REU: The Genetic Basis of Morphological Evolution in Maize

植物物种的形态分化通常与生殖隔离和遗传差异有关。长相非常不同的物种通常在生殖上也不相容（不能杂交，或者即使杂交了，也是不育的）。然而，一些在遗传意义上密切相关的植物物种，它们是完全干涉性的，在形态上可能看起来非常不同。这种情况迫使植物进化论者假设这些特殊的植物物种一定经历了快速的形态进化。在很短的时间内，它们的外表发生了很大的变化，以至于无法进化出生殖隔离的机制。与墨西哥的野生祖先大刍草玉米相比，栽培玉米就是这样一个特殊的物种。这种快速形态变化的遗传基础可能取决于两种截然不同的模式：极少量（1至4个）基因的变化产生重大影响，或许多基因的大量变化，每个基因都产生微小影响。John Doebley博士正开始在他对玉米穗形态变化的遗传影响的研究中检验这两种假设。玉米穗与其祖先墨西哥大刍草的穗明显不同，尽管事实上玉米和大刍草是完全相互干扰的，而且被认为是在1万年前印第安人驯化下才分化出来的。将对玉米和大刍草小种之间的后代进行试验，分析染色体上的分子标记与控制玉米和大刍草穗形态特征的基因之间的关系。通过这些方法，应该有可能确定控制玉米和大刍草之间关键形态差异的最小基因数量，这些基因的染色体位置，作用模式（显性或隐性），以及每个基因所占的表型变异百分比。这些数据将允许在两种相互竞争的理论之间进行区分，这些理论涉及最近玉米穗起源的形态学步骤。

Morphological differentiation in plant species is correlated in general with reproductive isolation and genetic differences. Very different-looking species are usually also reproductively incompatible (hybrids cannot be made, or if obtained, are sterile). Nevertheless, some plant species which are closely related in the genetic sense that they are completely interfertile may look very different morphologically. This situation has compelled plant evolutionists to hypothesize that these exceptional plant species must have undergone rapid morphological evolution. They have changed greatly in appearance in a period of time that was too short to allow them to evolve mechanisms of reproductive isolation. Cultivated maize is one such exceptional species, compared to its wild progenitors, the teosinte corns of Mexico. The genetic basis of such rapid morphological change may depend upon two contrasting modes: changes at a very small number (one to four) of genes with major effects, or numerous changes of many genes, each with small effects. Dr. John Doebley is beginning to test these two hypotheses in his study of genetic effects on morphological changes in the ear of corn, Zea mays subspecies mays. The ear of corn is remarkably different from that of its ancestor, Mexican teosinte, despite the fact that corn and teosinte are completely interfertile and are thought to have diverged only 10,000 years ago under Indian domestication. Tests will be conducted on progeny between corn and teosinte races, analyzing associations between molecular markers on the chromosomes and genes controlling morphological features in corn and teosinte ears. By these means it should be possible to determine the minimum number of genes controlling key morphological differences between corn and teosinte, the chromosomal location of these genes, the mode of action (dominant or recessive), and the percentage of phenotypic variation for which each gene accounts. The data will allow discrimination between two competing theories on the morphological steps involved in the recent origin of the corn ear.