Single or multiple trajectories for the spread of maize cultivation into South America?

玉米是一种被称为大刍草的草的栽培形式，这种草生长在墨西哥南部和西部的部分地区。史前美洲人最早种植玉米大约在9000年前，地点可能靠近墨西哥西部的巴尔萨斯河。当欧洲人在公元15世纪到达美洲时，玉米已经在整个美洲种植，并且是玛雅和印加等许多土著文明的主要作物。玉米是如何从它的原产地中心墨西哥传播到北美和南美的？第一个玉米传播到南美洲的模型来自于20世纪70年代对整个大陆不同品种的染色体结构进行的一项大型研究。染色体结构上的细微变化，特别是小旋钮的位置，可以推断出品种之间的遗传关系。然后通过比较这些遗传关系与每个品种的地理位置来推断传播模式。由此产生的模型表明，玉米最初被引入安第斯山脉中部，并从这里广泛传播到整个大陆的高地和低地地区，直到最近的一段时间，新品种沿着巴西东海岸向南传播，其他类型的玉米才得到了显著补充。这个模型一直被认为是普遍正确的，直到2002年，它被一项更详细的研究结果所取代，这项研究是在南美洲种植的不同品种的玉米中进行的，该研究被称为微卫星。这个新模型认为，玉米从墨西哥向南迁移首先进入南美洲的低地，然后才进入安第斯山脉。不幸的是，这两种模式都有一个问题。所研究品种的地理分布是分析的一个关键组成部分，因为当与不同植物之间的遗传关系进行比较时，这种分布揭示了传播模式。这意味着为了使分析有效，现在玉米品种的分布必须与玉米最初传播到南美洲时建立的模式相似。但情况不太可能是这样，因为欧洲人对南美农业产生了重大影响，特别是对玉米品种从一个地方到另一个地方的广泛传播负有责任。这种运动产生了“噪音”，模糊了史前的地理信号，掩盖了原始的传播模式。因此，我们需要去除现代的噪音，以更接近史前的地理信号。在之前的一个项目中，我们通过检查在欧洲人到来之前的考古玉米标本，以及从偏远的土著村庄收集的土著作物开始做这件事，这些村庄直到最近才受到欧洲影响。我们对这些考古标本和本地标本的遗传分析表明，之前的两种模型都可能是不正确的，相反，玉米种植传播到南美洲有两条独立的路线，一条沿着南美洲西部的安第斯山脉，另一条沿着东北海岸的低地。我们的项目范围非常有限，只检查了11个本土品种和10个考古标本的单个短DNA序列，其中大部分来自巴西，所以我们还不能确定我们的模型是否准确。拟议项目的目的是通过从更多的考古标本和本地品种中获得更多的遗传数据，从整个南美洲更广泛的地理范围中获得更多的遗传数据，来测试我们的模型。

Maize is the cultivated form of a grass called teosinte, which grows wild in parts of southern and western Mexico. The prehistoric Americans first cultivated maize about 9000 years ago, probably close to the Rio Balsas river in western Mexico. By the time that Europeans arrived in the 15th century AD, maize was being grown throughout the Americas, and was the staple crop for many indigenous civilisations such as the Maya and the Inca. How did maize spread from its centre of origin in Mexico throughout North and South America? The first model for the spread of maize into South America arose from a large study of chromosome structures in different varieties from all over the continent, carried out in the 1970s. Subtle variations in chromosome structure, in particular the positions of small knobs, enabled the genetic relationships between the varieties to be deduced. The pattern of spread was then inferred by comparing these genetics relationships with the geographic location of each variety. The resulting model suggested that maize was initially introduced into the central Andes and from this location spread extensively throughout the highland and lowland regions of the continent, not being significantly supplemented by other types of maize until new varieties spread southwards along the east coast of Brazil in relatively recent times. This model was held to be generally correct until 2002 when it was replaced by the results of a more detailed study of variable DNA sequences called microsatellites, again in different varieties of maize grown throughout South America. This new model holds that the southwards movement of maize from Mexico led first into the lowlands of South America, and only then into the Andes Mountains. Unfortunately, there is a problem with both these models. The geographical distribution of the varieties being studied is a critical component of the analysis, as this distribution, when compared with the genetic relationships between different plants, is what reveals the pattern of spread. This means that in order for the analysis to work, the present-day distribution of the maize varieties must be similar to the pattern set up when maize first spread into South America. But this is unlikely to be the case, because Europeans have had a major impact on South American agriculture, and in particular have been responsible for the extensive movement of maize varieties from one place to another. This movement has created 'noise', obscuring the prehistoric geographic signal and disguising the original pattern of spread. We therefore need to strip off the modern noise to get closer to the prehistoric geographic signal. In a previous project we started to do this by examining archaeological maize specimens that predate the arrival of Europeans, as well as indigenous crops collected from remote native villages which until recently were unaffected by European influences. Our genetic analysis of these archaeological and indigenous specimens suggested that both previous models might be incorrect, and that instead there were two independent routes for the spread of maize cultivation into South America, one along the Andes on the western side of South America, and the second along the lowlands of the northeast coast. Our project was very limited in scope, involving examination of a single short DNA sequence in just eleven indigenous varieties and ten archaeological specimens, most of these from Brazil, and so we cannot yet be sure if our model is accurate. The purpose of the proposed project is to test our model by obtaining substantially more genetic data from a greater number of archaeological specimens and indigenous varieties, from a much broader geographical range throughout South America.