Recent origin and phylogenetic utility of divergent ITS putative pseudogenes: a case study from Naucleeae (Rubiaceae)

Abstract

The internal transcribed spacer (ITS) of nuclear ribosomal DNA has been widely used by systematists for reconstructing phylogenies of closely related taxa. Although the occurrence of ITS putative pseudogenes is well documented for many groups of animals and plants, the potential utility of these pseudogenes in phylogenetic analyses has often been underestimated or even ignored in part because of deletions that make unambiguous alignment difficult. In addition, long branches often can lead to spurious relationships, particularly in parsimony analyses. We have discovered unusually high levels of ITS polymorphism (up to 30%, 40%, and 14%, respectively) in three tropical tree species of the coffee family (Rubiaceae), Adinauclea fagifolia, Haldina cordifolia, and Mitragyna rubrostipulata. Both secondary structure stability and patterns of nucleotide substitutions in a highly conserved region (5.8S gene) were used for distinguishing presumed functional sequences from putative pseudogenes. The combination of both criteria was the most powerful approach. The sequences from A. fagifolia appear to be a mix of functional genes and highly distinct putative pseudogenes, whereas those from H. cordifolia and M. rubrostipulata were identified as putative pseudogenes. We explored the potential utility of the identified putative pseudogenes in the phylogenetic analyses of Naucleeae sensu lato. Both Bayesian and parsimony trees identified the same monophyletic groups and indicated that the polymorphisms do not transcend species boundaries, implying that they do not predate the divergence of these three species. The resulting trees are similar to those produced by previous analyses of chloroplast genes. In contrast to results of previous studies therefore, divergent putative pseudogenes can be useful for phylogenetic analyses, especially when no sequences of their functional counterparts are available. Our studies clearly show that ITS polymorphism may not necessarily mislead phylogenetic inference. Despite using many different PCR conditions (different primers, higher denaturing temperatures, and absence or presence of DMSO and BSA-TMACl), we recovered only a few functional ITS copies from A. fagifolia and none from H. cordifolia and M. rubrostipulata, which suggests that PCR selection is occurring and/or the presumed functional alleles are located at minor loci (with few ribosomal DNA copies).