Chloroplast DNA Evidence on the Origin and Radiation of the Giant Lobelias in Eastern Africa

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Abstract
Chloroplast DNA restriction-site variation was surveyed for all 21 species, five non-autonymic subspecies, and six putative F1 hybrids of tetraploid giant lobelias from eastern Africa (Lobeliaceae, Lobelia subgenus Tupa section Rhynchopetalum; 95 accessions), the Brazilian L. organensis, and all four hexaploid Chilean species of section Tupa. The survey used 14 enzymes and revealed 275 synapomorphic mutations and 85 autapomorphic mutations. Morphological and biogeographical interpretation of the molecular phylogeny suggests that the giant lobelias arrived in eastern Africa on the ancient upland features of Tanzania, possibly the Uluguru Mts., as a colonist with a branched inflorescence from the Asia/Pacific region. One early lineage dispersed to southern Tanzania where it founded a clade of 14 extant species, all with unbranched inflorescences, that shows strong geographic patterns of phylogenetic relationship centered in the Western Rift, Eastern Rift, and Ethiopian mountains. Within the Branched Inflorescence clade, one lineage diversified to form five extant species restricted to the Eastern Arc mountains in Tanzania, and its sister-lineage gave rise to the morphologically and geographically heterogeneous group that includes L. organensis. Phylogenetic relationships show a pattern of movement from ancient upland features to Miocene, Pliocene, Pleistocene, and Recent volcanoes, with early diversification primarily geographic in nature, and recent speciation primarily altitudinal and predominantly upward. Simple interpretation of our evidence indicates putative instances of a relictual species, F1 hybrids, species of ancient hybrid origin, and extant ancestor-descendant (or progenitor-derivative) species pairs.

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