Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, USA.
Am J Bot. 2012 Apr;99(4):757-68. doi: 10.3732/ajb.1100472.
Plants and animals may experience reproductive Allee effects in fragmented populations, and obligate pollination mutualisms may be especially sensitive to extinction risk via this density-dependent process. In this study we examine how a shift from within-crown reproductive synchrony to asynchrony influences reproductive assurance through contributions to selfing and outcrossing in small, spatially isolated populations of Ficus.
The research focuses on the monoecious fig F. petiolaris and consists of phenological analyses and genetic assessments of selfing and outcrossing for populations located in Baja California's Sonoran Desert.
Phenological censuses of eight populations revealed within-crown asynchrony in 44% of reproducing trees, with 16% having sufficient overlap of male and female flowering phases to permit selfing via the cycling of pollinating fig wasps within natal trees. In mating system analyses of two of these populations, however, multilocus outcrossing rates (t(m)) were indistinguishable from 1. This result, combined with low levels of inbreeding, indicates selfing to be absent or at best a minor contributor to reproductive assurance.
The results indicate that the fitness benefits of within-crown asynchrony lie not with selfing, as commonly asserted, but with increased opportunities for outcross pollen transmission and receipt, changing our understanding of the mechanisms by which reproduction is facilitated and extinction risk minimized in naturally fragmented Ficus populations. Given the role of fig fruit as a keystone food resource in many tropical environments, trait variation leading to reproductive assurance in figs, such as within-crown asynchrony, has broader ecosystem-level implications.
植物和动物在种群破碎化的情况下可能会经历繁殖阿利效应，而强制性传粉共生关系可能特别容易受到这种密度依赖过程的灭绝风险的影响。在这项研究中，我们研究了从树冠内繁殖同步到异步的转变如何通过自我受精和异交对小的、空间隔离的榕属种群的繁殖保证产生影响。
该研究集中在雌雄同体榕属植物 F. petiolaris 上，包括对位于下加利福尼亚索诺兰沙漠的种群进行的物候分析和自我受精与异交的遗传评估。
对八个种群的物候学普查显示，44%的繁殖树存在树冠内异步现象，其中 16%的雄花和雌花开花阶段有足够的重叠，允许通过在母树内循环传粉榕小蜂进行自交。然而，在对其中两个种群的交配系统分析中，多基因座异交率（t(m)）与 1 无法区分。这一结果，加上低水平的近交，表明自交不存在，或者最多只是繁殖保证的一个次要贡献者。
研究结果表明，树冠内异步的适应优势不在于自交，如通常所认为的那样，而是在于增加了异交花粉传播和接收的机会，改变了我们对自然破碎化榕属种群中促进繁殖和最小化灭绝风险的机制的理解。鉴于榕果作为许多热带环境中的关键食物资源的作用，导致榕属植物繁殖保证的性状变异，如树冠内异步，具有更广泛的生态系统层面的意义。
