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Biochemistry - The Evolution of Poison-Oak

According to Considine (p. 1351-2) ferns, one of the earliest land plant groups, evolved 450 million years ago. Animals ate the ferns. The ferns evolved poisons, tall trunks sheathed with bark, and apical growth centers ringed with thorns, to discourage herbivory. Animals like Brontosaurus evolved long necks so they could still feed on the tender centers of the crowns.

With the evolution of flowering plants 150 mya, some animals were given pollen and/or nectar in exchange for functioning as pollinators; or were rewarded with fruit pulp in return for disseminating seed.  Other animals continued to just use the plants as food; and plants, particularly the Anacardiaceae family of flowering plants, continued to try poisons to discourage random herbivory.

Black stained Anacardiaceae appear in fossils 60 mya, in eastern Asia. They were the earliest Toxicodendra. Toxicodendra uniquely use urushiols, pronounced oo-RUE-she-alls, as herbivory deterrents.

The urushiol is stored in tubes that run next to the veins of leaflets; and just below the bark in stems and roots. It is stored under enough pressure that a drop immediately forms when a leaf stalk is severed; when roots are cut considerably more fluid is released. The storage tubing also excludes oxygen; once the air hits urushiol, the urushiol oxidizes, and then polymerizes into a black lacquer.

Toxicodendra have been successful as a group:  they spread west to the Urals, and south to the tectonic plate boundary in Indonesia. 55 mya Toxicodendron radicans ssp radicans evolved in central China, and crossed the Bering land bridge. In North America ssp radicans gave rise to T. diversilobum (poison-oak), T. rhydbergii (Rhydberg poison-ivy), T. toxicarium (eastern poison-oak), and eight other subspecies. Together, they inhabit all of North America except deserts and mountain tops!


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