人们普遍认为，农业独立起源于少数几个关键地区，然后传播到世界其他地方。北美东部被认为可能是这样一个地区，依据是三种主要作物的驯化过程的古植物学记录。但人们已对其中两种作物的地理起源提出了疑问，这两种植物是南瓜和藜。最近在墨西哥发现距今4000年的驯化的向日葵后，人们又对第三种作物的起源提出了疑问。对来自墨西哥和美国的野生和驯化向日葵种群所做的一项新的遗传调查，支持认为现存驯化的向日葵来自北美东部野生种群、在驯化过程中曾出现严重遗传瓶颈的假设。该结果与对其他大陆的研究结果是一致的，即向农业的过渡涉及复杂的区域模式，而不是简单的扩散。
Origin of extant domesticated sunflowers in eastern North America
Eastern North America is one of at least six regions of the world where agriculture is thought to have arisen wholly independently. The primary evidence for this hypothesis derives from morphological changes in the archaeobotanical record of three important crops—squash, goosefoot and sunflower—as well as an extinct minor cultigen, sumpweed. However, the geographical origins of two of the three primary domesticates—squash and goosefoot—are now debated, and until recently sunflower (Helianthus annuus L.) has been considered the only undisputed eastern North American domesticate. The discovery of 4,000-year-old domesticated sunflower remains from San Andrés, Tabasco, implies an earlier and possibly independent origin of domestication in Mexico and has stimulated a re-examination of the geographical origin of domesticated sunflower. Here we describe the genetic relationships and pattern of genetic drift between extant domesticated strains and wild populations collected from throughout the USA and Mexico. We show that extant domesticates arose in eastern North America, with a substantial genetic bottleneck occurring during domestication.
Figure 1 Map of sampling locations, archaeological sites and Native American groups. Shaded areas indicate centres of domestication with eastern North America1 to the north and Mesoamerica (as defined in ref. 26) to the south. Numbers indicate sampling locations of wild populations where 1 is Sinaloa, 2 is Sonora5, 3 is Sonora4, 4 is Sonora6, 5 is Tamaulipas, 6 is Zacatecas, 7 is Nuevo León, 8 is Chihuahua, 9 is Arizona, 10 is Texas, 11 is Oklahoma2, 12 is Kansas, 13 is Colorado, 14 is Montana1, 15 is Montana2, 16 is North Dakota, 17 is South Dakota, 18 is Iowa, 19 is Missouri, 20 is Oklahoma1 and 21 is Tennessee (colours correspond to local area model ancestry analysis). Letters indicate archaeological sites with the oldest remains of domesticated sunflower, where A is San Andrés, Tabasco, Mexico (4,130 40 radiocarbon years before present (BP)), B is Higgs, Tennessee, USA (2,850 85 radiocarbon years BP), C is Hayes, Tennessee, USA (4,265 60 radiocarbon years BP) and D is Marble Bluff, Arkansas, USA (2,843 44 radiocarbon years BP); and names indicate the historical locations of Native American groups. We note that although they were collected in Mexico, the identities of indigenous groups associated with Maíz de Tejas and Maíz negro are unknown. USDA and Mammoth are modern cultivars derived from Russian stock. Therefore, these strains do not appear on the map.
Figure 3 Comparisons of genetic drift in wild populations (colours correspond to local area model source clusters) and domesticated strains (in black). The lines represent a neighbour-joining tree summarizing the genetic distances, DA, between groups. Mean F values for each domesticated strain averaged across all wild comparisons and mean F values for each wild population averaged across all domesticate comparisons appear along lines. Numbers in brackets correspond to those on Fig. 1 for sampling locations.
