Mechanism of genetic differentiation of population repeatedly generated under high density

为探讨植物种群对高密度胁迫的适应策略，以芥菜（Brassica juncea） 2个品种C_<ZERN>为研究对象。以每平方米1万粒的高密度（H型）和每平方米1株（C型）的低密度（对照）连续栽培数代。H型和C型最初彼此的特征非常相似，但几代之后，它们在几个特征上开始出现分歧。H型比C型种子大，种子少，开花时间早。在高密度条件下，H型比C型具有更强的竞争能力。为了阐明植物对高密度胁迫的适应策略，研究了种子大小的变化及其在竞争能力中的作用。种子大小的选择是有效的，说明H型和C型都包含种子大小的遗传变异。在密播试验中，大粒种子的植株总比小粒种子的植株具有优势。但在相同大小种子的H型和C型混合林分中，H型比C型具有更高的竞争能力。由此可见，H型种子的竞争成功不仅受到种子大小的影响，还受到其他未知因素的影响，从而影响其竞争能力。虽然大种子的萌发速度比小种子慢，但C型种子在萌发时受种子大小的影响比H型更大。幼期种子大小与植株大小呈显著正相关。与C型相比，H型的种子大小与植株大小的关系维持的时间更长。结果表明，H型具有对高密度胁迫的综合适应特性。结果表明，该植物在高密度胁迫下具有较高的竞争能力。竞争能力可以通过包括种子大小在内的几个特征的组合来实现。种子大小性状具有较高的遗传力，在H型表现出显著的增大趋势。少

To look into the adaptive strategies of plant populations to high density stress, two cultivars in Brassica juncea C_<ZERN>.et Coss were grown for several generations at high density about 10,000 seeds sown per square meter ( H form ) and at low density, 1 plant / m^2 ( C form, as control).The H and C forms were initially very similar to characteristics each other, but after a few generations, they began to diverge in several characteristics. The H form had larger seeds, fewer seeds and earlier flowering time than the C form. Also, under high density conditions, the H form had stronger competitive ability than the C form.To clarify the adaptive strategies to high density stress, variations in seed size and its role in competitive ability were investigated.The selection for seed size was effective, suggesting that both the H and C forms contained the genetic variations of seed size.In dense sowing experiments, plants from larger seeds always had an advantage over those from smaller seed … More s. However, in mixed-stand of the H and C forms from the same size seeds, the H form had higher competitive ability than the C form.Therefore, it appeared that the competitive success of the H form could be affected not only by the seed size, but also by other unknown factors, contributing to the competitive ability.Although larger seeds germinated more slowly than smaller seeds, the C form was more strongly influenced by seed size at the time of germination than the H form. Also, seed size positively correlated to plant size during the juvenile phase. The attribution of seed size to plant size in the H form was maintained for longer growth periods than in the C form. It was found that the H form had a combination of adaptive characteristics to high density stress.These results showed that the plant was ensured survival by a high competitive ability under high density stress. The competitive ability could be achieved by a combination of several characteristics including seed size. Seed size characteristics had high heritability and then the remarkable increase of seed size seemed to be attained in the H form. Less