A phylogenomic study of African Impatiens (Balsaminaceae) using nuclear and chloroplast sequences.

Abstract

Background and aims: The large genus, Impatiens, is well known to vary excessively, presenting extensive modifications in floral structure among its 1100+ species. Phylogenetic relationships in the genus have historically been difficult to intuit based on morphology, given numerous occurrences of convergent evolution, but they are also difficult to determine based on molecular characters. In this study we focus on resolving the phylogentic relationships within the genus Impatiens, with an emphasis on continental African taxa.

Methods: A phylogenetic tree including 206 Impatiens taxa across the genus, 128 of which are continental African taxa, was inferred from a target enrichment dataset targeting the Angiosperms353 loci and from genome skimming for plastomes. To estimate divergence times within the genus, we first generated a phylogenetic tree for the order Ericales, then calibrated it using 13 Ericalean fossils.

Key results: Crown Balsaminaceae dates to 59.3 mya and crown Impatiens to 45.4 mya. This dates the dispersals into Africa and subsequent radiation to the Miocene. We confirm three African dispersals from Asia, and we find two major clades within the third African dispersal: the Pan-African clade diversified across all regions of tropical Africa, while the East Africa clade diversified within Tanzania and Kenya. We find extensive discordance between plastid and nuclear topologies, but both support the monophyly of the seven sections in subgenus Impatiens.

Conclusions: The radiation of Impatiens is not relatively recent, and the genus dates to the Eocene. Even with increased taxon sampling and gene coverage, there are numerous instances of convergence and rapid evolution of floral forms in Impatiens. Nuclear phylogeny of the genus tracks geography closer than chloroplast phylogeny. A robust framework has been generated for further phylogenetic studies on this megadiverse genus.