The subfamily Gomphrenoideae (Amaranthaceae) is characterized by having bilocular anthers. It includes 19 genera and 400 species predominantly from the Neotropics. The goals of this project were to test the monophyly of the subfamily Gomphrenoideae using chloroplast trnL-F regions and rpl16 intron; to resolve the position of Iresine, which was inconclusive based on previous molecular studies in the family; to test the existence of a Tidestromia-Alternanthera clade; to establish the sister relationships of Tidestromia within the Gomphrenoideae; to reconstruct the phylogeny of Alternanthera using trnL-F and rpl16; to investigate subgeneric classifications proposed in Alternanthera ; to determine the origin, colonization, and diversification of species of Alternanthera in the Galápagos Islands; to resolve the phylogenetic relationships within Tidestromia using rpl16, trnL-F, ITS, and morphology; to investigate the role of soils in the evolution of Tidestromia; and to explore evolutionary trends in anthers, pseudostaminodia, pollen, and gynoecium characters in the subfamily Gomphrenoideae.

This study resolved that subfamily Gomphrenoideae is monophyletic and includes Iresine. Three strongly supported clades were recovered in the consensus trees (Bayesian and Parsimony) distinguished by pollen types. The phylogeny suggested that there is not a Tidestromia-Alternanthera clade, but a Tidestromia-Alternanthera-Pedersenia clade. Tidestromia was resolved as sister to Alternanthera and Pedersenia.

The molecular phylogeny of Alternanthera supports Endlicher's subgeneric classification modified using the former sections Bucholzia, Brandesia, and Mogiphanes. In addition, Alternanthera, a large genus in the Galápagos, colonized the Archipelago at least three times with one clade radiating on the islands.

The monophyly of Tidestromia was supported in independent and combined analyses. The use of all sources of data provided resolution among taxa and identified a putative hybrid origin within the genus. Edaphic conditions play an important role in the evolution of Tidestromia .

The optimization of andrecium and gynoecium characters in the obtained phylogeny of the subfamily Gomphrenoideae suggested that there are not trends in evolution of pseudostaminodia as were suggested by Eliasson. Gynoecium characters were informative within the Gomphrenoideae to define generic circumscriptions. Bisporangiate anthers, metareticulate pollen, tecta on distal bands, and microspines distributed either around the apertures or in distal rows are derived characters in the subfamily Gomphrenoideae.