t perfect. It’s opportunistic. If that bee goes extinct, those orchids are in trouble. But—at least for now, in the opportunistic way that natural selection does its work—we have these incredible adaptations to behold.

    1. It’s not all doom and gloom: Some of the orchid species can persist after the loss of their pollinator by virtue of vegetative reproduction. They produce bulbils on the ends of their roots from which new plants spring — no pollinator needed. Only those species that lack this demographic back-up mechanism are predictably absent from small conservation areas where the bee has been extirpated.


  2. This is just SO cool! Thank you for the excellent summary and explanation. What a huge amount of difficult research went into uncovering this amazing mechanism

  3. A delightful tale! (I mean of course the good sense of that – as in, True Tales of Nature).

    How old comparatively are the bee and orchid family, generally – evolutionary time span wise? (I don’t know terminology on the spot).

    Example : the bee is X million years old, the orchids on average Y myo, so that’d mean the bee is younger… if that makes sense…

  4. This is great :

    “In P. catholicum, the pungent scent (described variously as ‘‘lemon,’’ ‘‘soap,’’ ‘‘wax,’’ ‘‘new car,’’ and ‘‘angry millipede’’) was found to be secreted primarily by the petals. ”

    New car smell! Now I know what those newer VW Beetles have on their dashboards – P. catholicum!

    I should put one on my bike, come to think of it…

    1. In auto supply stores, you can get a little spray can of New Car Smell. Handy to know when it’s time to sell the ‘ol car.

  5. I am glad to be sitting down. This is incredible. I am reluctant to suggest, but there may be non-native bee species that are attracted to the scent and form of these orchids. And if so, perhaps they could be introduced as a means to protect the flowers from local extinction.

  6. Fascinating! A sad conclusion to the story, though. Unfortunately, in a great many places around the world, nature is under severe stress from all kinds of anthropogenic pressures. Here in the UK the latest ‘State of Nature’ report has just been published and it shows that a cross a wide range of taxa including plants, the long-term population trends for may species in Britain continue to be downwards.

    Pauw highlights the particular vulnerability of this pollination guild to extinction from any cause but if we fail to do a better job of protecting nature than we have managed hitherto we risk wiping out many other wonderful products of evolution that have no obvious achilles heel beyond an inability to survive in an environment dominated by Homo sapiens.

    1. Sorry that should have said

      “…it shows that across a wide range of taxa including plants, fungi and vertebrate and invertebrate animals the long term population trends…”

  7. Clear explanation of an amazing example of divergent and convergent evolution coming together. Let’s hope habitat preservation saves the day for this wondrous bit of Nature.

  8. How easy extinction can happen and to many things at once. If I can throw in a bit of sarcasm here, isn’t this what g*d intended. Sure he did.

  9. That’s because if some environmental fluctuation or other contingency makes the pollinator rare (or even drives it extinct), the flowers wouldn’t get pollinated. This would imply that flowers should evolve to attract several species of pollinator, for those flowers that are generalists in this way are less likely to become rare or extinct themselves.

    I think this conundrum is easy to explain. Evolution can’t predict the future. If there is some evolutionary advantage to “betting the farm” on a single pollinator in the short term, it’s going to happen at least some of the time. Eventually, of course, the pollinator will go extinct and the pollinated will follow suit but evolution can’t “know” this in advance.

    Also, this state of affairs seems quite unstable to me. What if one of the species of orchid evolves an adaptation that makes it more attractive to the bees than the others? Would all the other fourteen species go extinct? Has this happened before?

  10. This is an astounding post. I am so fascinating by this delicate and finely tuned balance between the bees and the various pollens attaching to specific parts of the bee.
    Such a fragile relationship but so well engineered through evolution.

  11. A great story. Orchids are cool!

    Readers can learn more about orchid pollination in this 2004 article by my friend David Horak:


    The mechanisms described by Pauw are actually fairly common in orchids, though it is rare to find so many species sharing a single pollinator. David describes the phenomenon in his article: “The structural differences in the various flowers ensure that the pollinia are attached to a part of the bee specific to each orchid species: The pollinia of one may be attached to the insect’s eye, that of another to the top of the thorax, and that of still another to a foreleg. When the pollinia-loaded bee encounters an orchid flower, only the pollinia in the proper position for that species will come in contact with the stigma and accomplish pollination.”

  12. Fascinating, thanks!

    Side note: Is it just me, or are the pop-ups back?! On the bright side, our host’s tweets no longer seem to all be flagged as “sensitive content”, not even the Jesus ‘n’ Mo ones :o)

  13. Have you seen “Crime Pays But Botany Doesn’t” on YouTube? It’s by self taught botanist Joey Santone from Chicago. He uses words like “fuck” during his talks. He has, believe it or not a really good attitude and his stuff is informative and uplifting. He would get a big kick out of the bee and orchids paper.

  14. Just another proof that evolution (with its mechanism natural selection) is probably the greatest idea humankind has ever had. Think of how many avenues of enquiry open up because someone says, “I’ll bet natural selection explains that somehow,.” and seeks to find out the somehow.

Leave a Comment

Your email address will not be published. Required fields are marked *