School of Biological and Chemical Sciences, Queen Mary University of London, London, UK.
Key Laboratory of Southern Subtropical Plant Diversity, Fairy Lake Botanical Garden, Shenzhen and Chinese Academy of Sciences, Shenzen, PR China.
Ann Bot. 2019 May 20;123(5):767-781. doi: 10.1093/aob/mcy172.
Gnetophytes, comprising the genera Ephedra, Gnetum and Welwitschia, are an understudied, enigmatic lineage of gymnosperms with a controversial phylogenetic relationship to other seed plants. Here we examined the organization of ribosomal DNA (rDNA) across representative species.
We applied high-throughput sequencing approaches to isolate and reconstruct rDNA units and to determine their intragenomic homogeneity. In addition, fluorescent in situ hybridization and Southern blot hybridization techniques were used to reveal the chromosome and genomic organization of rDNA.
The 5S and 35S rRNA genes were separate (S-type) in Gnetum montanum, Gnetum gnemon and Welwitschia mirabilis and linked (L-type) in Ephedra altissima. There was considerable variability in 5S rDNA abundance, ranging from as few as ~4000 (W. mirabilis) to >100 000 (G. montanum) copies. A similar large variation was also observed in 5S rDNA locus numbers (two to 16 sites per diploid cell). 5S rRNA pseudogenes were interspersed between functional genes forming a single unit in E. altissima and G. montanum. Their copy number was comparable or even higher than that of functional 5S rRNA genes. In E. altissima internal transcribed spacers of 35S rDNA were long and intrinsically repetitive while in G. montanum and W. mirabilis they were short without the subrepeats.
Gnetophytes are distinct from other gymnosperms and angiosperms as they display surprisingly large variability in rDNA organization and rDNA copy and locus numbers between genera, with no relationship between copy numbers and genome sizes apparent. Concerted evolution of 5S rDNA units seems to have led to the amplification of 5S pseudogenes in both G. montanum and E. altissima. Evolutionary patterns of rDNA show both gymnosperm and angiosperm features underlining the diversity of the group.
买麻藤纲植物包括麻黄属、买麻藤属和百岁兰属，是一类研究较少、神秘的裸子植物谱系，其与其他种子植物的系统发育关系存在争议。本文研究了代表物种中核糖体 DNA（rDNA）的组织。
本文采用高通量测序方法分离和重建 rDNA 单位，并确定其基因组内的同源性。此外，还采用荧光原位杂交和 Southern 印迹杂交技术揭示 rDNA 的染色体和基因组组织。
在 Gnetum montanum、Gnetum gnemon 和 Welwitschia mirabilis 中，5S 和 35S rRNA 基因是分开的（S 型），而在 Ephedra altissima 中则是连接的（L 型）。5S rDNA 丰度存在很大差异，从少数的约 4000 个（W. mirabilis）到超过 100000 个（G. montanum）拷贝。5S rDNA 基因座数量也存在类似的大变异（每个二倍体细胞 2 到 16 个位点）。在 E. altissima 和 G. montanum 中，5S rRNA 假基因散布在功能基因之间，形成一个单位。它们的拷贝数与功能 5S rRNA 基因相当，甚至更高。在 E. altissima 中，35S rDNA 的内部转录间隔区较长且具有内在重复，而在 G. montanum 和 W. mirabilis 中则较短且没有亚重复。
买麻藤纲植物与其他裸子植物和被子植物不同，其 rDNA 组织和 rDNA 拷贝数以及基因座数量在属间存在惊人的大变异，而拷贝数与基因组大小之间没有明显的关系。5S rDNA 单位的协同进化似乎导致了 G. montanum 和 E. altissima 中 5S 假基因的扩增。rDNA 的进化模式既有裸子植物的特征，也有被子植物的特征，强调了该类群的多样性。
