人們普遍認(rèn)為,，農(nóng)業(yè)獨立起源于少數(shù)幾個關(guān)鍵地區(qū),，然后傳播到世界其他地方,。北美東部被認(rèn)為可能是這樣一個地區(qū),，依據(jù)是三種主要作物的馴化過程的古植物學(xué)記錄,。但人們已對其中兩種作物的地理起源提出了疑問,，這兩種植物是南瓜和藜。最近在墨西哥發(fā)現(xiàn)距今4000年的馴化的向日葵后,，人們又對第三種作物的起源提出了疑問,。對來自墨西哥和美國的野生和馴化向日葵種群所做的一項新的遺傳調(diào)查，支持認(rèn)為現(xiàn)存馴化的向日葵來自北美東部野生種群,、在馴化過程中曾出現(xiàn)嚴(yán)重遺傳瓶頸的假設(shè),。該結(jié)果與對其他大陸的研究結(jié)果是一致的，即向農(nóng)業(yè)的過渡涉及復(fù)雜的區(qū)域模式,，而不是簡單的擴散,。
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.
