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

玉米是一种名为teosinte的草的栽培形式，它生长在墨西哥南部和西部的部分地区。史前美洲人在大约9000年前第一次种植玉米，可能靠近墨西哥西部的里约巴尔萨斯河。到公元15世纪欧洲人到达时，整个美洲都在种植玉米，玉米是许多土著文明的主要农作物，如玛雅人和印加人。玉米是如何从墨西哥的原产地传播到北美和南美洲的？20世纪70年代，对南美洲各地不同玉米品种的染色体结构进行了大规模研究，从而产生了玉米向南美洲传播的第一个模式。染色体结构的细微变化，特别是小旋钮的位置，使人们能够推断出品种之间的遗传关系。然后通过将这些遗传关系与每个品种的地理位置进行比较来推断传播模式。由此产生的模型表明，玉米最初被引入安第斯山脉中部，并从这个地点广泛传播到整个大陆的高地和低地地区，直到最近新品种沿着巴西东海岸向南传播，其他类型的玉米才得到显著补充。这一模型一直被认为大体上是正确的，直到2002年，它被对称为微卫星的可变DNA序列的更详细研究的结果所取代，同样是在南美洲种植的不同玉米品种中。这一新模型认为，玉米从墨西哥向南移动首先进入南美洲的低地，然后才进入安第斯山脉。不幸的是，这两种模式都存在一个问题。被研究品种的地理分布是分析的关键组成部分，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，为了使分析发挥作用，目前玉米品种的分布必须类似于玉米第一次传播到南美洲时建立的模式。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，尤其是导致玉米品种从一个地方广泛转移到另一个地方。这场运动制造了“噪音”，掩盖了史前地理信号，掩盖了最初的传播模式。因此，我们需要去除现代的噪音，才能更接近史前的地理信号。在之前的一个项目中，我们通过检查早于欧洲人到来的考古玉米标本，以及从直到最近还没有受到欧洲影响的偏远原生村庄收集的本土作物，开始了这项工作。我们对这些考古和本土标本的遗传分析表明，之前的两种模式可能都不正确，相反，玉米种植传播到南美洲有两条独立的路线，一条沿着南美洲西侧的安第斯山脉，另一条沿着东北海岸的低地。我们的项目范围非常有限，只对11个本土品种和10个考古标本中的单个短DNA序列进行了检查，其中大部分来自巴西，因此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南美洲更广泛的地理范围内从更多的考古标本和土著品种中获得更多的基因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.