Fruit and seed morphology of Trichantherinae (Acanthaceae: Ruellieae) and its phylogenetic and ecological implications

Trichantherinae is the only exclusively Neotropical subtribe of Ruellieae (Acanthaceae), comprising approximately 80 species distributed from Mexico to western and northern South America. Although fruit and seed morphologies have provided important insights into the phylogeny and ecology of Ruellieae, these characters remain poorly documented for Trichantherinae. In this study, we analyzed the fruit and seed morphology of five out of the six genera within the subtribe, encompassing a total of 26 species, using stereomicroscopy and scanning electron microscopy. We present detailed morphological descriptions at the generic level, along with measurements and photographic illustrations for each species. Our results identify the reduction of seed trichomes as a newly reported synapomorphy for Trichantherinae. Furthermore, fruit and seed morphology proved effective in distinguishing among the genera of the subtribe. Louteridium, the sister-group to all other genera, is the most morphologically distinct, particularly due to the presence of marginal trichomes on the seeds. In contrast, these trichomes have been entirely lost in the core-Trichantherinae, supporting a synapomorphy for that clade. Members of the Bravaisia–Trichanthera clade are differentiated from those of the Sanchezia–Suessenguthia clade by their more robust capsules. Additionally, the presence of trichomes on the capsules distinguishes Suessenguthia and Trichanthera from their respective sister-genera. A notable and previously undescribed feature observed in this study is an atypical capsule dehiscence, here referred to as "fragmentary dehiscence." This novel characteristic in Trichantherinae may play a compensatory role in seed dispersal strategies, potentially offsetting the evolutionary loss of seed trichomes.