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The Origin of Flowering Plants

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Image: University of Wisconsin

Researchers have long puzzled over the origin of flowering plants, or angiosperms. Indeed, 125 years ago Darwin himself proclaimed their sudden appearance in the fossil record an "abominable mystery." Subsequent efforts to sort out floral emergence and diversification based on fossil evidence and comparisons of anatomy among living plants led to little consensus among researchers. But in recent years new data have provided insight, suggesting to a number of scholars that a genus of small flowering shrubs known as Amborella make up the first branch of the angiosperm family tree. According to a report published yesterday in the Proceedings of the National Academy of Sciences, however, Amborella may have to share that branch with the Nymphaeales, a group that includes waterlilies and lotuses.

Todd J. Barkman of Pennsylvania State University and his colleagues studied molecular sequence data from genes in all three of the genomic compartments found in plant cells--the nucleus, the mitochondria and the plastid--in order to generate a phylogeny>, or family tree. Their analyses of two different data sets, one composed of six genes and the other of nine, both indicated that Amborella and the Nymphaeales together form the "first-branching" angiosperm lineage. This expanded model differs from the earlier one in several important ways. For instance, whereas the model placing Amborella alone on the first branch suggests that the ancestor of the flowering plants had unisexual flowers and no vessels for nutrient transport, this Amborella + Nymphaeales model indicates that the ancestor may have had unisexual or bisexual flowers and vessels.

Such distinctions might seem arcane to the casual observer. But in a commentary accompanying the report William L. Crepet of Cornell University points out that making sense of angiosperm origins and evolution has modern relevance. "It would allow a better understanding of species distribution and their ecological implications. It would facilitate more efficient phylogenetic-context-guided searches for natural drugs and provide a precise framework within which to intelligently direct and ethically evaluate the inevitable, if controversial, bioengineering of plants for agricultural and medicinal purposes." In addition, he continues, "knowledge of relationships has the potential for allowing more informed decision making on biodiversity conservation issues by permitting comparisons of the explicit uniqueness of taxa in situations involving difficult choices."

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