Sun orchids are some of the most common and colourful orchids in Aotearoa New Zealand. Though their flowers are unusually simple for an orchid, their past is far from that. Postgraduate student Hayden Jones (Massey University/Te Papa) explains how old-school methods and new DNA technologies can help us understand the origin and diversity of our native sun orchids.
Thelymitra, an orchid named after a woman’s hat
New Zealand is home to over one hundred species of orchids. These come in a range of shapes and sizes, but arguably the most colourful are the sun orchids in the genus Thelymitra. Described by European naturalists Johan and Georg Forster (father and son), its scientific name makes reference to the fact the middle of the flowers looks a bit like a woman’s hat or turban. Their common name, on the other hand, makes reference to their flowers opening only when it is warm and sunny.
Sun orchids are also found throughout Australia, New Caledonia, and parts of Southeast Asia, and in total the genus is comprised of roughly 150 species. In New Zealand, there are 20 species, although there are some debates as to whether some species should be split up into several species, and whether a number of potentially new species are still waiting to be formally described.
As orchids go, Thelymitra is a bit unusual – forgoing the highly modified floral structures that most other orchid species have, in favour of flowers that are simple and relatively unmodified. Many species in this genus are also obligate self-pollinating, meaning that they will set seed regardless of if a pollinator visits them or not. This is another feature that makes this genus unusual, as most other orchids tend to have elaborate pollination strategies.
A crisscrossed history
Studies looking into the evolutionary history of sun orchids paint a complicated picture, although tend to agree that many species arose quite recently (within the last few million years, a short time in an evolutionary sense!). While it is not clear exactly what has driven the diversification of this group, one explanation is that some species originated from crosses between different species.
Natural hybridisation in Thelymitra is not a new idea, but arguably one of the most important contributions to it was made by New Zealand botanist Brian Molloy (1930-2022) in the 1990s. Molloy and collaborators looked at the physical characteristics of the suspected hybrids and counted their chromosomes (tiny structures found within cells where DNA is packaged up and stored) and concluded that at least six New Zealand species arose through hybridisation.
Molloy recognised that these hybrid species tended to have a mix of physical characteristics from their two parent species and a chromosome count that was the sum of each parent species. To confirm his hypothesis Molloy worked with another botanist, Doug McCrae, and artificially crossed two species (T. pulchella and T. longifolia) and produced plants that were consistent with a naturally occurring hybrid named Thelymitra x dentata, both in physical characters and chromosome count. A sample of the hybrid they have created is now at the Allan Herbarium (CHR) in Lincoln (near Christchurch) and its pictured below.
As part of my Masters thesis, I am investigating the relationships that Brian Molloy proposed surrounding the hybrid origin of some Thelymitra species in New Zealand. Specifically, I am looking for DNA evidence that supports his hypothesis. Work like this is important because it gives us insight into how these plants came to be and helps us decide what we should focus conservation efforts and funding on.
How can you spot a hybrid?
One characteristic true for all species, and not just hybrids, is the presence of DNA from both parents. The DNA from each parent may differ slightly from each other, with each different bit called an allele. If we look at a species that arose through a hybridisation, we might expect to find alleles that are very different from one-another as they have come from two completely different species. These could be compared against the alleles we find in the species we think might be the parents and we would expect them to be similar. This approach does not only allow us to find hybrid species, but to identify how they formed and who their parents are.
One way in which I can do this is by using nanopore sequencing. This is a relatively new technology that allows us to sequence all the DNA in a sample with relative ease. The exact specifics of this approach don’t matter too much, but for this application nanopore lets us separate the different DNA from each parent in a sample from one another very easily, and more importantly it is quite cheap!
Up close and personal with bundles of DNA
In addition to this molecular work, I am also interested in confirming the chromosome counts for the species of sun orchids with hybrid origin, as well as getting counts for some of the species this information is missing. Chromosomes are structures found within cells where DNA is packaged up and stored. Every cell (usually) has two copies of each chromosome, one from each parent. The number of chromosomes each species has in total varies, but in sun orchids most species have 26, 28, or 40.
Counting chromosomes is something that used to be very popular, but today this technique has largely fallen out of fashion and there not many people in New Zealand using it. Fortunately, after hours of practicing and testing, I have now learned this technique. So far I have managed to count the chromosomes of several sun orchids, including Thelymitra longifolia, one of New Zealand most common and variable sun orchids.
In the next couple of months I will focus on extracting DNA from samples of sun orchids both from New Zealand and Australia. Once this is finished, I will be able to study specific bits of their DNA and identify if they are hybrids too. This work will also include samples of the white sun orchid Thelymitra longifolia, which has been a focus of my previous work. This sun orchid comes in a range of shapes and sizes, and so it has been suggested that it might actually be several different species under one name. I want to confirm (or reject) this hypothesis by comparing the DNA of the different forms this species can take.
Acknowledgements
A special thanks to Te Papa Foundation and its generous donors for the Postgraduate Scholarship I have been awarded. This research has been partially funded by the Australasian Systematic Botany Society, and the Australia Pacific Science Foundation (Grant number APSF 19047). A huge thanks also goes to the New Zealand Native Orchid Group, New Zealand Orchid Society, Kew DNA Bank, the members of the public, and the staff of NCBI black mountain, for their help in sourcing samples in this project. Finally, I would like to thank my supervisors Dr Carlos Lehnebach (Te Papa) and Dr Jennifer Tate (Massey University) for giving me the opportunity to do this research.
Further reading
- Dawson, M. I., Molloy, B. P. J., & Beuzenberg, E. J. (2007). Contributions to a chromosome atlas of the New Zealand flora—39. Orchidaceae. New Zealand Journal of Botany, 45(4), 611-684. https://doi.org/10.1080/00288250709509743
- de Lange, P., & Rolfe, J. R. (2010). Illustrated guide to New Zealand sun Orchids, Thelymitra (Orchidaceae).
- de Lange, P. J., Rolfe, J. R., Barkla, J. W., Courtney, S. P., Champion, P. D., Perrie, L. R., Beadel, S. M., Ford, K. A., Breitwieser, I., Schönberger, I., Hindmarsh-Walls, R., Heenan, P. B., & Ladley, K. (2018). Conservation status of New Zealand indigenous vascular plants, 2017.
- Lehnebach, C. A., & Shepherd, L. D. (2020). DNA study helps to identify mystery winter-blooming sun orchid (Thelymitra) from Eastbourne. The New Zealand Native Orchid Journal, 159, 5-7.
- Molloy, B. P. J., & Dawson, M. I. (1998). Speciation in Thelymitra (Orchidaceae) by natural hybridism and amphidiploidy. The Royal Society of New Zealand Miscellaneous Series 48, 103-113.
- Nauheimer, L., Schley, R. J., Clements, M. A., Micheneau, C., & Nargar, K. (2018). Australasian orchid biogeography at continental scale: Molecular phylogenetic insights from the Sun Orchids (Thelymitra, Orchidaceae). Mol Phylogenet Evol, 127, 304-319. https://doi.org/10.1016/j.ympev.2018.05.031
Hi Hayden, it is great that the work you are involved in continues in NZ. I was interested in your note in the legend below Molloy’s hypothetical ancestry. I am wondering how a cross between T. formosa and T. longifolia could give rise to T. hatchii, whose chromosome #, as you put it, is the sum of the parents’. Is that a failure of meiosis (parents) or chromosome doubling (T. hatchii)?
Have you, in addition to chromosome counts, looked at the density and size of stomata of the different types/species? Cheers,
Don
Hi Don, thanks for your comment. While both situations are possible and would produce similar results, in the original publication by Molloy he suggests that it may have been a chromosome doubling in the F1 cross. It would be interesting to see if we could find evidence for either case, however I’m not sure how we’d go about that. I have not looked at stomata, that could be something that we look at next flowering season though. Cheers, Hayden