Comparative morphology and phylogeny of Nicotiana section Suaveolentes (Solanaceae) in Australia and the South Pacific

Abstract

Molecular phylogenies alone have failed to resolve evolutionary relationships of Nicotiana L. section Suaveolentes Goodsp. (Solanaceae), a section largely comprising Australian endemics. Comparative morphology of Suaveolentes is illustrated and characters, together with the chromosome number, coded for phylogenetic analysis. Morphological characters included discrete characters of seeds and trichomes studied by scanning electron and light microscopy, and gap-coded quantitative measurements of flowers and vegetative organs. These data were analysed using both maximum parsimony and splits network, and compared to and combined with molecular analyses based on published nuclear and chloroplast-DNA sequences. Among morphological characters, there was a high level of homoplasy, possibly attributable to convergent evolution, hybridisation and introgression, and the underlying polyploid origin of the group. There was some conflict between the morphological and molecular datasets; however, overall there was a level of concordance that identified a phylogenetic sequence of taxa that reflected a reduction in the chromosome number. With N. africana as the functional outgroup, analyses showed N. fragrans (New Caledonia and Tongatapu) and N. fatuhivensis (Marquesas islands, formerly N. fragrans var. fatuhivensis) to be a basal lineage in the Australian and South Pacific clade of Suaveolentes. N. forsteri (eastern mainland Australia, Lord Howe Island and New Caledonia) is the sister taxon to all other Australian species of Suaveolentes, which form a well supported monophyletic group. Within this Australian endemic group, species with a chromosome count of n = 24 or 23 are early lineages. Two clades with a reduced chromosome number are the ‘N. simulans clade’ (all taxa with n = 20) and the ‘N. suaveolens clade’ (n = 19, 18, 16, or 15). The position of N. cavicola (n = 20 or 23) is equivocal, and it and several other species require further study. Some widespread taxa require greater sampling of populations to test for variation in morphology, DNA sequences and chromosome number for further elucidation of the evolutionary history of Suaveolentes.