Analysis of plastid and nuclear DNA data in plant phylogenetics-evaluation and improvement

Correct combination of plastid (cp) and nuclear (nr) DNA data for plant phylogenetic reconstructions is not a new issue, but with an increasing number of nrDNA loci being used, it is of ever greater practical concern. For accurately reconstructing the phylogeny and evolutionary history of plant groups, correct treatment of phylogenetic incongruence is a vital step in the proper analysis of cpDNA and nrDNA data. We first evaluated the current status of analyzing cpDNA and nrDNA data by searching all articles published in the journal Systematic Botany between 2005 and 2011. Many studies combining cpDNA and nrDNA data did not rigorously assess the combinability of the data sets, or did not address in detail possible reasons for incongruence between the two data sets. By reviewing various methods, we outline a procedure to more accurately analyze and/or combine cpDNA and nrDNA data, which includes four steps: identifying significant incongruence, determining conflicting taxa, providing possible interpretations for incongruence, and reconstructing the phylogeny after treating incongruence. Particular attention is given to explanation of the cause of incongruence. We hope that our procedure will help raise awareness of the importance of rigorous analysis and help identify the cause of incongruence before combining cpDNA and nrDNA data.