Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins (2000)

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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.