The Gunnera flower:: Key to eudicot diversification or response to pollination mode?

Recent molecular phylogenetic studies have shown that the Gunnerales, including Gunnera and Myrothamnus, form a clade, sister to other core eudicots. This systematic position puts Gunnera in a pivotal position regarding the evolution of morphological characters in eudicots. In particular, the dimerous flower organization of Gunnera has recently been considered precursor to the pentamerous condition found in most core eudicots. In order to understand the role of Gunnera in the flower evolution of core eudicots, new detailed studies of the flowers of Gunnera are needed. In this study, the flower ontogeny and morphology of Gunnera monoica and G. dentata as well as of G. macrophylla were studied with SEM. The new observations are compared with previous studies of G. manicata and G. herteri. The first four species have dimerous perfect or staminate flowers with two small median sepals, followed by two large petals opposite the two stamens. In G. herteri, the staminate flowers are reduced to single stamens. In the pistillate flowers, there are two sepals, alternate to the two stylodia above the ovary. In G. monoica, two petal primordia initiate but abort early in floral development; in G. dentata, petal primordia arise after the sepals but remain smaller in the flower; in G. macrophylla petals are lacking. We focus on the floral reductions found in Gunnera and how these are connected to wind pollination common to the entire genus. Clear reductive trends in the perianth can be demonstrated in Gunnera, culminating in the completely perianthless G. herteri. A comparison is also made between the flowers of Gunnera and other simplified flowers such as those of Myrothamnus and other early-diverging eudicots. The floral reductions of dimerous Gunnerales cannot be related to the pentamerous core eudicots. Floral evolution of Gunneraceae, as well as that of other wind-pollinated, basally placed eudicots, is best seen as a unique specialization linked to this pollination mode.