The subfamily Apostasioideae belongs to the orchid family (Orchidaceae).
It is a monophyletic clade, but there is bootstrap support that it is a sister the other orchid subfamilies. A bootstrap is a method for evaluating the statistical importance of positions of different branches in a phylogenetic tree (= a treelike structure giving the evolutionary development of organisms).
The apostasioid orchids are the most primitive orchids with only two genera. Neuwiedia has 3 fertile, abaxial (= facing away from the stem) anthers, while Apostasia has two fertile abaxial anthers and a filamentous staminode (= a sterile stamen).
These primitive features makes them, according to some authorities, not true orchids but rather ancestors of modern orchids. However, modern studies (Stern, Cheadle and Thorsch, 1993) point out that the apostasioid orchids are uniquely defined ( = autapomorphous). This makes them unsuitable as models to be the ancestors of the orchids.
To study the position of Apostasioideae within Orchidaceae and their intra- and intergeneric relationships, a molecular phylogenetic analysis has been conducted on the nuclear ITS region and the two plastid DNA regions trn L-F intron and mat K. The two genera traditionally ascribed to Apostasioideae are each monophyletic.
One of these clades, Apostasioideae, is sister to the clade formed by Vanilloideae, Cypripedioideae, Orchidoideae and Epidendroideae.
High transition-transversion ratio and the absence of stop codons in the individual sequences suggest that mat K is at the transition from a possibly functional gene to a pseudogene in Apostasioideae, contrary to what is found in some other groups of Orchidaceae.
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