A Phylogenetic Study of the Old World Asclepiadinae (Apocynaceae) Based on Chloroplast and Nuclear DNA Sequence Data

Abstract

Relationships within the African Asclepias generic complex (Asclepiadinae, Apocynaceae) have for a long time been only a matter of intuitive speculation and generic delimitations have been diverse and rather contentious. Generic delimitation in this group has been based on morphological characters that are usually not exclusive to any particular clade or genus. The difficulty of identifying taxonomically useful morphological characters for inferring generic delimitations has led to differences in the emphasized characters by different taxonomists. This study is aimed at understanding phylogenetic relationships of species within the African Asclepias complex based upon the nuclear PgiC and three chloroplast DNA regions, rpll6 intron, trnC-rpoB spacer and trnS-G spacer/trnG intron. The data were analyzed by maximum parsimony, maximum likelihood and Bayesian inference methods and hypothesis tests were performed to test the monophyly of the putative genera using the Shimodaira-Hasegawa test. Lastly maximum parsimony was used to reconstruct ancestral character states for species habitat preference and some morphological characters previously used for generic delimitations in the African Asclepias complex. The African Asclepias complex is made up of two basal lineages, one containing only the Ethiopian highlands endemic species Trachycalymma pseudofimbriatum as the first diverging lineage and the other containing the rest of the African Asclepias sensu lato. Members of the Gomphocarpus integer group plus G. tomentosus and G. filiformis form the next diverging lineage sister to the remaining species. A number of genera are not monophyletic namely Gomphocarpus, Pachycarpus, Stathmostelma, Xysmalobium, Trachycalymma, Schizoglossum and Aspidoglossum. Glossostelma is monophyletic. Species still classified as Asclepias sensu lato show various, strongly supported affinities as follows: A. aurea is sister to A. cucullata, A. randii is sister to sample attributable to Gen. indet. aff. Asclepias (10917), A. macropus is sister to A. praemorsa, A. densiflora is sister to Xysmalobium aceratoides and A. gibba is sister to A. disparilis. The morphological characters optimized on the phylogenetic tree, were all homoplasious as indicated by multiple origins of character states. Ancestral characters states included stems branching, broad leaves, presence of nodal only inflorescences, pedunculate inflorescences and the Gomphocarpus type cucullate plus laterally flattened corona lobes.