CR2025_41 The diversity and evolutionary history of the shrimp plants (Acanthaceae: Justiciinae) in the palaeotropics.

Lead Supervisor: Iain Darbyshire, Royal Botanic Gardens, Kew

Email: i.darbyshire@kew.org

Co-supervisors: Alastair Culham, Department of Ecology and Evolutionary Biology, University of Reading; Alexandre Zuntini, Royal Botanic Gardens, Kew; Carrie Kiel, California Botanic Garden and Claremont Graduate University

The twin crises of extensive habitat loss and global climate change are driving many species – particularly those with restricted ranges – to imminent threat of extinction^1,2, hence it has never been more urgent to document global biodiversity and to understand which species are in most urgent need of conservation action. Effective species-based conservation is underpinned by accurate taxonomies and a robust understanding of the evolutionary history of species. This research project will use an integrative systematics approach, combining next generation sequencing (NGS) phylogenomic methods, macro- and micro-morphological analyses and phytochemistry, to study the evolutionary relationships within the palaeotropical clades of Justiciinae, to underpin the taxonomy and conservation of this important group.

With an estimated 4,900 species, the Acanthaceae are amongst the most diverse lineages of flowering plants globally, with concentrations in the tropics where they regularly fall within the 10 most species-rich families. Moreover, species of Acanthaceae can be locally abundant and form an important component of the ground flora. Conversely, an unusually high proportion of the species are very range-restricted and so face potentially heightened extinction risk. Hence, the family is of both ecological and conservation significance³. Despite this, our understanding of the evolutionary history and taxonomy of the Acanthaceae remains incomplete. Whilst the higher-level classification is now well documented, generic delimitation is problematic across the family^3,4, with widespread incongruence between the morphology-based classification and the molecular phylogeny^3,5. Nowhere is this more apparent than within the subtribe Justiciinae³, the most species-rich lineage of Acanthaceae, which includes the mega-diverse genus Justicia with potentially >1,000 species under its current circumscription including many range-restricted species. However, molecular phylogenetic evidence to date, based primarily on chloroplast and ITS data^6,7,8, has demonstrated that Justicia species do not belong to single lineage, but many lineages intertwined with a number of long-recognised and morphologically distinctive genera. Similarly, many of the sections within Justicia that were circumscribed based on morphology⁹ are not natural groupings^6,7. It is likely that a range of evolutionarily distinct and potentially threatened lineages are currently hidden within this broadly circumscribed Justicia. However, a revised classification of Justiciinae is hampered by the limited data resolution derived from sequencing techniques used to-date where many relationships remain unresolved and, moreover, from the insufficient sample size with only ca. 22% of the Justiciinae having available phylogenetic data. Research into the diversity of the Neotropical Justiciinae, which together form a clade distinct from the palaeotropical clades^7,8, is ongoing using

NGS techniques. However, apart from some studies on specific clades^10,11,12, the palaeotropical Justiciinae have not been studied further. There is, therefore, an urgent need to build a more comprehensive dataset using newly available, improved phylogenetic tools to gain a more complete understanding of this important lineage, to fuel taxonomic progress in light of the biodiversity crisis and inform taxon-based conservation planning based on accurate evolutionary data.

This study will sample across the full geographic range and morphological variation of the palaeotropical Justiciinae, with particular focus on poorly sampled groups and enigmatic species that are not readily placed using morphology. Molecular and phytochemistry sampling will be conducted primarily from herbarium specimens and tissue bank samples, supplemented by targeted fieldwork. By incorporating findings from pre-existing studies, we will aim for over 50% coverage of the palaeotropical taxa. Character evolution will be studied in herbaria and – where available – living collections, through systematic survey of a suite of morphological traits that are known to vary significantly within the lineage, subsequently mapping these onto the phylogeny and reconstructing putative ancestral states. This will also enable the identification of taxonomically informative morphological traits that will underpin a new classification of palaeotropical Justiciinae. Once a new phylogenetic framework is established, the student will focus their research in one of several integrative areas depending on their interest, including:

1. Combining their dataset with Neotropical data to assess diversification across the whole lineage and consider the evolutionary history of this group in the context of past environmental change to hypothesise what adaptations have driven such high rates of speciation, and how future humaninduced climate change may impact this diversity.
2. Linking the biogeography and diversification of this species rich lineage to conservation prioritisation, e.g., through an EDGE (Evolutionarily Distinct and Globally Endangered) approach.

Training opportunities: 

This project offers a range of training in both laboratory and collections-based scientific settings with a strong analytical component. Whilst the plant material for analysis will be derived primarily from herbaria, this will be supported by fieldwork to source fresh samples and observe species of Justiciinae in the wild, targeting a geographic region that is under-represented in existing collections such as Madagascar or southeast Asia. We will also explore the possibility of a placement at co-supervisor Carrie Kiel’s laboratory in California to learn first-hand some of the analytical techniques developed there to study the evolutionary history and biogeography of Acanthaceae.

Student profile:

This project would be suitable for students with a Master’s degree in botanical sciences or a closely related biological discipline, with a particular emphasis on biodiversity and/or biogeography. Strong analytical skills and a knowledge of molecular phylogenetic techniques are highly desirable.

Please note: Due to the nature of this project and to comply with visa regulations, only Home students should apply.

References:

1. Silva, J.M.C. da, Rapini, A., Barbosa, L.C.F. & Torres, R.R. (2019). Extinction risk of narrowly distributed species of seed plants in Brazil due to habitat loss and climate change. PeerJ 7: e7333 https://doi.org/10.7717/peerj.7333
2. Gaston, K.J., & Fuller, R.A. (2009). The sizes of species’ geographic ranges. Journal of Applied Ecology 46: 1 – 9. https://doi.org/10.1111/j.1365–2664.2008.01596.x
3. Manzitto-Tripp, E.A., Darbyshire, I., Daniel, T.F., Kiel, C.A. & McDade, L.A. (2022). Revised classification of Acanthaceae and worldwide dichotomous keys. Taxon 71: 103 – 153. https://doi.org/10.1002/tax.12600
4. Christenhusz, M.J.M., Fay, M.F. & Chase, M.W. (2017). Plants of the World: An Illustrated Encyclopedia of Vascular Plants. Royal Botanic Gardens, Kew. https://doi.org/10.7208/chicago/9780226536705.001.0001
5. McDade, L.A., Daniel, T.F. & Kiel, C.A. (2018). The Tetramerium Lineage (Acanthaceae, Justicieae) revisited: Phylogenetic relationships reveal polyphyly of many New World genera accompanied by rampant evolution of floral morphology. Systematic Botany 43: 97 – 116. https://doi.org/10.1600/036364418X697003
6. Kiel, C.A., Daniel, T.F., Darbyshire, I. & McDade, L.A. (2017). Unraveling relationships in the morphologically diverse and taxonomically challenging ‘justicioid’ lineage (Acanthaceae, Justicieae). Taxon 66: 645 – 674. https://doi.org/10.12705/663.8
7. Kiel, C.A., Daniel, T.F. & McDade, L.A.. (2018). Phylogenetics of New World ‘justicioids’ (Justicieae: Acanthaceae): major lineages, morphological patterns, and widespread incongruence with classification. Systematic Botany 43: 459 – 484. https://doi.org/10.1600/036364418X697201
8. McDade, L.A., Daniel, T.F., Darbyshire, I. & Kiel, C.A. (2020). Justicieae II: Resolved placement of many genera and recognition of a new lineage sister to Isoglossinae. Aliso 38: 1 – 31. https://doi.org/10.5642/aliso.20213801.02
9. Graham, V.A. (1988). Delimitation and infra-generic classification of Justicia (Acanthaceae). Kew Bulletin 551 – 624. https://doi.org/10.2307/4129957
10. Darbyshire, I., Kiel, C.A., Daniel, T.F., McDade, L.A. & Luke, W.R.Q. (2019). Two new genera of Acanthaceae from tropical Africa. Kew Bulletin 74: 39. https://doi.org/10.1007/s12225–019–9828–z
11. Darbyshire, I., Kiel, C.A., Astroth, C.M., Dexter, K.G., Chase, F.M. & Tripp, E.A. (2020). Phylogenomic study of Monechma reveals two divergent plant lineages of ecological importance in the African savanna and succulent biomes. Diversity 12: 1 – 26. https://doi.org/10.3390/d12060237 ¹² Lin, Z., Deng, Y. et al. (in prep.). Phylogenetic study of Rungia (Acanthaceae) [title tbc].