Thepparatia vines (Hibisceae subtribe Trionum) phylogenomics and evolution

Abstract

Hibisceae subtribe Trionum is the largest clade within Hibiscus s.l. and contains genera with a wide range of growth forms: herbaceous (Abelmoschus), shrubs (Malvaviscus), trees (Wercklea), and exceptionally lianas. The rare and localized Thepparatia species are unique in that they combine a lianescent habit and trioecy. Several problems have delayed the study of Hibisceae classification including low genetic variability, lack of synapomorphic morphological characters, transoceanic dispersal events, and allopolyploidy. Here, we present a comprehensive phylogenomic hypothesis of Thepparatia based on analyses of the maternally inherited chloroplast and biparentally inherited nuclear ribosomal cistron sequences, and compare these results with its speciation, karyology, and breeding system. Chloroplast genomes in subtribe Trionum were highly conserved except in Abelmoschus where four additional genes were duplicated. Recent diploid interspecific hybridization is suggested in Thepparatia sipsongpannaensis between T. fragrans and T. scandens during the Pleistocene, around 640 000 years ago. Our observations also indicate Thepparatia have a trioecious breeding system that combined with a unique habit, and a highly localized distribution pattern, have created cross-pollination barriers and ecological specialization. This suggests hybridization events are rare but evolutionary significant in subtribe Trionum. Finally, our study supports important nomenclature changes: the synonymization of Hibiscus austroyunnanensis with Thepparatia fragrans and the publication of a new species, Thepparatia sipsongpannaensis.