Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, UK.
School of Biological and Chemical Sciences, Queen Mary University of London, London, UK.
Ann Bot. 2018 Jun 28;122(1):133-150. doi: 10.1093/aob/mcy048.
Recent tissue-level observations made indirectly via flow cytometry suggest that endoreplication (duplication of the nuclear genome within the nuclear envelope in the absence of subsequent cell division) is widespread within the plant kingdom. Here, we also directly observe ploidy variation among cells within individual petals, relating size of nucleus to cell micromorphology and (more speculatively) to function.
We compared the labella (specialized pollinator-attracting petals) of two European orchid genera: Dactylorhiza has a known predisposition to organismal polyploidy, whereas Ophrys exhibits exceptionally complex epidermal patterning that aids pseudocopulatory pollination. Confocal microscopy using multiple staining techniques allowed us to observe directly both the sizes and the internal structures of individual nuclei across each labellum, while flow cytometry was used to test for progressively partial endoreplication.
In Dactylorhiza, endoreplication was comparatively infrequent, reached only low levels, and appeared randomly located across the labellum, whereas in Ophrys endoreplication was commonplace, being most frequent in large peripheral trichomes. Endoreplicated nuclei reflected both endomitosis and endocycling, the latter reaching the third round of genome doubling (16C) to generate polytene nuclei. All Ophrys individuals studied exhibited progressively partial endoreplication.
Comparison of the two genera failed to demonstrate the hypothesized pattern of frequent polyploid speciation in genera showing extensive endoreplication. Endoreplication in Ophrys appears more strongly positively correlated with cell size/complexity than with cell location or secretory role. Epigenetic control of gene overexpression by localized induction of endoreplication within individual plant organs may represent a significant component of a plant's developmental programme, contributing substantially to organ plasticity.
最近通过流式细胞术间接进行的组织水平观察表明，内复制（核膜内核基因组的复制，而没有随后的细胞分裂）在植物界中广泛存在。在这里，我们还直接观察到单个花瓣内细胞的倍性变化，将细胞核的大小与细胞微观形态（更推测性地）与功能相关联。
我们比较了两个欧洲兰科属的唇瓣（专门吸引传粉者的花瓣）：Dactylorhiza 具有已知的生物体多倍体倾向，而 Ophrys 则表现出异常复杂的表皮模式，有助于拟交配授粉。使用多种染色技术的共焦显微镜允许我们直接观察每个唇瓣中单个细胞核的大小和内部结构，而流式细胞术用于测试渐进性部分内复制。
在 Dactylorhiza 中，内复制相对较少，仅达到低水平，并且在唇瓣上随机定位，而在 Ophrys 中，内复制很常见，在大的外围毛状体中最常见。内复制核反映了内有丝分裂和内循环，后者达到第三轮基因组加倍（16C）以产生多线核。研究的所有 Ophrys 个体都表现出渐进性部分内复制。
对这两个属的比较未能证明在表现出广泛内复制的属中频繁多倍体物种形成的假设模式。Ophrys 中的内复制似乎与细胞大小/复杂性的相关性比与细胞位置或分泌作用更强。通过在单个植物器官内局部诱导内复制来控制基因过表达的表观遗传控制可能代表植物发育计划的重要组成部分，对器官可塑性有很大贡献。
