Phylogeny and Evolution in Lemnaceae (Duckweeds)

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Les DEB 9806537 Flowering plants (angiosperms) include a diversity of organisms of which the smallest are the aquatic duckweeds (family Lemnaceae). These plants may have a body less than 1 mm in size with flowers only 0.2 mm long. Several individual plants of the duckweed Wolffia would fit on the head of a pin, and the number of individual plants in a single pond might exceed the human population of North America. The greatly reduced size of duckweeds presents a formidable challenge to systematic botanists who identify species and classify these tiny aquatic plants. Accurate species identifications and knowledge of relationships are critical in this economically important family. Because of their minute size and ease of culture, duckweeds have been incorporated into numerous experiments and are used routinely as bio-monitors of pollution and toxic substances in public waters. Outcomes of such experiments can be seriously compromised if species have been misidentified. How can such small plants and flowers be studied in adequate detail to provide the level of confidence necessary for accurate species identifications and estimations of relationships? For more than a century, researchers have relied on light and electron microscopy to observe the few conspicuous features that define species and indicate their relationships; however, few living people possess the expertise to do so with confidence. In this study, molecular techniques are used to study duckweed relationships and to facilitate their identification. To supplement morphological observations, the structures of proteins and DNA molecules are compared to determine species relationships and to serve as molecular markers for identification. Many molecular data sets including flavonoids, allozymes, and sequences from nuclear and chloroplast DNA will provide the basis for a comprehensive reconstruction of duckweed relationships, which will serve as a framework for constructing a meaningful classification. The development of te sts using molecular markers will provide a simple means of identifying duckweed species and their geographical origin and save weeks of study. Furthermore, molecular markers will enable the geographical origin of duckweed clones to be determined, an achievement previously impossible using morphological markers. Molecular studies have also identified the existence of several cryptic duckweed species, i.e., populations that resemble one another morphologically, but are highly divergent genetically and isolated reproductively. The identification of cryptic species in several widespread taxa will provide important information to estimate duckweed biodiversity and assist with conservation.