Abstract Polyploidy is one of the most important evolutionary processes in plants. In natural populations, polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. The frequencies of these two pathways, and male versus female gamete contributions, however, are largely unexplored. Ranunculus kuepferi occurs with diploid, triploid and autotetraploid cytotypes in the Alps, whereby diploids are mostly sexual, while tetraploids are facultative apomicts. To test for the occurrence of polyploidization events by triploid bridge, we investigated 551 plants of natural populations via flow cytometric seed screening. We assessed ploidy shifts in the embryo to reconstruct female versus male gamete contributions to polyploid embryo and/or endosperm formation. Seed formation via unreduced egg cells (BIII hybrids) occurred in all three cytotypes, while only in one case both gametes were unreduced. Polyploids further formed seeds with reduced, unfertilized egg cells (polyhaploids and aneuploids). Pollen was highly variable in diameter, but only pollen >27 μm was viable, whereby diploids produced higher proportions of well-developed pollen. Pollen size was not informative for the formation of unreduced pollen. These results suggest that a female triploid bridge via unreduced egg cells is the major pathway toward polyploidization in R. kuepferi, maybe as a consequence of constraints of endosperm development. Triploids resulting from unreduced male gametes were not observed, which explains the lack of obligate sexual tetraploid individuals and populations. Unreduced egg cell formation in diploids represents the first step toward apomixis.

中文翻译：

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多倍体途径：高山物种毛茛属（毛茛科）中雌性三倍体桥的迹象

摘要 多倍体是植物最重要的进化过程之一。在自然种群中，多倍体通常来自未减少的配子，它们与减少的配子融合，产生三倍体后代（三倍体桥），或与其他未减少的配子融合，产生四倍体胚胎。然而，这两种途径的频率以及男性与女性配子的贡献在很大程度上尚未得到探索。Ranunculus kuepferi 在阿尔卑斯山以二倍体、三倍体和同源四倍体细胞型出现，其中二倍体主要是有性的，而四倍体是兼性无融合生殖体。为了通过三倍体桥测试多倍化事件的发生，我们通过流式细胞仪种子筛选调查了 551 株自然种群。我们评估了胚胎中的倍性变化，以重建雌性与雄性配子对多倍体胚胎和/或胚乳形成的贡献。通过未减少的卵细胞（BIII 杂种）形成的种子发生在所有三种细胞类型中，而只有在一种情况下，两种配子都未减少。多倍体进一步形成具有减少的未受精卵细胞（多单倍体和非整倍体）的种子。花粉的直径变化很大，但只有 >27 μm 的花粉是有活力的，因此二倍体产生了更高比例的发育良好的花粉。花粉大小对于未减少的花粉的形成没有提供信息。这些结果表明，通过未减少的卵细胞形成的雌性三倍体桥是 R. kuepferi 多倍化的主要途径，这可能是胚乳发育限制的结果。未观察到由未减少的雄配子产生的三倍体，这解释了缺乏专性四倍体个体和种群的原因。二倍体中未减少的卵细胞形成代表了无融合生殖的第一步。