Warty hammer orchids are sexual deceivers



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The Conversation, CC BY-SA

Ryan Phillips, La Trobe University

Orchids are famed for their beautiful and alluring flowers – and the great lengths to which people will go to experience them in the wild. Among Australian orchids, evocative names such as The Butterfly Orchid, The Queen of Sheeba, and Cleopatra’s Needles conjure up images of rare and beautiful flowers.

Yet there is a rich diversity of our orchids. Some are diminutive, warty, and unpleasant-smelling, bearing little resemblance to a typical flower.




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While many orchid enthusiasts have a soft spot for these quirky members of the Australian flora, what has brought them international recognition is their flair for using some of the most bizarre reproductive strategies on Earth.



The Conversation/Ryan Phillips/Suzi Bond., CC BY

Sexual mimicry

From the very beginnings of pollination research in Australia there were signs that something unusual was going on in the Australian orchid flora.

In the 1920s Edith Coleman from Victoria made the sensational discovery that the Australian tongue and bonnet orchids (Cryptostylis) were pollinated by males of a particular species of ichneumonid wasp attempting to mate with the flower.

But this was just the beginning.

The King-in-his-carriage, Drakaea glyptodon, is the most common species of hammer orchid. Here the flower is pictured next to the female of its pollinating thynnine wasp, Zaspilothynnus trilobatus.
Rod Peakall, Author provided

We now know that while the insect species involved may vary, many of our orchid species use this strategy. Australia is the world centre for sexual deception in plants.

Perhaps the most sophisticated flower of all sexually deceptive plants is seen in the hammer orchids, a diminutive genus that only grows in southwestern Australia. Their solitary stem reaches a height of around 40cm, and each stem produces a single flower no more than 4cm in length.

Even among sexually deceptive orchids, hammer orchids stand out from the crowd. They have a single heart-shaped leaf that sits flush with the soil surface, and grow in areas of dry inhospitable sand – an unusual choice for an orchid.

The thynnine wasp Zaspilothynnus nigripes is a sexually deceived.
pollinator of the Warty hammer orchid. Here they are pictured in copula, with the
flightless female having been carried to a food source by the male.

Keith Smith, Author provided

And then there is the flower. Not only does the lip of the flower more closely resemble an insect than a petal, but it is hinged partway along. All of which starts to makes sense once you see the pollinators in action.

Like many other Australian sexually deceptive orchids, they are pollinated by thynnine wasps – a unique group in which the male picks up the flightless female and they mate in flight.

In the case of hammer orchids, the male grasps the insect-like lip and attempts to fly off with “her”. The combination of his momentum and the hinge mechanism swings him upside down and onto the orchid’s reproductive structures.

It’s not me, it’s you (you’re a flower)

So, how do you trick a wasp?

Accurate visual mimicry of the female insect does not appear to be essential, as there are some sexually deceptive orchids that are brightly coloured like a regular flower.

Instead, the key ingredient for attracting pollinators to the flower is mimicking the sex pheromone of the female insect. And boy, is this pheromone potent.

Indeed, one of the strangest fieldwork experiences I’ve had was wasps flying through my open car window while stopped at traffic lights, irresistibly drawn to make love to the hammer orchids sitting on the passenger seat!

Pollination of the Warty hammer orchid by a male of the thynnine wasp Zaspilothynnus nigripes.
Suzi Bond, Author provided

While determining the chemicals responsible for attraction of sexually deceived pollinators is a laborious process, we now know that multiple classes of chemicals are involved, several of which were new to science or had no previously known function in plants.

What’s more, we are still discovering new and unexpected cases of sexual deception in orchids that don’t conform to the insect-like appearance of many sexually deceptive orchids.

A classic example is the case of the Warty hammer orchid and the Kings spider orchid – these two species have totally different-looking flowers, yet both are pollinated by the same wasp species through sexual deception.

While the ability to attract sexually excited males without closely resembling a female insect may partly explain the evolution of sexual deception, it does not explain the benefit of evolving this strategy in the first place.

A leading hypothesis for the evolution of sexual deception is that mate-seeking males be more efficient at finding orchid flowers than food-foraging pollinators – but this remains a work in progress.

The life cycle of the Warty hammer orchid and its pollinator species,
highlighting the complex ecological requirements needed to support a population of.
the orchid.

Martin Thompson, Author provided

From a conservation point of view, pollination by sexual deception has some interesting challenges. Female animals produce sex pheromones that only attract males of their own species. This means an orchid that mimics a sex pheromone typically relies on a single pollinator species. As such, conservation of any given orchid species requires the presence of a viable population of a particular pollinator.

Further, an interesting quirk of these sexually deceptive systems is the potential for cryptic forms of the orchid: where populations of orchids that appear identical to human observers actually attract different pollinator species through shifts in pheromone chemistry. Indeed, of the ten known species of hammer orchid, three contain cryptic forms.




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Not only does this create a major challenge for managing rare species, it raises the possibility that – should these forms prove to be separate species – the true diversity of sexually deceptive orchids could be greatly underestimated.


Sign up to Beating Around the Bush, a series that profiles native plants: part gardening column, part dispatches from country, entirely Australian.The Conversation

Ryan Phillips, Senior Lecturer in Ecology, Environment & Evolution, La Trobe University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Leek orchids are beautiful, endangered and we have no idea how to grow them



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Marc Freestone/The Conversation

Marc Freestone, Australian National University

Sign up to Beating Around the Bush, a series that profiles native plants: part gardening column, part dispatches from country, entirely Australian.


Leek orchids don’t have many friends. Maybe it’s because they lack the drop-dead gorgeous looks of many of their fellow family members. Or perhaps it’s because they’re always the first to leave the party: as soon as sheep or weeds encroach on their territory, they’re out of there. Whatever the reason, you don’t see leek orchids around very often.




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Leek orchids are small, ground-dwelling native Australian orchids, so called for their single spring-onion-like leaf, which shoots up from an underground tuber each autumn. In the spring, if there’s been enough rain, they produce a spike of small brown, green or white flowers.

Like many native orchids, they are battling extinction. My research involves trying to find the secret to propagating them – something we still don’t fully understand.


Marc Freestone/The Conversation

Extinction

Australia is quite rich in orchids with more than 1,300 native species (by contrast, there are only about 200 species in all of North America). About 140 of these are leek orchids, and most live in bushland remnants across the south of Australia.

With a preference for fertile soils and relatively high rainfall, these little plants suffered severely during the period of agricultural expansion in the southeast of the country during the first half of last century. Rabbits, weeds, inappropriate fire regimes, and declining rainfall patterns continue to plague those that survive, which often hang on in narrow roadsides, beside rail lines or in rural cemeteries – tiny pockets of land that were never ploughed.




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Almost one-third of all leek orchid species are at risk of extinction. Some are already extinct, such as the Lilac Leek-orchid (Prasophyllum colemaniae). It once grew in Melbourne’s outer eastern suburbs, but disappeared when an upgrade of a rail line in the 1970s destroyed the last population. Standing half a metre tall, with fragrant purple-white flowers, it was said to be the most beautiful of all leek orchids.

Collecting seed in the Alpine region.
Marc Freestone

Native orchids are rebounding – but not leek orchids

Fast forward to 2018 and things have changed. The Royal Botanic Gardens Victoria now hosts the largest orchid conservation program in the world. Dozens of critically endangered native orchids from the southeast mainland are being brought back from the brink of extinction through propagation and reintroduction programs.

But not leek orchids.

That’s because we still don’t know how to grow leek orchids successfully. In fact, growing any type of orchid is hard work. For a start, orchid seed is microscopic. It is so small it doesn’t contain any food for the germinating orchid seedling.

Instead, all orchids rely on symbiotic fungi that live in their roots and the surrounding soil and are required to inoculate the orchid seed – the fungus literally pumps food into the seeds to get them to germinate. We have no idea why these fungi do this, but we can replicate this scenario in the lab by carefully extracting fungi from the roots of a wild orchid plant, growing the fungi in a petri dish, and sprinkling in the orchid seed. But for some reason, leek orchid seed rarely germinates, and if it does, the young seedlings usually brown off and die.

Three month old baby leek orchid seedlings. Of the few seeds that germinate, most won’t survive past this point.
Marc Freestone

How to grow leek orchids is the subject of my PhD project with the Australian National University, based at the Royal Botanic Gardens Victoria. We have many theories about what might be going on and we’re looking at seed viability, growing conditions, and the relationship between leek orchids and their symbiotic fungi.

It’s a race against time to work out how to grow them before more species – like the Shelford Leek-orchid (Prasophyllum fosteri) from western Victoria, which is now down to only a handful of wild plants – go extinct.

Why should we care?

At first glance, leek orchids do not appear to be particularly useful for anything. They can’t cure cancer or be traded for Bitcoin. So who cares if they go extinct?

There are only a few hundred coast leek orchids remaining.
Marc Freestone

Well, the first point is we don’t know enough about leek orchids to be able to conclude that they are indeed completely useless to the human race. Second, leek orchids probably used to play an important ecosystem role in the lowland grasslands of southeastern Australia.

Up in the Australian Alps there are several species of leek orchid that are still very common, their flowers providing an important food source for insects. Seeing the massed flowering of the Alpine Leek-orchid (Prasophyllum tadgellianum) in summer really gives you a feel for what the lowland grasslands would have been like once upon a time, when species like the Gaping Leek-orchid (Prasophyllum correctum) would have numbered in the millions. Now there are perhaps 10 plants left.

If it goes extinct, Australia will have lost part of what makes it unique. A small part, perhaps, but when added to all the other threatened species in this country, a significant part.




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Personally, I find leek orchids delicate and utterly defenceless against humans, who have engulfed their world. Ironically, some species are now totally dependent on us for their survival. I feel a great sense of responsibility to help them.

Sign up to Beating Around the Bush, a series that profiles native plants: part gardening column, part dispatches from country, entirely Australian.The Conversation

Marc Freestone, PhD Candidate, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.