A role for environmental niche divergence in the rapid radiation of Nicotiana section Suaveolentes (Solanaceae)

Nicotiana sect. Suaveolentes is a group that has undergone recent and rapid radiation, a clade that originated ca. six million years ago and has since rapidly speciated throughout Australia, with a few taxa on Pacific Islands and one in Namibia (Africa). Post-tetraploidisation, species in N. sect. Suaveolentes vary in genome size and chromosome number (2.7 to 5.8 pg/1C nucleus; n = 15 to 24), which offers a novel opportunity to explore how underlying (and variable) polyploid genome structure may contribute to niche evolution during rapid radiation. Thus, we test whether there is a relationship between genome structure, and environmental niche variables, to observe if closely related taxa share more similar niches than more distantly related taxa in the section. We also test which environmental niche variables shape genome sizes or chromosome numbers of N. sect. Suaveolentes and could therefore play a potential role in speciation. We find that the species overall are undergoing niche divergence, which is likely a significant contributing factor to their radiation. Although genome sizes are conserved, we find they have a relationship with environmental niches, specifically edaphic, climatic and topographic variables, and therefore have a role in diversification. However, we do not find environmental relationships with chromosome number, which suggests this trait does not influence environmental niche evolution. This study deepens our understanding of niche evolution and the drivers of rapid speciation in Nicotiana, with potential implications in the nightshade family, Solanaceae. It also highlights the potential importance of genome size in angiosperm niche evolution and diversification.