Introducing… Thelymitra

A very “un-orchid like” orchid

Who is mimicking who? The two Thelymitra species are the mimics.

In my last blog, I introduced the concept of floral mimicry and left you with the tantalizing question: Who are the models and who are the mimics?

I am pleased to introduce you to Thelymitra, an orchid genus packed full of mimics.

Thelymitra ixioides

The genus Thelymitra contains terrestrial deciduous orchids that are widespread across Australasia.

Notable features of Thelymitra include a strongly actinomorphic perianth (all the tepals are the same colour and shape) and that the genus lacks distinct labellum ornamentation that so many other orchid genera possess.

All the tepals of Thelymitra look similar compared to other orchid genera, which tend to have a highly modified main tepal.

It is considered an unusual genus within the Orchidaceae, largely due to the prevalence of this very “un- orchid like” morphology (Dressler 1981).

The distinguishing feature used to characterise between species of Thelymitra relates to the central column structure. The column is produced from the fusion of the style, two staminodes and a single fertile anther.

Thelymitra aristata flower and column structures. P = where the orchid pollen is, S = stigmatic surface. Don’t worry about the other labels. All photos by Freya Thomas.

Most species have flowers that are blue/purple, an unusual colour for terrestrial orchids though a few distinct colour morphs exist such as red and yellow (Jones 2006).

Thelymitra iconically display nastic movements, which has influenced the common name for this genus of ‘Sun Orchids’. The flowers typically open on bright sunny days; the length of time that flowers are open differs between species (Jones 2006).

Thelymitra is a prominent food deceptive genus in the Australian Diuridae and all species are rewardless (Jones 2006). The symmetrical form, together with its lack of reward and close resemblance too many Australian lily-like taxa, suggest that many Thelymitra species are food deceptive mimics of local, co-blooming and pollen rewarding guilds (Bates and Weber 1990).

Thelys

References:

Dressler RL (1981) Biology of the orchid bees (Euglossini). Annual Review of Ecological Systematics 13, 373 – 394.

Bates RJ, Weber JZ (1990) Orchids of South Australia. (South Australia Government Printer: Adelaide).

Jones DL (2006) “A complete guide to native orchids of Australia, including the island territories‟. (Reed New Holland: Sydney).

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2 Responses to Introducing… Thelymitra

  1. MB says:

    Why are they only suggesting, rather than concluding? What sort of / level of evidence would be considered final?

    • It is relatively easy to hypothesise a mimicry system, but actually untangling these systems proves difficult and contentious. There are a few reasons for this, but two main points spring to mind.

      Deciding how similar two flowers are to the pollinator is a complex task (and another story). Even if this was established adequately, a morphological similarity between model and mimic is nowhere near enough to prove or disprove mimicry.

      Another key criterion for mimicry systems, which is lacking in many mimicry studies, is that mimic reproductive capacity should be higher in the presence of the model. This condition acts to establish if resemblance of the mimic has a fitness benefit directly related to the model.

      I guess one way to establish this ‘fitness benefit’ would be to have a very well designed and well-replicated experiment. At the moment most mimicry studies for Thelymitra are observational and often anecdotal. Most likely because outbreeding Thelys often occur in small, isolated numbers, in small relatively isolated populations. Thely’s also only flower for a very short time period (~12 days for T. aristata), which makes for quite an intense field ‘season’. Additionally, it is notoriously difficult to propagate orchids.

      If a solid morphological similarity could be justified between model and mimic from the pollinators perspective and if there was evidence to suggest that the mimic has higher fitness in the presence of the model, then there would be very solid evidence to conclude mimicry.

      In short, proving or disproving a mimicry system in a ‘final’ way is a complex task. At the moment we have a solid hypothesis for mimicry in Thelymitra, but more study is needed to cement this. Stay tuned for my paper.

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