The Relatives of Nicotiana benthamiana - the Suaveolentes
The Suaveolente genomes range from 16 to 24 chromosome pairs and their evolution has involved genome duplication, addition and reduction. Goodspeed postulated that the original ancestral Nicotiana (now extinct) had a genome of 6 chromosome pairs that duplicated once, to give the predominantly 12 chromosome pair genomes of the South American Tabacum, Rustica and Petunioides subgenera, and occasionally twice to produce some 24 chromosome pair members. He suggested a number of possible paths for the evolution of the fifteen then-known Australian Suaveolente species and the essence of his favoured scenario shown in "Evolution of Benthamiana".
The ancient progenitor of the 24 chromosome pair N. debneyi (derived from an ancient 24 chromosome pair member of the South American Accuminate) and the ancient progenitor of the 16 chromosome pair N. suaveolens (derived from a genome-duplicated then chromosome-reduced Noctifloare or Alatae progenitor) spread into Australia before the continent separated from Antarctica. These two, by now diverged, species crossed and as a result of an unusual meiotic/fertilization event produced a 20 chromosome pair amphidiploid plant (12 chromosome pairs from N. debneyi and 8 from N. suaveolens) which became the progenitor of many members of the Suaveolentes.