BREEDING SYSTEMS, POLLEN FLOW, AND CONTINUING EVOLUTION IN AVON GORGE SORBUS (WHITEBEAMS, ROWANS, AND SERVICE TREES)

雅芳峡谷是英国最丰富的植物遗址之一，其Sorbus多样性在国际上具有重要意义，在19个分类群中记录了3种特有物种和至少3种特有杂交物种（Rich et al. 2008）。目前被列入IUCN濒危物种红色名录的有4种：S. bristooliensis、S. wilmottiana、S. anglica和S. eminens （IUCN, 2007）。Hiscock和CASE的合作伙伴Rich最近的研究表明，峡谷中的Sorbus多样性主要是由两性二倍体S. aucuparia、S. torminalis和（特别是）S. aria与多倍体兼性单性体S. porrigentiformis及其衍生物S. eminens和S. bristooliensis （Robertson等人正在准备中）杂交产生的。不同分类群所采用的交配系统的类型对这一持续的进化过程至关重要。这3个有性类群是专性异交，而杂交多倍体要么是专性单性异交（产生无性种子），要么是兼性单性异交（通过无融合和有性生殖的结合繁殖）。在多倍体中，无融合是通过假交配进行的，需要授粉来诱导无性种子发育并产生有性胚乳。然而，重要的是，我们最近的研究表明，由于自交不亲和，罕见的无融合多倍体S. bristooliensis和S. wilmottiana不能利用自花粉或来自其他同特异性个体（克隆）的花粉。这就需要它们接受来自其他Sorbus物种的花粉，特别是s.a aria，以便通过假配子产生种子（Robertson等人在准备中）。因此，授粉对Sorbus类群的所有生殖过程至关重要，因此对其种群动态和持续进化至关重要。本研究旨在量化雅芳峡谷Sorbus类群的花粉运动和花粉限制，并将其与新Sorbus类群的种群动态和持续进化联系起来。然后，这些数据将用于通知我们的合作者，布里斯托尔市议会，自然英格兰和国家信托基金，最适当的保护策略，以保护稀有的特有的Sorbus物种，并维持在雅芳峡谷内创造Sorbus多样性的过程。我们的初步研究结果（Robertson等人正在准备中）表明，对于罕见的无融合的Sorbus物种来说，S. aria是种子生产所需的重要花粉来源，这表明S. aria需要在所有Sorbus保护策略中加以考虑。本文的研究主要有两个目的：1)表征5个Sorbus新分类群的交配系统；2)量化Avon Gorge不同Sorbus分类群的开花时间、花粉运动和花粉限制，并将其与生殖产出、种群动态和Sorbus新分类群的持续进化联系起来。特别是，这项工作将证实s.a aria花粉是否确实推动了Avon Gorge Sorbus的繁殖和持续多样化，并提供维持这些过程所需的s.a aria的数量和密度，并保证稀有特有物种的成功繁殖。这将通过实现以下目标来实现：在尚未测试的新描述的Sorbus分类群中建立繁殖模式（交配系统）。2. 确定所有Sorbus分类群开花时间的年度和季节变化。3. 确定Sorbus传粉者的主要传粉者。4. 测定两性和无融合花楸属分类群的花粉限制。5. 利用荧光尘埃和多态微卫星标记估计单株Sorbus树（特别是S. aria）的花粉流距离。6. 计算花粉邻域和预测在罕见的假配子单配体中影响种子生产所需的锦葵密度。

The Avon Gorge is one of the richest botanical sites in Britain, and is internationally important for its Sorbus diversity, with 3 endemic species and at least 3 endemic hybrids amongst the 19 taxa recorded (Rich et al. 2008). Four species are currently on the IUCN Red List of endangered species: S. bristoliensis, S. wilmottiana, S. anglica and S. eminens (IUCN, 2007). Recent research by Hiscock and CASE partner, Rich, has shown that Sorbus diversity in the gorge is generated primarily by hybridisation between sexual diploids S. aucuparia, S. torminalis, and (especially) S. aria, and the polyploid facultative apomict S. porrigentiformis and its derivatives, S. eminens and S. bristoliensis (Robertson et al. in preparation). The type of mating system employed by the various taxa is pivotal to this continuing evolutionary process. The 3 sexual taxa are obligate outcrossers whereas the hybrid polyploids are either obligate apomicts (producing asexual seed), or facultative apomicts (reproducing by a combination of apomixis and sexual reproduction). In the polyploids, apomixis is via pseudogamy with pollination being required to induce asexual seed development and produce a sexual endosperm. Importantly, however, our recent studies have shown that the rare apomictic polyploids, S. bristoliensis and S. wilmottiana, cannot utilize self-pollen or pollen from other con-specific individuals (clones) because of self-incompatibility. This necessitates that they receive pollen from other Sorbus species, in particular S. aria, in order to produce seeds via pseudogamy (Robertson et al. in preparation). Thus pollination is essential for all reproductive processes among the Sorbus taxa and consequently for their population dynamics and ongoing evolution. This research will aim to quantify pollen movement and pollen limitation among Sorbus taxa in the Avon Gorge and relate this to population dynamics and ongoing evolution of new Sorbus taxa. These data will then be used to inform our collaborators, Bristol City Council, Natural England, and The National Trust, on the most appropriate conservation strategies to preserve rare endemic Sorbus species and to maintain the process responsible for creating Sorbus diversity within the Avon Gorge. Our preliminary findings (Robertson et al. in preparation) indicate that S. aria is an essential source of pollen needed for seed production by the rare apomictic Sorbus species suggesting that S. aria will need to be considered in all Sorbus conservation strategies. The research proposed here has two main aims: 1) to characterize the mating system(s) of the 5 new Sorbus taxa and 2) to quantify flowering time, pollen movement and pollen limitation among the different Sorbus taxa in the Avon Gorge and relate this to reproductive output, population dynamics, and ongoing evolution of new Sorbus taxa. In particular, this work will confirm whether S.aria pollen is indeed driving reproduction and ongoing diversification of Sorbus in the Avon Gorge, and provide estimates of the number and density of S. aria needed to maintain these processes and guaratee successful reproduction of the rare endemic species. This will be accomplished through delivery of the following objectives: 1. Establish modes of reproduction (mating system) in newly described Sorbus taxa not already tested. 2. Determine annual and seasonal variation in flowering time for all Sorbus taxa. 3. Determine the primary pollinators of Sorbus pollinators. 4. Measure pollen limitation among sexual and apomictic Sorbus taxa. 5. Estimate distances of pollen flow from individual Sorbus trees (particularly S. aria) using fluorescent dust and polymorphic microsatellite markers. 6. Calculate pollen neighbourhoods and predict densities of S. aria required to effect seed production in rare pseudogamous apomicts.