Herbarium and Department of Botany, Charles University, Prague, Czech Republic.
Ann Bot. 2019 Jan 23;123(2):391-403. doi: 10.1093/aob/mcy137.
Apomixis or asexual seed reproduction is a key evolutionary mechanism in certain angiosperms providing them with reproductive assurance and isolation. Nevertheless, the frequency of apomixis is largely unknown, especially in groups with autonomous apomixis such as the diploid-polyploid genus Hieracium.
Using flow cytometric analyses, we determined the ploidy level and reproductive pathways (sexual vs. apomictic) for 7616 seeds originating from 946 plants belonging to >50 taxa sampled at 130 sites across Europe.
Diploid seeds produced by diploids were formed exclusively by the sexual pathway after double fertilization of reduced embryo sacs. An absolute majority of tri- and tetraploid seeds (99.6 %) produced by tri- and tetrapolyploid taxa were formed by autonomous apomixis. Only 20 polyploid seeds (0.4 %) were formed sexually. These seeds, which originated on seven polyploid accessions of four taxa, were formed after fertilization of either unreduced embryo sacs through a so-called triploid bridge or reduced embryo sacs, and frequently resulted in progeny with an increased ploidy. In addition, the formation of seedlings with increased ploidy (4x and 6x) was found in two triploid plants. This is the first firm evidence on functional facultative apomixis in polyploid members of Hieracium sensu stricto (s.s.).
The mode of reproduction in Hieracium s.s. is tightly associated with ploidy. While diploids produce seeds exclusively sexually, polyploids produce seeds by obligate or almost obligate apomixis. Strict apomixis can increase the reproductive assurance and this in turn can increase the colonization ability of apomicts. Nevertheless, our data clearly show that certain polyploid plants are still able to reproduce sexually and contribute to the formation of new cytotypes and genotypes. The finding of functional facultative apomicts is essential for future studies focused on evolution, inheritance and ecological significance of apomixis in this genus.
无融合生殖或无性种子繁殖是某些被子植物的关键进化机制，为它们提供了生殖保障和隔离。然而，无融合生殖的频率在很大程度上是未知的，特别是在具有自主无融合生殖的类群中，如二倍体-多倍体属 Hieracium。
使用流式细胞分析，我们确定了来自欧洲 130 个地点的 946 株植物的 7616 粒种子的倍性水平和生殖途径（有性与无融合生殖）。这些植物属于 >50 个分类群。
由二倍体产生的二倍体种子是通过减数胚囊的双受精形成的，完全由有性途径形成。绝大多数由三倍体和四倍体类群产生的三倍体和四倍体种子（99.6%）是通过自主无融合生殖形成的。只有 20 粒多倍体种子（0.4%）是有性形成的。这些种子起源于四个类群的七个多倍体材料，是通过所谓的三倍体桥或减数胚囊受精形成的，并且经常导致后代的倍性增加。此外，在两个三倍体植物中发现了增加倍性（4x 和 6x）的幼苗形成。这是首次在 Hieracium s.s. 的多倍体成员中发现功能上的兼性无融合生殖的证据。
Hieracium s.s. 的繁殖方式与倍性密切相关。虽然二倍体只产生有性种子，但多倍体只产生强制性或几乎强制性的无融合生殖种子。严格的无融合生殖可以提高生殖保障，从而提高无融合生殖体的定居能力。然而，我们的数据清楚地表明，某些多倍体植物仍然能够有性繁殖，并有助于新细胞型和基因型的形成。功能上的兼性无融合生殖体的发现对未来研究该属无融合生殖的进化、遗传和生态意义至关重要。
