Botanic Gardens Trust, Sydney, Australia

 

Evolution of Vallisneria

Dr Surrey Jacobs - Senior Principal Research Scientist

Vallisneria L. (Hydrocharitaceae) is a genus of monocotyledons estimated to include from four to ten species worldwide. Vallisneria has low regional diversity throughout its range with the highest number of species reported in Australia. A recent phylogenetic study of Hydrocharitaceae indicated that Vallisneria, Maidenia Rendle and Nechamandra Planch. are closely related. Vallisneria species are annual or perennial, dioecious, submersed, freshwater aquatics that are highly valued commercially as ornamental aquarium specimens. Because Vallisneria is cultivated so widely, the potential for introductions to nonindigenous regions is high. Ecologically, the plants are an important source of food for a variety of wildlife.

The family Hydrocharitaceae is primitively unisexual but contains species with derived bisexual as well as dioecious and monoecious sexual conditions. Hydrocharitaceae also has a spectrum of pollination methods including entomophily, hydrophily, and a unique system found only in Hydrocharitaceae; this system involves the complete detachment of staminate flowers from a spathe at the base of the submersed male plants; the flowers rise to the surface, where they open to form free-floating, raft-like structures that disperse on the water surface by wind and currents. Despite its unorthodox nature, this system has evolved independently several times within the family. Despite the attention focused on the reproductive peculiarities, the taxonomy of Vallisneria has remained poorly understood, and species limits require further clarification. For many years, the original Linnaean name Vallisneria spiralis was applied indiscriminately to similar rosette plants that grew in Asia, Australia, Europe and North America.

Eventually, numerous differences were detected among plants from all four continents, prompting a call for a revision of the genus. More detailed observations have led several authors to distinguish a number of taxa from North America, Asia and Australia from what originally was regarded as V. spiralis. The Australian genus Maidenia resembles Vallisneria but differs conspicuously by its vittate (caulescent) habit. There are two vittate Australian species (V. caulescens, V. triptera). Preliminary phylogenetic studies indicated that Maidenia probably is not distinct from Vallisneria. Our survey includes specimens collected in Asia, Australia, Europe and North America as well as a variety of material in cultivation worldwide. Our main objectives were to

  1. establish a sound phylogenetic framework for Vallisneria and related genera;
  2. use this framework to assess the monophyly of the genus and relationships among the species;
  3. determine whether morphological convergence has influenced the taxonomic utility of characters currently used to circumscribe taxa;
  4. use the results of phylogenetic analysis to improve the taxonomy of Vallisneria; and,
  5. determine the origins of cultivated material and evaluate introductions of nonindigenous species where possible.

Phylogenetics of Vallisneria

All three molecular datasets provided highly congruent reconstructions of phylogenetic relationships in Vallisneria. All molecular data resolved species with a rosette growth form as basal elements in Vallisneria and embedded the vittate taxa between two main groups of rosette species. Because these analyses consistently place Maidenia within Vallisneria we concluded that Maidenia should be merged with Vallisneria. The combined molecular data provide a fairly comprehensive phylogenetic overview for the remainder of the genus. A clade consisting of the North American V. americana, V. neotropicalis (and associated umbellate taxon) is supported strongly as the basal element of the rosette clade that arises above the embedded vittate species. Next to diverge in that clade is the Australian V. australis, followed by a clade that divides the remaining rosette Australian species from an Asian clade that includes V. asiatica and V. natans. The Asian V. asiatica and V. natans represent one of the most derived groups within the genus and appear to have arisen from the rosette Australian taxa.

Systematics of Vallisneria (Hydrocharitaceae) Donald H. Les, University of Connecticut, Department of Ecology and Evolutionary Biology, Storrs, CT 06269-3043, USA Surrey W. L. Jacobs, Royal Botanic Garden, Sydney, Australia, 2000 Nicholas Tippery, University of Connecticut, Department of Ecology and Evolutionary Biology, Storrs, CT 06269-3043, USA Lei Chen, Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Michael L. Moody, Indiana University, Department of Biology, Bloomington, IN 47405-7000, USA Maike Wilstermann, Ludwigsburger Steige 119, 71686 Remseck/Neckar, Germany

 

 

 

Vallisneria australis
Vallisneria. Australis Photos: Surrey Jacobs

Vallisneria caulescens
Vallisneria caulescens

Vallisneria triptera
Vallisneria triptera

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