Revisiting convergence: another example (Introduction)

by David Turell , Friday, October 21, 2016, 01:32 (2738 days ago) @ David Turell

Plants make caffeine in different ways with different enzymes, but with similar pathways:

http://www.the-scientist.com/?articles.view/articleNo/47087/title/How-Plants-Evolved-Di...

"Plant species belonging to divergent branches of the evolutionary tree are known to have independently evolved caffeine production. According to scientists at Western Michigan University, caffeine-producing plants have taken a number of different biochemical routes to synthesize the stimulant. Coffee, tea, cocoa, orange, and guaraná plants make caffeine using an array of enzymes and substrates,

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"Caffeine is produced by approximately 30 of the world’s 300,000 or so different species of flowering plants, estimated Todd Barkman of Western Michigan, who led the new study. The divergent nature of these species and of the functions the molecule performs—attracting pollinators, deterring pests, and more—has indicated that caffeine production independently evolved multiple times, Barkman said.

"Indeed, studies have shown that coffee (Coffea arabica) and tea (Camellia sinensis) use different enzymes to generate caffeine—xanthine methyltransferases (XMT) and caffeine synthases (CS), respectively. Furthermore, comparisons of the coffee genome (Coffea canephora) to genes from cocoa (Theobroma cacao) and tea have provided genetic confirmation of the convergent rather than divergent evolution of caffeine production.

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"Barkman considered that the enzymes might utilize alternative substrates. “For every enzyme we tested every possible substrate related to the caffeine biosynthetic network,” he told The Scientist. And, at last, the experiments worked. “We all just jumped for joy,” said Barkman.

"It turned out that while the tea and coffee pathways begin with xanthosine, the other plants studied make use of a similar molecule, called xanthine. Some or all of the intermediate molecules in the pathways also differed in cocoa, orange, and guaraná plant caffeine synthesis.

“'It really illustrates the idiosyncratic nature of how selection can pick and choose,” Barkman said.

"The team went on to investigate how the enzymes might have evolved, focusing on the XMT enzyme from the Citrus lineage. By comparing the sequences of orthologous XMTs from approximately 500 other flowering species, and by calculating probabilities of amino acid substitutions over millions of years, the researchers recreated putative ancestral enzymes and tested them in biochemical assays.

They found that the earliest synthetic ancestor, which the Citrus lineage would have shared with coffee, could methylate benzoic acid and salicylic acid (carboxylic acids involved in floral scent, pathogen defense, and more), but could not methylate xanthine or xanthosine. A more recent synthetic Citrus XMT ancestor, however, could utilize both salicylic acid and xanthine, the team found.'"

Comment: A clear example of plant convergence. Might this come from guidelines in an inventive mechanism.