Hydatellaceae are a
family of small, aquatic
flowering plants. The family consists of tiny, relatively simple plants occurring in Australasia and India. It was formerly considered to be related to the grasses and sedges (order
Poales), but has been reassigned to the order
Nymphaeales as a result of DNA and morphological analyses showing that it represents one of the earliest groups to split off in flowering-plant phylogeny, rather than having a close relationship to
monocots, which it bears a superficial resemblance to due to
convergent evolution.[4] The family includes only the
genusTrithuria, which has at least 13 species, although species diversity in the family has probably been substantially underestimated.[5]
Description
Plants are submerged and emergent aquatic plants, rooted in the substrate below the water. They are tiny plants, just a few cm tall. Most species are ephemeral aquatics that flower in vernal pools when the water draws down, but several species are submerged perennials found in shallow lakes. The simple leaves are concentrated basally around a short stem. Individual species are cosexual (with several types of
hermaphroditic conditions) or
dioecious, and are either
wind-pollinated (anemophilous) or self-pollinating (
autogamous). Two predominantly
apomictic species are also known.[6] Flower-like reproductive units are composed of small collections of minute stamen- and/or pistil-like structures that may each represent very reduced individual flower, so that the reproductive units may be
pseudanthia. The non-fleshy fruits are
follicles or
achenes.[2]
Taxonomy
The family was for many years assumed to be a close relative of the
grasses and sedges and was even sometimes lumped under the
poalean family
Centrolepidaceae. Even as recently as 2003, the
APG II system assigned Hydatellaceae to the grass order Poales in the
commelinidmonocots. However, research based on DNA sequences and morphology by Saarela et al. indicates that Hydatellaceae is the living sister group of the water lilies (
Nymphaeaceae and
Cabombaceae) and thus represents one of the most ancient lineages of flowering plants.[7] Developers of earlier classifications were misled by the apparently reduced vegetative and reproductive morphology of these plants. As aquatic herbs, Hydatellaceae have environmental adaptations leading to
derived characteristics that create a morphological similarity to the more distant taxon. Careful reanalysis of their morphological traits and comparisons with other so-called 'basal' angiosperms have supported this "dramatic taxonomic adjustment".[8][9] This realignment is now recognized in the
APG III and
APG IV systems of classification.[1][10]
The family now includes only the
genusTrithuria Hook.f., which in 2008 was re-defined to include the genus Hydatella Diels. The type species is Trithuria submersa Hook.f.[11]
^
abMacfarlane, T.D.; Watson, L. & Marchant, N.G., eds. (2000).
"Hydatellaceae U Hamann". FloraBase: Flora of Western Australian. Archived from
the original on 14 July 2007. Retrieved 15 November 2013.
^Smissen, Rob D.; Ford, Kerry A.; Champion, Paul D.; Heenan, Peter B. (2019). "Genetic variation in Trithuria inconspicua and T. filamentosa (Hydatellaceae): a new subspecies and a hypothesis of apomixis arising within a predominantly selfing lineage". Australian Systematic Botany. CSIRO Publishing.
doi:
10.1071/sb18013.
ISSN1030-1887.
S2CID91261563.
^Rudall, Paula J.; Sokoloff, Dmitry D.; Remizowa, Margarita V.; Conran, John G.; Davis, Jerrold I.; Macfarlane, Terry D. & Stevenson, Dennis W. (2007). "Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early-divergent angiosperm lineage". American Journal of Botany. 94 (7): 1073–1092.
doi:
10.3732/ajb.94.7.1073.
hdl:2440/44423.
PMID21636477.
S2CID10405562.
^Friis, Else Marie; Crane, Peter R. & Pederses, Kaj Raunsgaard (2011). Early Flowers and Angiosperm Evolution. Cambridge University Press.
ISBN978-1-139-12392-1.