Seed morphology of the Eriostemon-Phebalium clade (Rutaceae-Zanthoxyloideae) and implications for systematics

The Eriostemon-Phebalium clade (Rutaceae) includes c. 200 species across 16 genera, and is distributed in Australia (including Tasmania), New Caledonia, New Hebrides, New Zealand, and New Guinea. Phylogenetic analyses based on DNA sequences have consistently shown that the Eriostemon-Phebalium group is robustly monophyletic. However, despite this, taxonomy within the group is still uncertain: relationships between the genera are not well understood, and the genus Philotheca is polyphyletic. Seed morphological characters can be useful in taxonomy in angiosperms, and in this study, 21 seed morphological characters for 29 taxa of all the 16 genera circumscribed in the Eriostemon-Phebalium clade were examined using stereo and scanning electron microscopy. Seed morphological traits such as testa type and hilum were informative for some of the robustly supported clades in molecular phylogenetic studies, and a combination of seed characters could be useful for species identification. This study highlights the potential of seed morphological traits for the systematics.

Automated summary

The Eriostemon-Phebalium clade is a group of Rutaceae plants with approximately 200 species distributed in Australia, New Caledonia, New Hebrides, New Zealand, and New Guinea. While DNA sequence analyses confirm the monophyletic nature of the group, the taxonomy within the clade remains uncertain. This study explores the potential of seed morphological traits for species identification and highlights their usefulness in systematic research.