Summer scholar turns the spotlight on native tree-perching orchids

Summer scholar turns the spotlight on native tree-perching orchids

Because of their modest flowers and small size, New Zealand epiphytic orchids are rarely talked about and even more rarely studied. This is about to change as summer research intern Joe Dillon (Victoria University of Wellington) spends his summer at Te Papa and Ōtari Wilton’s Bush native botanic garden researching an orchid that’s struggling to survive.

There are more than 100 species of orchids in Aotearoa New Zealand, but only a few are epiphytic. These epiphyte orchids live perched on the trunks and branches of shrubs and trees. Sometimes they are also attached to large rocks and found hanging from cliff faces.

Epiphytic orchids are represented in Aotearoa New Zealand by nine species in five genera. Drymoanthus is a small genus of four epiphytic orchids, two of which are endemic to Aotearoa New Zealand (D. adversus and D. flavus). The other two species are endemic to Australia (D. minutus) and New Caledonia (D. minimus). The name Drymoanthus comes from the Greek words drymos, meaning forest, and anthos, meaning flower. This name was created in 1943 by the Australian amateur botanist William Henry Nicholls (1885-1951) and refers to the preferred habitat of the genus.

First collections and the stories behind a name

What we now know as Drymoanthus adversus was first collected by European botanists Joseph Banks and Daniel Solander during Captain James Cook’s first voyage to Aotearoa New Zealand more than 250 years ago. Solander created a name for it then (Epidendrum adversum), but this was never validly published, so the species went without a name for almost a century.

Specimens of the Drymoanthus adversus (Hook.f.) Dockrill collected by Joseph Banks and Daniel Solander in 1769 in New Zealand. The labels indicate taxonomic changes this orchid has gone through over the years. CC BY 4.0. Te Papa (SP063870)

Finally, in 1853, Joseph Dalton Hooker created a new name for this species which was published in a book, and it is therefore considered a valid name. He called the orchid Sarcochilus adversus. The epithet adversus comes from a Latin word meaning “turned towards”, probably referring to the habit of the orchid bending towards light. Later, when Nicholls created the genus Drymoanthus for the Australian species D. minutus, he included our Sarcochilus adversus in this genus too. To do this he had to change its name to Drymoanthus adversus, a name that has stayed the same since.

Drymoanthus adversus growing on mingimingi (Leucopogon fasciculatus) near Wellington, New Zealand, 21 December 2021. Photo by Joe Dillon

Drymoanthus adversus remained the sole endemic species recognised in Aotearoa New Zealand until 1994, when a previously undescribed taxa that had been included within D. adversus was described by NZ orchidologists Brian Molloy and Ian St George and named Drymoanthus flavus. The species was named after its yellow flowers (flavus means pale yellow), and is usually smaller than D. adversus, with purple spotted leaves. The flowers of D. adversus are green with purplish flecking and there are rarely spots on the leaves.

Drymoanthus flavus growing on kāmahi (Pterophylla racemosa) near Wellington, New Zealand, 8 November 2022. Photo by Joe Dillon

Half the genome and half as common

Researchers have counted the number of chromosomes in our Drymoanthus species. Chromosomes are tiny structures inside the cells that carry genetic information. They found that D. flavus has 38 chromosomes while D. adversus has twice that, 76! This means that at some point in the past, a dramatic event called a genome doubling, occurred. This difference may seem insignificant, but it makes reproduction between the two species impossible.

For some reason, D. adversus is much more abundant than D. flavus, being common throughout most of the North Island and upper South Island. It seems to be adapted to a wider range of environments, and many tree hosts. Drymoanthus flavus however, is very sparse. It is absent from the northern end of the North Island, but extends to the far south of the South Island, where it is more common than D. adversus.

Distributions of Drymoanthus adversus (A) and D. flavus (B) based on herbarium records from the Australasian Virtual Herbarium. AVH (2023). The Australasian Virtual Herbarium, Council of Heads of Australasian Herbaria, https://avh.chah.org.au, accessed 20/01/2023

In terms of conservation, Drymoanthus adversus is classified as Not Threatened, which means it’s doing okay. Some populations of D. flavus however are believed to be declining, the reason for which is not really known. Poaching has largely been implicated as the cause, but other suggestions include anything from deer and possum browse, to climate change-induced drought, to pollination failure.

Currently, D. flavus is classified as At Risk- Declining. This means there are less than 20,000 plants of this species in the wild across Aotearoa New Zealand and it is expected this number will decline between 10-30% in the coming years. Clearly, we need to find out exactly what the problem is, and work out insurance in case it gets worse, or even better, solutions.

How to save an orchid

Orchids stick out as some of the most specialised and unusual plants. They require specific fungal partners for their seeds to germinate, often have very restrictive environmental requirements, and may have highly specific interactions with pollinators. This all makes conserving them, or even growing them, very difficult. As part of my Summer Research Scholarship, I will be working at Te Papa’s Herbarium and DNA Lab, Ōtari Wilton’s Bush Plant Conservation Lab and a patch of native forest near Wellington researching each of these aspects.

Joe photographing Drymoanthus flavus plants perching on the trunk of kāmahi (Pterophylla racemosa) and the moss and lichens growing next to them. Photo by Carlos Lehnebach, Te Papa

Orchid seeds are quite bizarre. If you imagine a bean, only a small amount of it actually becomes the plant – this bit is called the embryo, which is essentially some very young roots (called a radicle) and a growing tip (called a plumule). The vast majority of it is taken up by two large seed leaves (called cotyledons). The cotyledons contain all of the food the embryo needs to grow and form its first leaves and roots. This is the way the vast majority of plant seeds work, however, orchids have a different playbook.

Orchid seeds are so small that they contain no cotyledons, and therefore have no stored source of food. Even their embryo is reduced to the point that it has no radicle or plumule – really it’s just a very small blob of cells. This presents some obvious challenges, but small seeds can be made very cheaply, which allows some orchids to create literally millions of them (one species from overseas can produce up to 4 million seeds per flower). Their small size also allows them to travel far on the wind.

Thousands of tiny orchid seeds collected from one orchid capsule (fruit) to be used in germinations experiments at the Plant Conservation Lab at Ōtari Wilton, Wellington, New Zealand. 12 December 2022. Photo by Joe Dillon

To overcome the challenges presented by their limited resources, orchids rely on specialised fungi (orchid mycorrhizal fungi or OMF) which provide the embryo with the energy and nutrients necessary to germinate. So far, the OMF of only a handful of our terrestrial orchids has been studied. However, we know nothing about our epiphytic species, including Drymoanthus. If we are to ever cultivate Drymoanthus for conservation, identifying and isolating the right fungal partner is indispensable

Fungal isolation and identification

Fungi are able to interact with orchids by growing microscopic coils of fungal hyphae inside the orchid roots. These coils are called pelotons. One of the main goals of my summer scholarship is to learn how to extract these pelotons and culture the fungi.

In the earlier half of my scholarship, I learned how to do this in other orchids. The first step is to carefully slice off the surface of the root and scrape the inner layer out with a scalpel. The pelotons come loose and can be sucked up with a pipette and placed on plates with a nutritious medium. We use a special kind of substrate called fungal isolation media (FIM), which is specially formulated to grow most fungi.

Agar plates with fungi isolated from Drymoanthus flavus growing at the Plant Conservation Lab at Ōtari Wilton, Wellington, New Zealand, 2022. Photo by Jennifer Anderton-Moss

Contamination by bacteria and the wrong fungi (like mould) can become a problem on the agar, so we do this step in a sterile room in the Ōtari lab. This room uses hospital-grade machines to blow sterile air at our workbench, preventing unwanted spores from landing on our samples. Then we wait.

If the fungi grows, we can subculture it and isolate DNA for sequencing and identification. We also use some of it to inoculate seeds of our orchids of interest. If the fungi interacts with the seeds and they germinate, we know we’ve found the right fungal partner. If not, then we’re back to square one and have to either try different fungi, or try again the whole process under different conditions.

Finding the perfect habitat and pollinators

The ultimate goal of my project is to find a method to allow us to grow these orchids from seed and transplant them into the wild. To succeed in this goal, however, we first have to understand their habitat preferences and identify what might be the factors causing decline and limiting their distribution.

So far, I’ve spent several days in the bush measuring environmental conditions around some plants to understand for example their light requirements. Separately, I’ve examined the moss, liverwort, and lichen communities on their host trees, to try to understand if they have specific micro-habitat requirements related to them.

The blue arrows indicate seedlings of Drymoanthus adversus growing on the trunk of mingimingi (Leucopogon fasciculatus) and Joe’s fingers as scale. Photo by Carlos Lehnebach, Te Papa

I have also conducted a few pollination experiments to determine whether these orchids require a pollinator in order to create seeds. During these preliminary observations, we’ve found a tiny wasp, likely in the Dendrocerus genus visiting a D. adversus flower – a promising lead, but likely too small to pollinate the flower.

Wasp observed visiting flowers of Drymoanthus adversus and feeding on nectar. This wasp is likely a member of the genus Dendrocerus. Photo by Joe Dillon

The answers to these and many other questions will help us to eventually grow these orchids at Ōtari and reintroduce them to the wild, allowing us to support existing declining populations, or insure the futures of populations which might disappear. Over the course of my master’s, starting this year, I’ll be looking into these and other questions.

A special thank you has to go out to Carlos Lehnebach and Karin Van Der Walt for supervising this summer project, and Jennifer Alderton-Moss for showing me the ropes in the lab. I’d also like to especially acknowledge the Deane Endowment Trust for funding my research scholarship, without whom this would not have been possible. Research on these two species is funded by a grant through Te Tahua Taiao Nga Taonga Lotteries Environment and Heritage Fund.

References

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