Justicieae II: Resolved Placement of Many Genera and Recognition of a New Lineage Sister to Isoglossinae
Authors: McDade Lucinda , Kiel Carrie , Daniel Thomas , Darbyshire Iain 
Published: August 2021

We present phylogenetic results for Justicieae, the largest of the major lineages of Acanthaceae. With 300% more sequence data and a similarly increased but also more geographically representative taxon sample compared to the only previous study of the lineage as a whole, we confirm the existence of the Pseuderanthemum Lineage, Isoglossinae, the Tetramerium Lineage and the Justicioid Lineage. To the last three lineages, we add a number of taxa with the goal of advancing our knowledge of genera (e.g., Isoglossa, Rhinacanthus) and of geographic areas (e.g., Malagasy and Asian Acanthaceae). These added taxa are accommodated within the phylogenetic framework for the lineages established in our earlier work with one exception. The monospecific genus Ichthyostoma is not placed in any of the established lineages but is instead sister to (Tetramerium Lineage + Justicioid Lineage). As this result is unexpected, we examined topologies both including and excluding this plant which revealed that other aspects of relationships remain stable whether Ichthyostoma is included or not. We point to Ichthyostoma as one of several Justicieae that will benefit from additional study. One species of Isoglossa is placed in the Tetramerium Lineage instead of with all other sampled members of the genus in Isoglossinae. Pollen morphology of I. variegata is more consistent with its phylogenetic placement here than with its taxonomic assignment to Isoglossa. As suggested in our earlier work, pollen morphology can be a powerful signal of phylogenetic relationships in Justicieae. With our increased sampling, we show that the Ptyssiglottis Lineage, a relatively small group of African and Asian plants, warrants recognition separate from Isoglossinae. With the addition of the Ptyssiglottis Lineage and excluding Ichthyostoma for now, the lineages of Justicieae are related as follows: (Pseuderanthemum Lineage ((Isoglossinae + Ptyssiglottis Lineage) (Tetramerium Lineage + Justicioid Lineage))). The Pseuderanthemum Lineage has been understudied to date and we here attempt to include representatives of all genera that are putatively part of this lineage and also to test monophyly of the larger genera. We identify a number of strongly supported clades but some aspects of relationship remain unresolved. Asystasia, Graptophyllum, Oplonia, and Pseuderanthemum are not monophyletic although, with a small number of nomenclatural changes, the first and second can be rendered monophyletic. With a range essentially as extensive as that of Justicieae as a whole, the Pseuderanthemum Lineage is biogeographically complex with patterns of distribution suggestive of multiple shifts between continents including Australia and parts of the Pacific basin. The calibrated phylogeny confirms that intercontinental shifts in distribution across Justicieae must have been by long-distance dispersal rather than by vicariance or stepwise dispersal over land bridges and also points to a number of groups that have potentially undergone rapid diversification. The Pseuderanthemum Lineage, in particular, requires considerable additional research to both understand relationships and achieve an informative taxonomy for the group. Although we here establish a phylogenetic framework across Justicieae, Next Generation Sequence data will be necessary to elucidate details of relationships in most lineages. Additional study of structural characters is also warranted as we continue to be unable to identify structural synapomorphies for a number of aspects of phylogenetic relationships that are very strongly supported by molecular data.