Trithuria is a genus of small ephemeral aquatic herb that represent the only members of the family Hydatellaceae found in India, Australia, and New Zealand.[1][2] All 13 described species of Trithuria are found in Australia, with the exception of T. inconspicua and T. konkanensis, from New Zealand and India respectively.[3][4] Until DNA sequence data and a reinterpretation of morphology proved otherwise, these plants were believed to be monocots related to the grasses (Poaceae). They are unique in being the only plants besides two members of Triuridaceae (Lacandonia schizmatica and L. braziliana) in which the stamens are centred and surrounded by the pistils; in Hydatellaceae the resulting 'flowers' may instead represent condensed inflorescences or non-flowers.[5]
These diminutive, superficially moss-like, aquatic plants are the closest living relatives of a clade comprising two closely related water-lily families Nymphaeaceae and Cabombaceae.[6] Together, these three families compose the order Nymphaeales in the APG III system of flowering plant classification. Trithuria (Hydatellaceae) diverged from the rest of Nymphaeales soon after Nymphaeales diverged from its sister taxon, although the crown clade evolved relatively recently, in the early Miocene (~19 Ma;[7]). The order as a whole is the sister group of all flowering plants except Amborellales.
Trithuria exhibits a remarkable similarity to Centrolepis and species of both genera were mistaken for members of the other genus.[8]
The genus Hydatella was subsumed into Trithuria as its members are phylogenetically nested in it. The family as a whole shares the following features (morphological synapomorphies[3])
The generic name Trithuria is composed of two parts: the Greek words treis meaning "three", and thyris meaning "window". It references the dehiscence of the fruit.[12] Specifically, it references the pericarp valves of some Trithuria species.[13]
The diploid chromosome count of Trithuria inconspicua subsp. inconspicua is 2n = c. 24. The diploid chromosome count of Trithuria submersa is 2n = 56.[13] The diploid chromosome count of the tetraploid species Trithuria konkanensis 2n = 40.[14] The diploid chromosome count of Trithuria australis is 2n = 14.[15]