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Classification of the early-divergent angiosperm family Hydatellaceae: one genus instead of two, four new species and sexual dimorphism in dioecious taxa

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Hydatellaceae was traditionally circumscribed as a family of two genera, Hydatella and Trithuria. We place Hydatella in synonymy of Trithuria. We describe three new species of Hydatellaceae from northern Australia and one new species from south-western Western Australia. Thus, we distinguish a single genus of Hydatellaceae, Trithuria, which consists of one species in New Zealand, ten species in Australia and one species in India. Two main characters were formerly used to distinguish between Hydatella and Trithuria: (1) reproductive units unisexual (homogamous) vs. bisexual (heterogamous) and (2) fruits with three prominent ribs, typically dehiscent vs. without prominent ribs, indehiscent. New evidence shows that the type of reproductive unit does not correlate with fruit morphology in Hydatellaceae. We hypothesize that two south-western Australian endemics known as Hydatella dioica and Trithuria occidentalis represent male and female individuals, respectively, of the same biological species. All four dioecious species of Hydatellaceae show similar sexual dimorphism. Male plants have reproductive units with longer and fewer involucral bracts than female ones. Anthers are much longer in dioecious species than in the morphologically closest cosexual species. The evolutionary significance of dioecy in Hydatellaceae is discussed.
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Keywords: AUSTRALIA; DIOECY; EVOLUTION; FLOWER; HYDATELLA; HYDATELLACEAE; INFLORESCENCE; MORPHOLOGY; TAXONOMY; TRITHURIA

Document Type: Research Article

Affiliations: 1: Department of Higher Plants, Biological Faculty, Moscow State University, 119992 Moscow, Russia 2: Western Australian Herbarium, Science Division, Department of Environment & Conservation, Brain Street, 6258 Manjimup, WA, Australia 3: Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, U.K.

Publication date: 2008-02-01

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