Structure and development of flowers in<i>Lepidagathis</i>and implications for systematics of the genus and floral evolution in Acanthaceae

Molecular studies have been important to understanding relationships in Acanthaceae, but morphological synapomorphies are only known for some infrafamilial clades. Recently, phylogenetic analysis of tribe Barlerieae indicated the inclusion of Lophostachys and Teliostachya in Lepidagathis, a genus with diverse floral morphology. This study examined floral structure and development in three species representing each of the three genera now combined into the revised Lepidagathis and aimed to test its broader circumscription and find new synapomorphies. The results reveal all three species share common features in their structure and development. The sepals develop unequally, and the pronounced growth of the first sepal in posterior (adaxial) position is associated with inhibition of the fifth stamen formation. The gynoecium of the three species is formed by two congenitally united carpels which form a ring-like primordium developing initially into a non-septate tube; it is divided only later into two locules by secondary inward growth and post-genital fusion of their margins. The data corroborate the inclusion of Lophostachys and Teliostachya in Lepidagathis and provide new insight into the floral evolution in Lamiales. The developmental pattern of sepals and the progressive reduction of stamens observed in Lepidagathis provide new insights into the evolution of androecium reduction in the family. In addition, the unusual mode of secondary formation of a septum in the symplicate ovary is a new synapomorphy for the broader circumscription of Lepidagathis and represents an evolutionary novelty reported for the first time in Acanthaceae.