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.

Aim  Nicotiana section Suaveolentes is largely endemic to Australia but includes one species endemic to Africa, one to New Caledonia and Tongatupa, and one to the Marquesas Islands in the Pacific. Other sections of Nicotiana are found in the New World. In Australia, Suaveolentes is widespread across the continent, with many taxa adapted to the Eremean zone. We aim to analyse the biogeography of the Australian clade, both to shed light on the evolution of the group and to determine general area relationships that provide insight into the history of the arid‐zone biota. Location  Mesic and arid regions of continental Australia, the Central–South Pacific and Namibia, Africa. Methods  A phylogeny of Suaveolentes, based on morphology and molecular data, was used to analyse the relationships of areas in which the taxa occur. The section is monophyletic, and all but three taxa were included (25). The method of paralogy‐free subtree analysis was employed, with the basal taxon Nicotiana africana used as the outgroup. Results  Paralogy‐free subtree analysis found five area subtrees that, when combined, resulted in a minimal area cladogram with six resolved nodes. Pacific and mesic eastern Australia (including Lord Howe Island) are at the base of the area cladogram, followed by the differentiation of North West Australia and later South East Australia. Arid regions of Australia are related, revealing three biogeographical tracks: a northern track including the Great Sandy Desert and Tanami, which are related to the Pilbara; a central track relating the Western Desert, Central Ranges, Eastern Desert and North East Interzone; and a southern track relating the South West Interzone, Nullarbor, Adelaide/Eyre and the South East Interzone. Plesiomorphic taxa with chromosome number n = 24–23 occur on the periphery of the continent, and derived taxa with n = 21, 20, 18, 16–15 identify the tracks across arid Australia. Main conclusions  The patterns of distribution and differentiation of Suaveolentes in Australia show that the age of the clade is at least Early Miocene, dating to before the onset of aridification in Australia about 15 Ma. The patterns are also interpreted as evidence that it was vicariance that largely shaped speciation in the Eremean zone, with range expansion of some widespread taxa probably occurring in the most recent cycles of severe drying and mobilization of desert dune sands.

Nicotiana section Suaveolentes (Solanaceae) currently includes 28 species and subspecies that are endemic to Australasia and the South Pacific and one African species, N. africana. The section is monophyletic and of allotetraploid origin, but relationships among the species in it and its diploid progenitors are poorly understood. Here we report chromosome numbers for 20 of the 29 taxa from the Suaveolentes, including a count for one recently proposed species for which no number has previously been available. Many of the published chromosome numbers for the Suaveolentes are confirmed in this study. However, six counts were different from the published numbers including n = 15 for N. maritima and N. suaveolens, which is a new chromosome number for the genus. Nicotiana goodspeedii and N. rotundifolia were n = 16, and the same number was found in the suggested species N. sp. 'Corunna'. Nicotiana suaveolens contains polyploid races of n = 32 and here we report the probable existence of an n = 31 race as well. Karyotypic variation within species and within the section is apparently much greater than previously thought and further investigation is warranted.

Nicotiana is found predominantly in the Americas and Australia, but also has representatives in Africa and the Pacific Islands. All native Australian Nicotiana species belong to section Suaveolentes. The number of species in this section is uncertain and subject to revision. An example of this uncertainty is the taxonomic status of a South Australian Nicotiana accession colloquially termed 'Corunna'. Here, we report sequences for nuclear and plastid markers for N. sp. Corunna (D.E. Symon 17088) and accessions of two other Australian species, N. burbidgeae and N. benthamiana. Phylogenetic comparison of these sequences with those of other members of Nicotiana places all three taxa in N. section Suaveolentes and shows that 'Corunna' represents a distinct phylogenetic lineage in a well supported clade along with N. goodspeedii, N. maritima, N. amplexicaulis and N. suaveolentes. Phenetic analysis of floral characters also supports recognition of N. sp. Corunna (D.E.Symon 17088) as a distinct species, which we describe here as Nicotiana paulineana Newbigin & P.M.Waterh., sp. nov. The enlarged molecular dataset described here contributes to a better understanding of taxonomic relationships within the section.

The taxonomy of South Pacific species of Nicotiana requires clarification. Examination of type and other herbarium specimens supports recognition of three species: N. forsteri Roem. & Schult., N. fragrans Hook., and N. fatuhivensis F.Br. Nicotiana forsteri is synonomised with N. debneyi Domin. In agreement with previous authors, N. forsteri is considered to be validly published and the name takes precedence over N. debneyi. Plants from the Marquises Islands are distinct from N. fragrans Hook., and thus are recognised at species level as N. fatuhivensis F.Br. rather than N. fragrans var. fatuhivensis (F.Br.) Goodsp. Nicotiana fatuhivensis is treated as a member of section Suaveolentes Goodsp., not section Paniculatae Goodsp., but more data are required to resolve its relationships. Key features for discriminating taxa are the presence of a caudex, ellipsoid-headed glandular hairs and flower size.