Documenting domestication in a lost crop (Polygonum erectum L.): evolutionary bet-hedgers under cultivation

Abstract

This study uses morphometrics and digital image analysis to document domestication syndrome in an annual seed crop, Polygonum erectum L. (erect knotweed), which was cultivated by Native Americans for c. 2,500 years in eastern North America. This plant is one of several seed crops referred to as the Eastern Agricultural Complex, a pre-maize agricultural system that supported societies in a core area centred on the central Mississippi valley for millennia. The extinct domesticated subspecies P. erectum ssp. watsoniae N. G. Muell. described here, exhibits some classic markers of domestication, including larger fruits and reduced germination inhibitors in comparison to its wild progenitor. Domesticated P. erectum also exhibits greatly reduced germination heteromorphism. Germination heteromorphism is the classic example of evolutionary bet-hedging in plants: wild P. erectum sacrifices maximum fitness per generation for a reduction in fitness variance over many generations. It does so by producing two different types of fruits: ones that germinate immediately in the spring after they are produced (smooth morphs), and ones that remain in the soil seed bank for one or more growing seasons before germinating (tubercled morphs). Tubercled morphs allow populations to recover after adverse events. Under cultivation, the selective pressures that maintained this strategy were relaxed as humans saved seeds and created predictable microenvironments for seedlings, resulting in homogenous harvests and reliable germination for ancient farmers.