Mayaca is a monogeneric family of aquatic herbs with approximately four species in subtropical to tropical America and west Africa. The reduced size and adaptation to the aquatic habit has made the placement of Mayaca within the commelinid clade difficult. In traditional systems of classification it has been suggested that this genus is related to Commelinaceae based on poricidal anther dehiscence and the operculate seeds, or to Xyridaceae based on parietal placentation and orthotropus, tenuinucellate ovules. However, subsequent data has shown that seed operculum development differs between Mayaca and Commelinaceae, and that Xyridaceae are variable in ovule development and orientation, as well as placentation. Despite this, Mayaca and Xyridaceae share an exothecium, a feature not known to occur in Commelinaceae. Recent analyses of ndhF and rbcL sequence data allied Mayaca to Bromeliaceae and Rapateaceae; the latter also have poricidal anther dehiscence. In our on-going analysis of rbcL, atpA, and cob sequence data representing all orders of monocots, Mayaca has been placed in a variety of positions, appearing either as sister to Eriocaulaceae or to Xyridaceae. These alternative hypotheses will be evaluated with respect to morphological data.

Key words: Bromeliaceae, commelinids, Eriocaulaceae, Mayaca, Rapateaceae, Xyridaceae