Long-distance dispersal was responsible for allotetraploid Nicotiana reaching Australia from South America about six million years ago (mya) based on molecular clock analyses. This group of species, recognized taxonomically as N. section Suaveolentes (native Australian tobacco) originated from diploids now found only in South America and comprises species found in Africa (one species, N. africana), several on Pacific islands and c. 50 in Australia (more than half of which have been newly identified in this project), principally found in the deserts of central Australia.
Using standard phylogenetic markers, we established that N. africana is sister to the rest, followed by N. fatuhivensis (French Polynesia), but in terms of understanding phylogenetic relationships in the core group of Australian species and how they reached the central deserts, we soon reached an impasse. The level of variation was too low to obtain well-supported results, so we used restriction site-associated DNA sequencing (RADseq).
We further made use of a reference genome of N. bethamiana to extract over 100K high-quality variant positions from accessions representing all but one of the described species to determine relationships and how genetically distinct the new species are from those previously known. We then used unlinked biallelic data (SNPs) with a full coalescent method to sample phylogenetic trees from the posterior distribution of species trees and parameters. Our species tree produced a set of relationships nearly identical to those observed in the standard ML tree based on concatenated SNPs.
Finally, we rescaled the species tree, applying a generation time of one year and a general genome-wide mutation rate, which produced an estimate of one million years for their radiation in the Australian desert, agreeing in general with our previous molecular clock estimates. This is noteworthy due to its recentness, severity of habitats colonized and number of species involved.
Mark Chase, is a Senior Research Professor at Royal Botanic Gardens, Kew. He is visiting the Herbarium at CSIRO from 10-13 September and is very happy to meet with researchers and students during his time in Canberra. We will have lunch at ANU following Mark's seminar.