Selection for Oil Content During Soybean Domestication Revealed by X-Ray Tomography of Ancient Beans

Yunbing Zong,Hui Fang,Fengshi Luan,Guangzhao Zhou,Xuexiang Chen,Yang Liu,Xiulan Duan,Jianfeng Lang,Jianhua Zhang,Tiqiao Xiao,Jiadong Fan,Qing Wang,Huaidong Jiang,Shengkun Yao,Gary W Crawford,Zhibin Sun

doi:10.1038/srep43595

Abstract

When and under what circumstances domestication related traits evolved in soybean (Glycine max) is not well understood. Seed size has been a focus of archaeological attention because increased soybean seed weight/size is a trait that distinguishes most modern soybeans from their ancestors; however, archaeological seed size analysis has had limited success. Modern domesticated soybean has a significantly higher oil content than its wild counterpart so oil content is potentially a source of new insight into soybean domestication. We investigated soybean oil content using X-ray computed tomography (CT; specifically, synchrotron radiation X-ray CT or SRX-CT) of charred, archaeological soybean seeds. CT identified holes in the specimens that are associated with oil content. A high oil content facilitates the development of small holes, whereas a high protein content results in larger holes. The volume of small holes increased slowly from 7,500 to 4,000 cal B.P. We infer that human selection for higher oil content began as early as 7,500 cal B.P. and that high oil content cultivars were well established by 4,000 cal B.P.

Highlights

Soybean (Glycine max) is one of the world’s most important crops largely because of its high protein (40%) and oil (20%) content[1,2]
Accelerator mass spectrometer (AMS) radiocarbon dates were obtained for soybeans from each assemblage to confirm the ages of the archaeological beans (Table 1)
We suggest that the Houli soybeans had already undergone some selection for higher oil content by 7,500 B.P

Summary

Introduction

Soybean (Glycine max) is one of the world’s most important crops largely because of its high protein (40%) and oil (20%) content[1,2]. We explore (1) X-ray computed tomography (CT) as a tool to examine a charred seed structure related to oil content and (2) how the results inform soybean domestication. Four other taxa with oil-rich seeds were examined in order to determine whether oil content influences their structures after charring: hemp (Cannabis sativa), castor (Ricinus communis), perilla (Perilla frutescens), rapeseed (Brassica napus) and safflower (Carthamus tinctorius). These specimens were compared to archaeological seeds of four crops with low to no oil content but rich in starch or polysaccharides: bread wheat (Triticum aestivum), rice (Oryza sativa), buckwheat (Fagopyrum esculentum) and foxtail millet Synchrotron X-rays have higher flux and brightness than a conventional X-ray tube source, permitting

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Discussion

Conclusion