A little Christmas treat for you: lie back and enter a world of passion and deception, as orchid expert Prof Richard Bateman takes over the BSBI News & Views blog to ponder the enduring appeal of bee orchids
that achieve pollination through sexual deceit. All images are courtesy of Prof Bateman.
You've recently been peppering the scientific literature
with articles about bee orchids; what's so special about them?
Well, orchids always attract a lot of attention, but you'd
be hard-pressed to find a more charismatic group of orchids than the bee
orchids (Ophrys). They look bizarre, but once you learn more about their
biology in general and their reproductive biology in particular you realise
just how bizarre they really are. And they're widespread across Europe – it's
impossible to take a spring holiday in the Mediterranean without bumping into
at least one kind of bee orchid on your travels.
"Reproductive biology"? You mean plant sex? How
can that possibly be exciting?
The way that bee orchids are pollinated is so extraordinary
that even Charles Darwin reputedly refused to believe it! Male solitary bees
emerge in the spring and immediately start looking for a suitable female with a
view to starting a family. But the females emerge later than the males, so all
a promptly arriving male can find to cool his ardour is a flower with one petal
– the lip – that resembles his true paramour. So he attempts to mate with the
lip instead.
So that's what you meant by 'sex between kingdoms'? But even
male bees can't be that stupid?
You have to understand the effort that evolution has
invested in making the orchid sufficiently attractive to the bee. Perhaps most
remarkably, the flower exudes a cocktail of chemicals closely resembling that
emitted by the female insect to attract a mate – chemicals called pheromones that
can be detected from a considerable distance. Then the complex patterns and
often highly reflective wing-like surfaces of the orchid flower act as a
beacon, encouraging the insect to home in on the lip. And once the bee has
landed, the diverse surface textures of the lip persuade the naïve male that it
is indeed gripping its one true love. During the resulting sexual frenzy the
orchid glues pollen masses firmly onto the bee (while on some occasions the bee
retaliates by ejecting sperm onto the lip!).
And that's how the orchid is fertilised and produces vast
numbers of seeds? How clever.
Well, no, not that clever, actually. Transferring pollen
from one orchid plant to another means that the bee has to be fooled not once
but twice. You'll recall the old adage "fool me once, shame on you, fool
me twice, shame on me." And it's doubtful that the insect benefits from this
X-rated relationship – it is in practice being parasitised by the orchid! Also,
the orchid is sufficiently precisely adapted that it attracts very few kinds of
insect, so its range of suitors is somewhat limited. Consequently, fewer bee
orchid flowers set seed than in almost any other kind of orchid. Producing
nectar – or even just pretending to produce nectar – to feed the visiting
insect is far more effective. Even for a bee, dinner can trump sex.
Just how faithful is the relationship between a bee orchid
and its chosen insect 'mates'?
Ah well, this is where the current debates begin. It is
certainly true that a bee orchid attracts far fewer kinds of insect than a
typical orchid that provides a nectar reward. But in my opinion, some orchid
researchers have themselves become so seduced by the elegance of this 'pseudo-copulatory'
pollination mechanism that, for them, observing a particular kind of insect
visiting a particular kind of bee orchid means that, by definition, they have discovered
a new species of bee orchid.
You sound sceptical...
Well, the detailed genetic work pursued in institutes across
Europe tells us much about the genealogy of the orchids we study, including who
their ancestors were and, to some extent, what kind of life those ancestors
experienced. As with human genealogy, we can identify mothers, fathers and more
distant relatives, whereas a few visits to the orchid flower from a particular
kind of insect represent the most transient kind of scientific observation.
Certainly, we've gathered enough genetic information to be confident that
'mistakes' are often made by bees who attempt to mate with two different
supposed species of bee orchid and so transport pollen masses between them. Such
mis-matings blur the boundaries between the supposed species.
You keep saying "supposed" species? Now you're
sounding very sceptical!
Well, researchers who classify and name species often
disagree about the number of species that should be recognised within a
particular group. But for bee orchids the arguments have become exceptionally
polarised. It is clear that only nine widespread groups are reliably
genetically distinct, yet there exist monographs that illustrate more than 350
supposed species of bee orchid, many of which are confined to very small areas
of Europe such as single small Mediterranean islands. My colleagues and I
believe that current evidence is far too weak to justify viewing these local
variants as species.
Well, I guess I see your problem, but isn't this an 'angels
on the head of a pin' argument, of no real practical importance?
If only this was a purely intellectual exercise. But if we
were to agree that there are at least 350 species of bee orchid it would mean
that they represent about half of all orchid species found in Europe and Asia
Minor. Combine this over-inflated species number, the rarity of many of those
supposed species, and the unparalleled charisma of bee orchids and you inevitably
create a massive draw for conservation attention. But if these local variants
are indeed not real species, they should not be allowed to draw scarce
conservation resources away from other, less charismatic orchids (or indeed,
other kinds of plant) – genuine species that are in truth far more deserving of
conservation.
So there are ethical considerations as well as scientific
ones?
Absolutely. This fact was brought home to me most vividly
when studying orchids on a small island in the Aegean. The mayor of one of the towns
on the island offered me attractive 'inducements' if I were to state publicly
that the bee orchids in his district were species unique to his island! Clearly,
he had become aware that orchids in general, and bee orchids in particular, are
a significant draw for spring tourism, as orchids attract the same kind of obsessive
attention as birds. Box-ticking enthusiasts like many kinds of birds or orchids
to be recognised and named by taxonomists so that they have a good excuse for
tracking them all down to their lairs.
In that case, why not just give the public what they seem to
want?
In my eyes, the extensive variation in appearance and
remarkable pollination mechanism of bee orchids are no less interesting if they
are viewed as actively evolving, rather than having yielded numerous finished products
in the form of genuine, stable species that can readily be 'discovered' and
immediately awarded formal names. It would be far better if naming of plants
were to be prevented until adequate scientific evidence has first been gathered.
I admit that in practice scientific research never achieves its goal of being
entirely objective, and I recognise that in recent years pressures –
particularly those involving funding – have increased on scientists to
'discover' particular results that were desired from the outset. But even
speculations should be rooted in substantial bodies of relevant information,
otherwise the entire scientific enterprise would become pointless and we could
instead simply adhere to lazy, unsubstantiated beliefs.
Acknowledgements
My research interest in bee orchids was sparked by a welcome
grant from the John Spedan Lewis Foundation, which provided my former student Dion
Devey with his doctorate. The more recent NGS-based studies benefitted greatly
from the expertise and drive of Gabór Sramkó (University of Debrecen) and
Ovidiu Paun (University of Vienna).
Further Reading
Bateman, R. M. (2018). Two bees or not two bees? An overview
of Ophrys systematics. Berichte aus den Arbeitskreisen Heimische Orchideen 35:
5–46.
Bateman, R. M., Sramkó, G. & Paun, O. (2018).
Integrating restriction site-associated DNA sequencing (RAD-seq) with
morphological cladistic analysis clarifies evolutionary relationships among
major species groups of bee orchids. Annals of Botany 121: 85–105.
Breitkopf, H., Schlüter, P. M., Xu, S., Schiestl, F. P.,
Cozzolino, S. & Scopece, G. (2013). Pollinator shifts between Ophrys
sphegodes populations: might adaptation to different pollinators drive
population divergence? Journal of Evolutionary Biology 26: 2197–2208.
Claessens, J. & Kleynen, J. (2011). The flower of the
European orchid: form and function. Published by the authors, Voerendaal.
Darwin, C. (1862). On the various contrivances by which
British and foreign orchids are fertilised by insects. Murray, London.
Delforge, P. (2016). Orchidés d'Europe, d'Afrique du Nord et
do Proche-Orient, 4th edn. Delachaux et Niestle, Paris.
Devey, D. S., Bateman, R. M., Fay, M. F. & Hawkins, J. A.
(2008). Friends or relatives? Phylogenetics and species delimitation in the
controversial European orchid genus Ophrys. Annals of Botany 101: 385–402.
Pedersen, H. & Faurholdt, N. (2007). Ophrys: the bee
orchids of Europe. Kew Publishing.
Sedeek, K. E. M., Scopece, G., Staedler, Y. M.,
Schönenberger, J., Cozzolino, S., Schiestl, F. P. & Schlüter, P. M. (2014).
Genic rather than genomewide differences between sexually deceptive Ophrys
orchids with different pollinators. Molecular Ecology 23: 6192–6205.
