Department of Biology, University of Minnesota Duluth, Duluth, MN, 55812, USA.
Department of Biology, Duke University, Durham, NC, 27708, USA.
Am J Bot. 2021 Feb;108(2):263-283. doi: 10.1002/ajb2.1611. Epub 2021 Feb 23.
Xeric environments impose major constraints on the fern life cycle, yet many lineages overcome these limitations by evolving apomixis. Here, we synthesize studies of apomixis in ferns and present an evidence-based model for the evolution and establishment of this reproductive strategy, focusing on genetic and environmental factors associated with its two defining traits: the production of "unreduced" spores (n = 2n) and the initiation of sporophytes from gametophyte tissue (i.e., diplospory and apogamy, respectively).
We evaluated existing literature in light of the hypothesis that abiotic characteristics of desert environments (e.g., extreme diurnal temperature fluctuations, high light intensity, and water limitation) drive the evolution of obligate apomixis. Pellaeid ferns (Cheilanthoideae: Pteridaceae) were examined in detail, as an illustrative example. We reconstructed a plastid (rbcL, trnG-trnR, atpA) phylogeny for the clade and mapped reproductive mode (sexual versus apomictic) and ploidy across the resulting tree.
Our six-stage model for the evolution of obligate apomixis in ferns emphasizes the role played by drought and associated abiotic conditions in the establishment of this reproductive approach. Furthermore, our updated phylogeny of pellaeid ferns reveals repeated origins of obligate apomixis and shows an increase in the frequency of apomixis, and rarity of sexual reproduction, among taxa inhabiting increasingly dry North American deserts.
Our findings reinforce aspects of other evolutionary, physiological, developmental, and omics-based studies, indicating a strong association between abiotic factors and the establishment of obligate apomixis in ferns. Water limitation, in particular, appears critical to establishment of this reproductive mode.
干旱环境对蕨类植物的生命周期造成了重大限制，但许多谱系通过进化出无融合生殖来克服这些限制。在这里，我们综合了蕨类植物无融合生殖的研究，并提出了一个基于证据的模型，用于研究这种生殖策略的进化和建立，重点关注与无融合生殖的两个定义特征相关的遗传和环境因素：产生“未减数”孢子（n = 2n）和从配子体组织启动孢子体（即分别为二倍体孢子和无配子生殖）。
我们根据以下假设评估了现有文献：即沙漠环境的非生物特征（例如，极端的昼夜温度波动、高强度的光照和水分限制）推动了强制性无融合生殖的进化。我们详细研究了 Pellaeid 蕨类植物（Cheilanthoideae：Pteridaceae），作为一个说明性的例子。我们重建了该类群的质体（rbcL、trnG-trnR、atpA）系统发育，并在生成的树中映射了生殖模式（有性生殖与无融合生殖）和倍性。
我们提出的蕨类植物强制性无融合生殖进化的六阶段模型强调了干旱和相关非生物条件在建立这种生殖方式中的作用。此外，我们更新的 Pellaeid 蕨类植物系统发育揭示了强制性无融合生殖的多次起源，并显示出在栖息于越来越干燥的北美的蕨类植物中，无融合生殖的频率增加，有性生殖的罕见。
我们的研究结果强化了其他进化、生理、发育和基于组学的研究的某些方面，表明非生物因素与蕨类植物中强制性无融合生殖的建立之间存在强烈关联。特别是水分限制似乎对建立这种生殖方式至关重要。
