CRB: Shifts in the Genetic Load in Response to Inbreeding and Population Size in Brassica rapa

9728855 Waller小种群面临许多危险，包括遗传变异的丧失和近亲繁殖的遗传效应。当积累的有害突变通过近亲繁殖暴露出来时，大多数种群都会经历中度到严重的近亲繁殖抑制。然而，有近亲繁殖历史的种群，如果他们的突变负荷已经被表达并被选择出种群，可能会经历较少的近亲繁殖抑制。一些科学家认为，这种选择可以有效地消除近亲繁殖的抑制，从而有机会通过有意的近亲繁殖来清除受威胁的种群。另一些人则从理论上认为，在大多数情况下，基因净化是微弱和无效的，实际上可能会增加在小群体中固定的轻度有害突变的数量。这个项目将探索自然选择如何有效地清除实际种群的近亲繁殖萧条。为了节省空间和加快研究进度，研究人员将使用卷心菜科（Brassica rapa）中一种小型植物的菌株，这种植物已经在实验室中培育得生长迅速。多系将通过自交受精进行最大程度的近交，而其他不同大小的群体将经历自交和异交交替或纯异交6代。然后，研究者将通过比较第一代、第三代和第六代自交和异交受精种子的幼苗的大小和性能，估计每种处理下近交抑制的减少程度。此外，他还将研究自交系间杂交产生的高适应性是否会在随后的几代随机交配中保持（结果表明净化是有效的），还是会恶化（反映出暂时的杂交活力的衰减）。为了解决另一个争议，研究人员还将比较在不同生长条件下种植的植物所观察到的近亲繁殖衰退的数量。这些实验应该证明近亲繁殖是如何迅速而彻底地清除种群的遗传负荷的。像这样的实证结果应该有助于解决关于清除的有效性和操作速度的争议。这些结果将帮助我们理解包含自交和异交混合的交配系统是如何进化的。它们还将阐明小种群面临的遗传危害，并证明有意近亲繁殖是否可以为受威胁或濒危物种的小种群提供有用的繁殖策略。

9728855 Waller Small populations face many hazards including the loss of genetic variation and the genetic effects of inbreeding. Most populations experience moderate to severe inbreeding depression which occurs when accumulated deleterious mutations are exposed via inbreeding. Populations with a prior history of inbreeding, however, may experience less inbreeding depression if their load of mutations has already been expressed and selected out of the population. Some scientists argue that such selection can effectively eliminate inbreeding depression, allowing opportunities to purge threatened populations of their load via intentional inbreeding. Others argue from theory that purging will be weak and ineffective under most circumstances and may actually increase the number of mildly deleterious mutations fixed in small populations. This project will explore just how effectively selection can purge real populations of their inbreeding depression. To save space and allow rapid progress, the investigator will use strains of a small plant in the cabbage family (Brassica rapa) that has been bred to grow quickly in the laboratory. Multiple lines will be subjected to maximal inbreeding via self-fertilization, while other populations of various size will experience alternating selfing and outcrossing or pure outcrossing for six generations. The investigator will then estimate how much inbreeding depression has declined in each treatment by comparing the size and performance of seedlings derived from self- and cross-fertilized seeds from the first, third, and sixth generations. In addition, he will examine whether the high fitness expected in seedlings derived from crosses among the inbred lines is maintained over subsequent generations of random mating (a result suggesting that purging was effective) or deteriorates (reflecting the decay of temporary hybrid vigor). To address another controversy, the investigator will also compar e the amount of inbreeding depression observed when plants are grown under different growing conditions. These experiments should demonstrate how quickly and completely inbreeding can purge populations of their genetic load. Empirical results like these should help to resolve the controversy over how effective purging can be and how quickly it can operate. Such results will help us to understand how mating systems that involve a mixture of selfing and outcrossing evolve. They will also illuminate the genetic hazards faced by small populations and demonstrate whether intentional inbreeding can provide a useful breeding strategy for small populations in threatened or endangered species.