Department of Botany, Natural History Museum, London SW7 5BD, UK.
BMC Plant Biol. 2013 Dec 20;13:219. doi: 10.1186/1471-2229-13-219.
Homosporous ferns are distinctive amongst the land plant lineages for their high chromosome numbers and enigmatic genomes. Genome size measurements are an under exploited tool in homosporous ferns and show great potential to provide an overview of the mechanisms that define genome evolution in these ferns. The aim of this study is to investigate the evolution of genome size and the relationship between genome size and spore size within the apomictic Asplenium monanthes fern complex and related lineages.
Comparative analyses to test for a relationship between spore size and genome size show that they are not correlated. The data do however provide evidence for marked genome size variation between species in this group. These results indicate that Asplenium monanthes has undergone a two-fold expansion in genome size.
Our findings challenge the widely held assumption that spore size can be used to infer ploidy levels within apomictic fern complexes. We argue that the observed genome size variation is likely to have arisen via increases in both chromosome number due to polyploidy and chromosome size due to amplification of repetitive DNA (e.g. transposable elements, especially retrotransposons). However, to date the latter has not been considered to be an important process of genome evolution within homosporous ferns. We infer that genome evolution, at least in some homosporous fern lineages, is a more dynamic process than existing studies would suggest.
同型孢子蕨类在陆地植物谱系中因其染色体数目高和神秘的基因组而独具特色。基因组大小测量在同型孢子蕨类中是一种未被充分利用的工具，它们在这些蕨类中提供了对定义基因组进化机制的全面了解，具有巨大的潜力。本研究的目的是调查同型孢子蕨类的基因组大小演化以及在无融合生殖的 Asplenium monanthes 蕨类复合体及其相关谱系中基因组大小与孢子大小之间的关系。
对孢子大小与基因组大小之间关系进行的比较分析表明，它们之间没有相关性。然而，这些数据确实为该组中物种之间存在明显的基因组大小变异提供了证据。这些结果表明，Asplenium monanthes 的基因组大小已经经历了两倍的扩张。
我们的研究结果挑战了一个广泛持有的假设，即孢子大小可用于推断无融合生殖蕨类复合体中的倍性水平。我们认为，观察到的基因组大小变异很可能是由于染色体数目的增加（多倍体）和重复 DNA（如转座元件，特别是反转录转座子）的扩增导致的。然而，到目前为止，后者尚未被认为是同型孢子蕨类中基因组演化的一个重要过程。我们推断，基因组演化，至少在某些同型孢子蕨类谱系中，是一个比现有研究所表明的更为动态的过程。
