National Engineering Laboratory for Tree Breeding; Key Laboratory of Genetics and Breeding in Forest Trees and Ornamental Plants, Ministry of Education, College of Biological Sciences and Technology, Beijing Forestry University, Beijing 100083, China.
BMC Evol Biol. 2014 Mar 21;14(1):53. doi: 10.1186/1471-2148-14-53.
Pollen donor compositions differ during the early stages of reproduction due to various selection mechanisms. In addition, ovules linearly ordered within a fruit have different probabilities of reaching maturity. Few attempts, however, have been made to directly examine the magnitude and timing of selection, as well as the mechanisms during early life stages and within fruit. Robinia pseudoacacia, which contains linear fruit and non-random ovule maturation and abortion patterns, has been used to study the viability of selection within fruit and during the early stages of reproduction. To examine changes in the pollen donor composition during the early stages of reproduction and of progeny originating from different positions within fruit, paternity analyses were performed for three early life stages (aborted seeds, mature seeds and seedlings) in the insect-pollinated tree R. pseudoacacia.
Selection resulted in an overall decrease in the level of surviving selfed progeny at each life stage. The greatest change was observed between the aborted seed stage and mature seed stage, indicative of inbreeding depression (the reduced fitness of a given population that occurs when related individual breeding was responsible for early selection). A selective advantage was detected among paternal trees. Within fruits, the distal ends showed higher outcrossing rates than the basal ends, indicative of selection based on the order of seeds within the fruit.
Our results suggest that selection exists both within linear fruit and during the early stages of reproduction, and that this selection can affect male reproductive success during the early life stages. This indicates that tree species with mixed-mating systems may have evolved pollen selection mechanisms to increase the fitness of progeny and adjust the population genetic composition. The early selection that we detected suggests that inbreeding depression caused the high abortion rate and low seed set in R. pseudoacacia.
由于各种选择机制的作用，花粉供体在生殖早期的组成会有所不同。此外，果实内线性排列的胚珠到达成熟的概率也不同。然而，很少有尝试直接检查选择的幅度和时间，以及生命早期阶段和果实内的机制。紫穗槐，其果实呈线性且胚珠的成熟和败育模式呈非随机分布，已被用于研究果实内和生殖早期阶段的选择活力。为了研究生殖早期阶段花粉供体组成的变化以及来自果实不同位置的后代的变化，对虫媒授粉的紫穗槐的三个生命早期阶段（败育种子、成熟种子和幼苗）进行了亲权分析。
选择导致每个生命阶段存活的自交后代水平总体下降。在败育种子阶段和成熟种子阶段之间观察到的变化最大，表明存在近交衰退（当相关个体繁殖导致早期选择时，给定种群的适应性降低）。父本树木表现出选择性优势。在果实内，远轴端的异交率高于近轴端，表明基于果实内种子的顺序进行选择。
我们的结果表明，选择既存在于线性果实内，也存在于生殖早期阶段，这种选择可以影响雄性生殖成功在生命早期阶段。这表明具有混合交配系统的树种可能已经进化出花粉选择机制，以提高后代的适应性并调整种群遗传组成。我们检测到的早期选择表明，近交衰退导致了紫穗槐高的败育率和低的结实率。
