Phylogenetic evaluation of 5S ribosomal RNA gene and spacer in the Callistachys group (Fabaceae: Mirbelieae)

Sequences of 5S rDNA from an Australian group of papilionoid legumes were evaluated for phylogenetic informativeness. Twenty four sequences were sampled from ten species in five closely related genera: Brachysema, Callistachys, Jansonia, Nemcia and Podolobium. These sequences fell into two size classes, 200 bp and 600 bp, which appear to represent paralogous copies of the 5S unit at different loci. As in previous studies, the 5S rRNA gene and both ends of the inter-gene spacer are found to be conserved and almost uninformative about phylogeny at this taxonomic level. By contrast, the middle part of the spacer is highly variable, both in substitutions at individual sites and in length. The short sequences contain virtually no duplications. The long sequences contain numerous short repeats (c.20 bp) which form a regular pattern in the middle section. Phylogenies estimated from these data support monophyly of the study group, and of species. Relationships among species and genera are less clean perhaps because divergence of the 5S spacer between species is too great. However, non-monophyly of Nemcia and Brachysema, indicated by other data, is corroborated.