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

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Part IV POPULATION VARIATION N ina Fedoroff (“Transposons and Genome Evolution in Plants,” Chapter 10) notices that the publication 50 years ago of Stebbins’ Variation and Evolution in Plants roughly coincides with the first reports by Barbara McClintock that there are genetic elements capable of transposing from one to another chromosomal location in maize. Today we know that transposable elements make up a large fraction of the DNA of agriculturally important plants, such as corn and wheat, and of animals such as mice and humans, and perhaps all species of mammals and many other vertebrates. Fedoroff reviews the history of the discovery of trans- posing elements and advances the hypothesis that the mechanisms con- trolling transposition are an instance of “the more general capacity of eukaryotic organisms to detect, mark, and retain duplicated DNA through regressive chromatin structures.” Grasses (family Poaceae) and their cultivated relatives encompass a gamut of genome size and structural complexity, that extends from rice at the lower end to wheat and sugarcane at the higher end, having nuclear DNAs more than 30 times larger than rice’s. Maize is towards the middle, with about six times more nuclear DNA than rice, embodied in 10 pairs of chromosomes. The maize genome is replete with chromosomal duplica- tions and repetitive DNA sequences, as Brandon S. Gaut and his collabora- tors tell us (“Maize as a Model for the Evolution of Plant Nuclear Genomes,” Chapter 11). This complexity has motivated these authors to focus on maize as a model system for investigating the evolution of plant nuclear genomes. More than 11 million years ago, but after the sorghum and maize lineages 165 

166 / Stephen M. Rich and Francisco J. Ayala had split, the maize genome became polyploid, which accounts for much of the difference in DNA content between these related species. The polyploid event was followed by diploidization and much rearrangement of the ge- nome, so that maize is now a diploid. But there remains much “extra” DNA in maize, mostly consisting of multiple repetitions of retrotransposons that account for 50% of the nuclear genome. This multiplication has occurred within the last 5–6 million years and has also contributed to the genome differentiation between maize and sorghum. The evolutionary complexi- ties of cultivated maize extend to individual genes that have been variously impacted by domestication and intensive breeding. Michael T. Clegg and Mary L. Durbin (“Flower Color Variation: A Model for the Experimental Study of Evolution,” Chapter 12) trace the development of flower color in the morning glory, from the molecular and genetic levels to the phenotype, as a model for analyzing adaptation. Most mutations determining phenotypic differences turn out to be due to transposon insertions. Insect pollinators discriminate against white flow- ers in populations where white flowers are rare. This would provide an advantage to white genes through self-fertilization in white maternal plants. The pattern of geographic distribution of white plants indicates that such advantage is counteracted by definite, but undiscovered disad- vantages of the white phenotype. The authors conclude by proposing that floral color development is an area of special promise for understanding the complex gene interactions that impact the phenotype and its adapta- tion, precisely because “the translation between genes and phenotype is tractable . . . [and] the translation between environment and phenotype is more transparent for flower color than in most other cases.” Barbara A. Schaal and Kenneth M. Olsen point out, in “Gene Gene- alogies and Population Variation in Plants” (Chapter 13), that it was largely due to Stebbins that the investigation of individual variation within populations become part and parcel of the study of plant evolution. For many years beyond 1950, the focus of investigation was the phenotype: morphology, karyotype, and fitness components. Protein electrophoresis opened up the identification of allozyme variation and thus the study of allelic variation at individual genes. Restriction analysis and DNA se- quencing have added the possibility of reconstructing the intraspecific genealogy of alleles. The mathematical theory of gene coalescence has provided the analytical tools for reconstruction and interpretation. Schaal and Olsen put all these tools to good use in several model cases: the recent rapid geographic expansion of Arabidopsis thaliana, with little differentia- tion between populations; the recolonization of European tree species from refugia created by the Pleistocene glaciation; the origin and domes- tication of cassava (manioc), the main carbohydrate source for 500 million people in the world tropics.