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Citation for Study 18129

About Citation title: "Threonine Deaminase Positive Selection".
About Study name: "Threonine Deaminase Positive Selection".
About This study is part of submission 18129 (Status: Published).

Citation

Rausher M., & Huang J. 2015. Threonine Deaminase Positive Selection. Molecular Biology and Evolution, .

Authors

  • Rausher M.
  • Huang J.

Abstract

Although plants and their natural enemies may coevolve for prolonged periods, little is known about how long individual plant defensive genes are involved in the coevolutionary process. We address this issue by examining patterns of selection on the defensive gene threonine deaminase (TD). Tomato (Solanum lycopersicum) has two copies of this gene. One performs the canonical housekeeping function in amino-acid metabolism of catalyzing the first reaction in the conversion of threonine to isoleucine. The second copy has evolved to function as an anti-nutritive defense against lepidopteran herbivores by depleting the essential nutrient threonine in the insect gut. Wild tobacco (Nicotiana attenuata) also contains a defensive copy. We show that a single copy of TD underwent two or three duplications near the base of the Solanaceae. One copy retains the housekeeping function, while a second copy evolved defensive functions. Positive selection occurred on the branch of the TD2 gene tree subtending the common ancestor of the Nicotianoideae and Solanoideae. It also occurred within the Solanoideae clade but not within the Nicotianoideae clade. Finally, it occurred on most branches leading from the common ancestor to Solanum lycopersicum. Based on recent calibrations of the Solanaceae phylogeny, TD2 experienced adaptive substitutions for a period of 30? 50 million years. We suggest that the most likely explanation for this result is fluctuating herbivore abundances: when herbivores are rare, relaxed selection increases the likelihood that slightly disadvantageous mutations will be fixed by drift; when herbivores are common, increased selection causes the evolution of compensatory adaptive mutations. Alternative explanations are also discussed.

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  • Canonical resource URI: http://purl.org/phylo/treebase/phylows/study/TB2:S18129
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