Abstract
Melolobium is a relatively small southern African papilionoid genus of the family Fabaceae. Although 20 species have previously been described, in the present revision only 15 of them are recognized. It appears to have close affinities with four other African genera of the tribe Genisteae: Adenocarpus, Argyrolobium, Dichilus and Polhillia, from which it can be distinguished by the often spiny habit, auriculate stipules and presence of glands (stalked and stalkless) in many species of the genus.
The type and distribution of trichomes were found to be the most useful characters in the classification of Melolobium, such that it is possible to construct a key to the species based mainly on these characters. The anatomical structure of these trichomes, particularly of the glands, has given an insight into taxonomic relationships between species of Melolobium.
Species and subspecies nomenclature, complete typification, descriptions and geographic distribution for each taxon are presented and a key to the species of the genus, based mainly on the type and distribution of trichomes, is presented. A synopsis of extra-southern African species of the genus Argyrolobium, including a key to the species, is presented. Brief synopses of the genera Dichilus and Polhillia are also given.
Phylogenetic relationships among the five African genera of Genisteae, using morphological and DNA data, were explored. Cladistic analysis of morphological data at generic level indicates that either Melolobium or Dichilus occupies a basal position in the tribe Genisteae and that Argyrolobium and Polhillia are sister taxa. Analysis of DNA sequences show that there is congruence between the results from the two plastid sequences. In both analyses two groups of sister taxa: Adenocarpus and Lupinus, and Polhillia and Argyrolobium, are seen. The Argyrolobium and Polhillia sister relationship is also supported by the ITS analysis. TrnL-F analysis groups Dichilus and Melolobium together as sister taxa, but the bootstrap support is very low (52%). ITS analysis has resulted in very low bootstrap values (except for the Argyrolobium/Polhillia clade which is supported by 100% bootstrap). Morphological data also supports the sister-group relationships between Adenocarpus and Lupinus, and Argyrolobium and Polhillia. All the analyses however, point to a basal position for Melolobium and Dichilus within the tribe Genisteae (they are sister to the rest of the tribe). The exact sequence of branching (whether Dichilus or Melolobium branches off first) remains unresolved. Generic relationships are discussed, and a key to the genera of the African Genisteae is presented.
At species level in Melolobium, morphological data resulted in a fully resolved cladogram in which virgate species are basal to the divaricately branched group. The latter group is further divided into spiny and non-spiny groups. Morphological data is also analysed phenetically. Analysis of all three DNA sequences (rbcL, trnL-F and ITS) on the other, hand could not resolve phylogenetic relationships between species of Melolobium but indicated that speciation in Melolobium occurred relatively recently or perhaps that hybridisation has resulted in a reticulate pattern. Preliminary hypotheses of species relationships in the genera Polhillia and Argyrolobium are also presented, based on cladistic analyses of morphological data.
Prof. B.E. van Wyk