Olsen K M, Schaal B A
Department of Biology, Washington University, St. Louis, MO 63130,
Proc Natl Acad Sci U S A. 1999 May 11;96(10):5586-91. doi: 10.1073/pnas.96.10.5586.
Cassava (Manihot esculenta subsp. esculenta) is a staple crop with great economic importance worldwide, yet its evolutionary and geographical origins have remained unresolved and controversial. We have investigated this crop's domestication in a phylogeographic study based on the single-copy nuclear gene glyceraldehyde 3-phosphate dehydrogenase (G3pdh). The G3pdh locus provides high levels of noncoding sequence variation in cassava and its wild relatives, with 28 haplotypes identified among 212 individuals (424 alleles) examined. These data represent one of the first uses of a single-copy nuclear gene in a plant phylogeographic study and yield several important insights into cassava's evolutionary origin: (i) cassava was likely domesticated from wild M. esculenta populations along the southern border of the Amazon basin; (ii) the crop does not seem to be derived from several progenitor species, as previously proposed; and (iii) cassava does not share haplotypes with Manihot pruinosa, a closely related, potentially hybridizing species. These findings provide the clearest picture to date on cassava's origin. When considered in a genealogical context, relationships among the G3pdh haplotypes are incongruent with taxonomic boundaries, both within M. esculenta and at the interspecific level; this incongruence is probably a result of lineage sorting among these recently diverged taxa. Although phylogeographic studies in animals have provided many new evolutionary insights, application of phylogeography in plants has been hampered by difficulty in obtaining phylogenetically informative intraspecific variation. This study demonstrates that single-copy nuclear genes can provide a useful source of informative variation in plants.
木薯（Manihot esculenta subsp. esculenta）是一种在全球具有重要经济意义的主食作物，但其进化和地理起源仍未得到解决且存在争议。我们在一项基于单拷贝核基因甘油醛-3-磷酸脱氢酶（G3pdh）的系统地理学研究中，对这种作物的驯化进行了调查。G3pdh基因座在木薯及其野生近缘种中提供了高水平的非编码序列变异，在检测的212个个体（424个等位基因）中鉴定出了28个单倍型。这些数据代表了单拷贝核基因在植物系统地理学研究中的首次应用之一，并对木薯的进化起源产生了几个重要的见解：（i）木薯可能是沿着亚马逊盆地南部边界的野生M. esculenta种群驯化而来的；（ii）这种作物似乎并非如先前所提出的那样源自多个祖先物种；（iii）木薯与近缘的、可能杂交的物种Manihot pruinosa没有共享单倍型。这些发现提供了迄今为止关于木薯起源最清晰的图景。从谱系学的角度考虑，G3pdh单倍型之间的关系在M. esculenta内部和种间水平上都与分类学界限不一致；这种不一致可能是这些最近分化的分类群之间谱系分选的结果。尽管动物的系统地理学研究提供了许多新的进化见解，但植物系统地理学的应用因难以获得系统发育信息丰富的种内变异而受到阻碍。这项研究表明，单拷贝核基因可以为植物提供有用的信息变异来源。
