6-Year Periodicity and Variable Synchronicity in a Mass-Flowering Plant

Periodical organisms, such as bamboos and periodical cicadas, are very famous for their synchronous reproduction. In bamboos and other periodical plants, the synchronicity of mass-flowering and withering has been often reported indicating these species are monocarpic (semelparous) species. Therefore, synchronicity and periodicity are often suspected to be fairly tightly coupled traits in these periodical plants. We investigate the periodicity and synchronicity of Strobilanthes flexicaulis, and a closely related species S. tashiroi on Okinawa Island, Japan. The genus Strobilanthes is known for several periodical species. Based on 32-year observational data, we confirmed that S. flexicaulis is 6-year periodical mass-flowering monocarpic plant. All the flowering plants had died after flowering. In contrast, we found that S. tashiroi is a polycarpic perennial with no mass-flowering from three-year individual tracking. We also surveyed six local populations of S. flexicaulis and found variation in the synchronicity from four highly synchronized populations (>98% of plants flowering in the mass year) to two less synchronized one with 11–47% of plants flowering before and after the mass year. This result might imply that synchrony may be selected for when periodicity is established in monocarpic species. We found the selective advantages for mass-flowering in pollinator activities and predator satiation. The current results suggest that the periodical S. flexicaulis might have evolved periodicity from a non-periodical close relative. The current report should become a key finding for understanding the evolution of periodical plants.以竹类、周期蝉为代表的周期性生物，以同步繁殖行为享誉学界。在竹类及其他周期性植物中，集体开花与枯萎的同步性现象屡被报道，这提示此类物种属于单次结实（semelparous，终生一次繁殖）类群。因此，这类周期性植物的同步性与周期性往往被视为紧密耦合的关键性状。本研究以日本冲绳岛的曲茎紫云菜（Strobilanthes flexicaulis）及其近缘物种田代紫云菜（S. tashiroi）为研究对象，探究二者的周期性与同步性特征。紫云菜属（Strobilanthes）已知包含多个周期性物种类群。基于32年的长期观测数据，我们证实曲茎紫云菜为6年生周期性集体开花单次结实植物，所有开花个体在完成开花后均会死亡。与之形成鲜明对比的是，通过3年的个体跟踪观测，我们发现田代紫云菜为多次结实多年生植物，无集体开花现象。我们还对曲茎紫云菜的6个本地居群开展了调查，结果显示其同步性存在显著变异：4个种群为高度同步化类群（集体开花年份中开花植株占比超98%），剩余2个种群同步性较低，在集体开花年份前后共有11%~47%的植株开花。本研究结果暗示，在单次结实物种中，若已演化出周期性特征，集体开花的同步性可能会受到自然选择的青睐。我们发现集体开花在传粉者活动效率与捕食者饱和效应两方面均具有选择优势。本研究结果表明，周期性的曲茎紫云菜可能从不具有周期性的近缘祖先物种演化而来。本报告将为阐释周期性植物的演化机制提供关键研究依据。