IngentaConnect Structure, molecular evolution, and phylogenetic utility of the 5...

Abstract:

The 18S-26S nuclear rDNA external transcribed spacer (ETS) has recently gained attention as a region that is valuable in phylogenetic analyses of angiosperms primarily because it can supplement nucleotide variation from the widely used and generally shorter internal transcribed spacers (ITS-1 and ITS-2) and thereby improve phylogenetic resolution and clade support in rDNA trees. Subrepeated ETS sequences (often occurring in the 5 region) can, however, create a challenge for systematists interested in using ETS sequence data for phylogeny reconstruction. We sequenced the5ETS for members ofLessingia (Compositae, Astereae) and close relatives (26 taxa total) to characterize the subrepeat variation across a group of closely related plant lineages and to gain improved understanding of the structure, molecular evolution, and phylogenetic utility of the region. The5ETS region ofLessingia and relatives varied in length from sim 245 to 1009 bp due to the presence of a variable number of subrepeats (one to eight). We assessed homology of the subrepeats using phylogenetic analysis and concluded that only two of the subrepeats and a portion of a third ( sim 282 bp in total) were orthologous across Lessingia and could be aligned with confidence and included in further analyses. When the partial5ETS data were combined with3ETS and ITS data in phylogenetic analyses, no additional resolution of relationships among taxa was obtained beyond that found from analysis of3ETS + ITS sequences. Inferred patterns of concerted evolution indicate that homogenization is occurring at a faster rate in the3ETS and ITS regions than in the 5ETS region. Additionally, homogenization appears to be acting within but not among subrepeats of the same rDNA array. We conclude that challenges in assessing subrepeat orthology across taxa greatly limit the utility of the5ETS region for phylogenetic analyses among species of Lessingia.

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Structure, molecular evolution, and phylogenetic utility of the 5 region of the external transcribed spacer of 18S-26S rDNA in Lessingia (Compositae, Astereae)