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Suggested Citation:"Part III PROTOCTIST MODELS." National Academy of Sciences. 2000. Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins. Washington, DC: The National Academies Press. doi: 10.17226/9766.
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Page 115
Suggested Citation:"Part III PROTOCTIST MODELS." National Academy of Sciences. 2000. Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins. Washington, DC: The National Academies Press. doi: 10.17226/9766.
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Page 116

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Part III PROTOCTIST MODELS T he mitochondrial genome of kinetoplasts is a highly derived ge- nome in which frameshift errors in reading frames are corrected at at the messenger RNA level. “RNA editing” refers to these post- transcriptional modifications, of which two types are known. One con- sists of the precise insertion or deletion of U residues, so as to produce open reading frames in the messenger RNAs encoded in the organelle DNA known as the maxicircle. The other editing system is a modification of 34 C’s into 34 U’s in the anticodon of transfer RNA molecules that thus can decode the UGA stop codon as tryptophan. Larry Simpson and col- leagues (“Evolution of RNA Editing in Trypanosome Mitochondria,” Chapter 8) seek to unravel the evolution of these two peculiar genetic systems. With support from computer simulations, the authors elaborate an evolutionary scenario that proposes an ancient but unique evolution- ary origin for both systems, which may have arisen shortly after the di- vergence of the trypanosomes and their relatives from the euglenoids. Stephen M. Rich and Francisco J. Ayala (“Population Structure and Recent Evolution of Plasmodium Falciparum,” Chapter 9) summarize data showing absence of synonymous nucleotide polymorphisms in diverse genes from Plasmodium falciparum, the agent of malignant malaria. The inference is that the extant world populations of P. falciparum originated from a single ancestral cell in recent times, estimated to be less than 50,000 years. This inference seems at first incompatible with the existence of numerous amino acid and other polymorphisms in the antigenic genes of 115

116 / Larry Simpson et al. the parasite. Rich and Ayala analyze allelic sequences of antigenic genes and conclude that they are consistent with a recent origin of the world populations of P. falciparum. The antigenic polymorphisms come about rapidly by mass natural selection acting on sequence variations origi- nated at high rates by intragenic recombination of short DNA repeats.

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Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins Get This Book
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"The present book is intended as a progress report on [the] synthetic approach to evolution as it applies to the plant kingdom." With this simple statement, G. Ledyard Stebbins formulated the objectives of Variation and Evolution in Plants, published in 1950, setting forth for plants what became known as the "synthetic theory of evolution" or "the modern synthesis." The pervading conceit of the book was the molding of Darwin's evolution by natural selection within the framework of rapidly advancing genetic knowledge.

At the time, Variation and Evolution in Plants significantly extended the scope of the science of plants. Plants, with their unique genetic, physiological, and evolutionary features, had all but been left completely out of the synthesis until that point. Fifty years later, the National Academy of Sciences convened a colloquium to update the advances made by Stebbins.

This collection of 17 papers marks the 50th anniversary of the publication of Stebbins' classic. Organized into five sections, the book covers: early evolution and the origin of cells, virus and bacterial models, protoctist models, population variation, and trends and patterns in plant evolution.

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