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Remarkable variation of ribosomal DNA organization and copy number in gnetophytes, a distinct lineage of gymnosperms

Gnetophytes, comprising the genera Ephedra, Gnetum and Welwitschia, are an understudied, enigmatic lineage of gymnosperms with a controversial phylogenetic relationship to other seed plants. Wang et al. examined the organization of ribosomal DNA (rDNA) across representative species.

Projections of RepeatExplorer graphs representing rDNA units in gnetophytes
Projections of RepeatExplorer graphs representing rDNA units in gnetophytes. Graphs show rDNA paths in (A) G. montanum 35S rDNA, (B) E. altissima linked 35S–5S rDNA, (C) W. mirabilis35S rDNA, (D) G. montanum 5S rDNA and (E) W. mirabilis 5S rDNA. Individual subregions of 35S rDNA are highlighted (see legend in the centre of the figure). The 5S subregions indicated are as follows: (B) 5Sp1 and 5Sp2 correspond to 5S pseudogene 1 and 2, respectively; (D) NTS, non-transcribed spacer (region); 5Sc, 5S rDNA coding sequence; 5Sp, 5S rDNA pseudogene; (E) NTS1, NTS2, NTS3, putative NTS categories 1, 2 and 3; 5Sc, 5S rDNA coding sequence.

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.

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