THE MEGAGAMETOPHYTE IN ANARTHRIA (ANARTHRIACEAE, POALES) AND ITS IMPLICATIONS FOR THE PHYLOGENY OF THE POALES

The development and structure of the megagametophyte of Anarthria (Anarthriaceae), Aphelia, and Centrolepis (Centrolepidaceae) are described. Anarthriaceae has tenuinucellate ovules and the Polygonum type of megagametophyte development, both characters typical of the Poales. However, it lacks the anticlinally elongated nucellar epidermis and numerous large starch bodies observed in the megagametophyte of Centrolepidaceae, both characters also present in Restionaceae. This relatively generalized megagametophyte structure is consistent with data from the chloroplast genome, which suggest that Anarthriaceae are not as closely related to Restionaceae as previously assumed. New data from the megagametophyte are analyzed cladistically together with other available information on the poalean families. The results show that there are two possible positions for Anarthriaceae: either as sister to Poaceae, Joinvilleaceae, Restionaceae, Ecdeiocoleaceae, and Restionaceae, or as sister to only the latter three families. The new data also allow a critical reevaluation of the phylogenetic position of Centrolepidaceae, which is either basal to the poalean clade (based on microgametophyte data), or embedded in the Restionaceae (based on anther structure and megagametophyte data).