Sex, Lies and Orchids

This post is primarily about a technical talk given to the Plant Science group of the Friends of the ANBG.  I found it an engrossing talk and will try to summarise it here for the benefit of others.

On arrival we were given yellow ribbons in recognition of it being Wattle Day.

The background story is that the City of Hiroshima, Japan has a Wattle Appreciation Society, reflecting a wattle being the first blossom seen after the atom bombing of the city.  Every 1 September since 1992 they have provided 1000 ribbons to the Gardens.  They were originally made by children at an orphanage but are now made in primary schools.  The media release says to put the ribbon on a favourite tree so here it is on our huge Yellow Box (Eucalyptus meliodora).

In getting to the talk, it is crucial to note that while the presenter of the talk was Professor Rod Peakall from the  ANU College of Medicine, Biology and Environment what follows is my interpretation of his points and not necessarily his views.  I do hope I haven't misrepresented hims points too badly as they were extremely interesting. 

Rod went to pains to point out that a large number of other folk, in various institutions around the world,  have participated in this work.  The presentation included many great photographs and astonishing video clips but this will be largely a textual summary of the talk.  Apart from anything else (for example, politeness) I was too engrossed - and busy taking notes - to take screengrabs during the talk.  However this image was up before the talk:

It shows the range of organisms which the unit works on.  Orchids have not - as far as known to me -colonised Hump-backed Whales!

A starting point was to note the diversity of orchids: some people regard them as the most diverse family in Australia while others rate them second in number of species to Asteraceae (and I'm  not sure how that would vary depending on one's view of nomenclature wars in the orchidverse).  Plants which are insect pollinated need to follow one of three strategies to attract insects and get fertilised:

1. Food provision: the plant pays off the insect with food - nectar;
2. Food deception: they appear to offer food but don't deliver.  This is about 1/3rd of orchids and includes Diuris.
3. Sexual deception: they appear to offer a chance of sex, but don't deliver.

Group 3 are the topic of the talk.  The strategy is followed by some plants that are not orchids, and has been observed in most parts of the world except sub-Saharan Africa (although known in South Africa) and much of Asia. [Comment by MB: possibly this reflects the distribution of biologists as much as distribution of the phenomenon.]

The phenomenon was discovered in the 1930s  by Edith Coleman noting that Ichneumonid wasps  pollinate Cryptostylis sp.  As she was a polite lady her papers apparently didn't mention the words 'sex' or 'copulation' which must have been quite a challenge in the circumstances, and 'pseudocopulation' only appears to have been invented as a consequence of her work.  (Much of her material was published in "The Victorian Naturalist " but their on-line archive appears to be a work in progress so I haven't been able to find an electronic example of her actual words.  She also published internationally but I don't have a copy of them either, and the Vic. Naturalist work seems to be the most cited in biographical material. )

The genus covered at the start of the talk was Chiloglottis.  This is a sample of C. trilabra .. 

.. which shows what the genus looks like.  All Chiloglottis practise sexual deception (except C. cornuta from NZ and sub-antarctica which self pollinates)..

The species of wasps seemed to be highly specific to the species of orchid, although it has recently been discovered that some wasps in WA will have a shot at two species of orchid, and some Chiloglottis are now known to share pollinators.

There are two possible ways the orchids could persuade a wasp that they were a hot prospect for breeding:

• Visual cues (ie mimicry); or
• Olfactory cues (eg generate pheromones).

It was found by Edith Coleman that if the orchid flowers were covered with stockings - so the flowers were invisible but air could still circulate- the wasps still turned up.  Thus it appeared that the olfactory cues were the main process, although if looked at with the correct mind set the labellum of the orchids did look somewhat like the female wasp (and were presented in such a way that they resembled the pose of an 'interested' female wasp).

OK that is the (relatively) easy part.  There are 1700 chemicals in odours from flowers.  Which of these presses the hot buttons for a male Thynnine wasp?  That was worked out by feeding the odours from a Chiloglottis into a gas chromatograph while simultaneously observing reaction from wasp antennae (through which scent signals are processed).  At points in the process both gave a response.  Through very technical "chemist business" the correct chemical was identified.  This was then confirmed by field tests in which the wasps went beserk doing the business - or least trying to - with a small black bead impregnated with the chemical.

This only took 5 years work by a multidisciplinary team across the world.  For one species each of wasp and orchid!

The chemicals concerned are now known as Chiloglottones and represent a new class of chemical.  This is equivalent to discovering a new Family of Plants and quite justifiably got folk a bit excited.   The process of synthesising a chiloglottone (Chiloglottone 4) is described in a blog post and summarised in a diagram on that post and reproduced below. 

The process is referred to as simple, but is way beyond my comprehension.  However the orchids seem to have no trouble in generating it through their calli (with a need for sunlight).  Some 7 chiloglottones have been identified and in some cases it has been found that like a good red wine a blend is what works.

Work has also been undertaken on Drakaea, a WA genus of orchids.  I don't have a personal image of same, so have acquired this of Drakaea glyptodon from the Atlas of Living Australia.

The common name of this genus is 'Hammer Orchids' and when a wasp lands on the labellum the wasp becomes pressed against the column before flying off laden with pollen.  Again a chemical has been identified but in this case it is a pyrazine - with no oxygen atoms but some Nitrogen.  Here is its structure:

Unlike chilogottones, pyrazines are widespread.

Another species emits a compound with sulphur in the molecule, but about this point my brain started to run up a white flag and notes became fewer.  The final element was about a Caladenia - possibly C. pectinata [MB comment: still without wishing to get too deeply into  nomenclature wars, in many places East of the Nullabor that would be known as Arachnorchis pectinata as it is a spider orchid.] which is pictured below

The interesting bit of this is that the chemical is produced in the tips of the sepals rather than the labellum.

There is currently no information as to how all these mechanisms have evolved.  Working that out will take time - 10+ years!

BTW if you were lured here by the first word in the Subject: sorry.  But at least you were interested enough to get this far!