Closest relatives of Pistia stratiotes resolved with combined chloroplast and mitochondrial DNA sequences
about the work of Dr. Susanne Renner, Menzinger Strasse 67, D-80638 Munich, Germany, renner@lrz.uni-muenchen.de, and Dr. Li-Bing Zhang, Department of Biology, Colorado State University, Fort Collins, CO 80523, Libing.Zhang@ColoState.edu
At long last, the closest relatives of Pistia stratiotes (water lettuce) have been identified. Previous hypotheses about the plant relationships of Pistia had to be based on morphology alone, which in its case is difficult because of the plant's much-condensed flower and other reproductive structures. The great morphological distinctness of Pistia is reflected in classifications of Araceae, which place this single species in a subfamily or tribe by itself. This work was conducted by S. Renner and L.-B. Zhang, systematists at the Missouri Botanical Garden and the University of Missouri-St. Louis.
The scientists used sequences from three sections of chloroplast DNA, called ‘introns' and ‘spacers' because they are inserted between and inside genes, and one intron in a mitochondrial gene. (A manuscript on their discovery has been submitted and the sequences have been made public in the genetic sequence database GenBank, an annotated collection of all publicly available DNA sequences which also contains the human sequence.)
The figure is a phylogenetic ‘tree' (drawn as a circle) based on all combined sequences. The tree represents the most likely relationships between Pistia and its closest relatives in the Araceae family, given the data and a model of sequence evolution based on the specific sequences in the analysis. The numbers on the branches represent statistical confidence (100 is the highest possible level).
Pistia stratiotes appears in the lower right, and it is the sister group to the entire circle of genera ‘above it' in the tree. In other words, Pistia is not closely related to any single living species. Rather, its ancestor diverged from the ancestor of all the genera in the tree before those other genera had diversified.
Most of the genera in the ‘tree' have but a few species, but a few, such as jack-in-the-pulpit (Arisaema, with 150 species), are species rich. The distribution maps next to the genera show that the species related to Pistia all occur in the Old World tropics. The only exceptions are three species of jack-in-the-pulpit that entered North America across the Bering land bridge, sometime in the Miocene as indicated by 16-18 million year old fossils from Spokane. The oldest fossils of Pistia are seeds from the Late Oligocene/Early Miocene (24 million years) of Europe and Russia. These fossils, however, ‘underestimate' the true age of Pistia because some of the genera in the Pistia sister group have 45 million year old fossils. Also, the group at the very bottom of the tree, Peltandra (in Florida) and Typhonodorum, are known from 60 million year old leaves from the Late Paleocene/Eocene of eastern Europe, Kazakhstan, North Dakota, and Tennessee (Wilde et al., in press).
The combined molecular and fossil evidence led the researchers to infer that the early evolution of Pistia took place between 60 and 45 million years ago somewhere around the Tethys sea, that is the proto-Mediterranean sea which opened into the Indian Ocean, and that Pistia in geologic terms may be a relative newcomer to the New World tropics. More detailed comparisons of gene sequences from different populations of Pistia are needed to test whether New World Pistia populations on average are younger than Old World Pistia populations. It is tricky, however, because Pistia is so mobile, and there is likely to have been much local extinction, followed by re-invasion.
References cited: Renner, S.S., L-B. Zhang. Submitted. Phylogeny and evolution of the pantropical aquatic weed Pistia stratiotes (Araceae).
Wilde, V., Z. Kvacek, and J. Bogner. In press. Fossil leaves of the Araceae from the European Eocene and notes on other aroid fossils. Int. J. Plant Sciences.
Aquaphyte Contents | Aquaphyte page | Home
CAIP-WEBSITE@ufl.edu
Copyright 2003 University of Florida