This week, a local radio station introduced a segment with the phrase “our rarest sun orchid” and that it was called Thelymitra irregularis or Spotted Pink Sun Orchid. However it certainly is not our rarest sun orchid.
True, it is not common, but that is partly because it is a hybrid and, correctly speaking, the name should be written as Thelymitra x irregularis (the “x” indicates that it is a hybrid).
For a hybrid to occur, the two parent species need to grow in close proximity, the flowers need to open at the same time that the pollinator is visiting flowers, either to collect or to deposit the pollen and, in the case of self pollinating species, before the individual flower has pollinated itself.
The majority of hybrids are sterile, but occasionally some are fertile. When hybrids occur the majority will only last a few years before disappearing although sometimes colonies are formed which may last for decades. Hence, it is not usual to name hybrids, but the more common and recurring ones have been named formally. T. x irregularis is one of them.
Several species of Thelymitra have been proposed as parent species of Thelymitra x irregularis. Jeanes & Backhouse (2006) give T. ixioides and T. carnea as parents; Weber & Entwisle (1996) and Jones (2006) suggest T. ixioides and T. carnea and/or T. rubra; Bates & Weber (1990) state that in South Australia the parents are T. ixioides and T. rubra, but T. ixioides and T. carnea in the Eastern states; in contrast, Bates (2011) states that in South Australia it is a hybrid between T. juncifolia and T. rubra. However, without detailed genetic studies or breeding experiments these all remain suggestions.
Due to the transient nature of hybrids and the conditions needed to produce them, the named hybrids are not common, but since the 1890s specimens of Thelymitra x irregularis have been collected in every decade, which suggest that this hybrid readily occurs. The 71 specimens held in the Australian herbaria have been collected from four states – which gives a good indication of the distribution but not necessarily the frequency of occurrence. See Australia’s Virtual Herbarium (AVH) for details. The AVH lists 12 herbarium records for South Australia with specimens collected from the Adelaide Hills, the Barossa Valley, Kangaroo Island and near Naracoorte.
The botanical name “irregularis” refers to the description of the column. With many sun orchids, it is often necessary to observe the column to distinguish one species from another. In this instance the top of the column is irregularly toothed. Retired Aussie has some very good photographs with one in particular showing the column detail.
Thanks to Juergan Kellermann, State Herbarium of South Australia for his help with this post.
References
Bates (2011). South Australia’s Native Orchids. DVD-ROM
Bates & Weber (1990). Orchids of South Australia.
Jeanes & Backhouse. Wild Orchids of Victoria, Australia
Jones (2006), Native Orchids of Australia, 2nd edn.
Weber & Entwisle (1996). Thelymitra. In: Flora of Victoria, Vol 3.
Australia’s Virtual Herbarium accessed 24th September 2014
NOSSA 
